:After recapping, rewiring (carefully checking wire dress), cleaning variable condenser plates, and retesting all tubes, I still can't get the set aligned. Here's the problem...IF (455KC) can be easily set, but that's it. On the BC band, what I'm getting is pretty good reception and dial (LO) tracking from about 800KC to 1200KC. I get no reception above 1200KC. Below 800KC I get image frequencies for the upper third of the dial. So that, for example, with the dial set at 600KC, I'm receiving a clear station at 1510KC! With my signal generator set for frequencies above 1200KC, the signal comes through weakly with the dial set at those freqs, but come through much stronger with the dial set below 800KC at the image freq. I have checked all trimmers for shorts, could they still be bad? No amount of tweaking them helps. Somehow the detector (6J5G) is allowing the image freqs to come through and override the dial setting. HELP please, this is affecting my sleep!
:
:Hi Marc...triple check your wiring. I once had similiar problems that turned out to be a wire to chassis gnd that was left off...Neal
:
::After recapping, rewiring (carefully checking wire dress), cleaning variable condenser plates, and retesting all tubes, I still can't get the set aligned. Here's the problem...IF (455KC) can be easily set, but that's it. On the BC band, what I'm getting is pretty good reception and dial (LO) tracking from about 800KC to 1200KC. I get no reception above 1200KC. Below 800KC I get image frequencies for the upper third of the dial. So that, for example, with the dial set at 600KC, I'm receiving a clear station at 1510KC! With my signal generator set for frequencies above 1200KC, the signal comes through weakly with the dial set at those freqs, but come through much stronger with the dial set below 800KC at the image freq. I have checked all trimmers for shorts, could they still be bad? No amount of tweaking them helps. Somehow the detector (6J5G) is allowing the image freqs to come through and override the dial setting. HELP please, this is affecting my sleep!
::
:
:
Hey Neal, thanks for the reply. This problem has consumed me for over 2 months! I think I'm at wiring/cap check #47!! Did not fire up the radio before rewiring and recapping due to total deterioration of original wiring. Am begining to think I have a defective LO...I have been able to "program" the pushbuttons with their separate oscillators pretty much all up and down the dial. Any suggestions on how to check the LO?? I think I may try a loose coupling to my freq counter and see what the LO is reading. I've done this on other radios with mixed results - sometimes it works sometimes not. Wish I had a grid-dip meter. Anyway, I will go back and check all circuit grounding points. Marc
::Hi Marc...triple check your wiring. I once had similiar problems that turned out to be a wire to chassis gnd that was left off...Neal
::
:Hey Neal, thanks for the reply. This problem has consumed me for over 2 months! I think I'm at wiring/cap check #47!! Did not fire up the radio before rewiring and recapping due to total deterioration of original wiring. Am begining to think I have a defective LO...I have been able to "program" the pushbuttons with their separate oscillators pretty much all up and down the dial. Any suggestions on how to check the LO?? I think I may try a loose coupling to my freq counter and see what the LO is reading. I've done this on other radios with mixed results - sometimes it works sometimes not. Wish I had a grid-dip meter. Anyway, I will go back and check all circuit grounding points. Marc
:
::Hi Marc...triple check your wiring. I once had similiar problems that turned out to be a wire to chassis gnd that was left off...Neal
::
:Hey Neal, thanks for the reply. This problem has consumed me for over 2 months! I think I'm at wiring/cap check #47!! Did not fire up the radio before rewiring and recapping due to total deterioration of original wiring. Am begining to think I have a defective LO...I have been able to "program" the pushbuttons with their separate oscillators pretty much all up and down the dial. Any suggestions on how to check the LO?? I think I may try a loose coupling to my freq counter and see what the LO is reading. I've done this on other radios with mixed results - sometimes it works sometimes not. Wish I had a grid-dip meter. Anyway, I will go back and check all circuit grounding points. Marc
:
Signal generator is directly coupled to antenna post thru 200pf cap, so should not be getting any stray signals. Alas, my freq counter will not register LO freq loose coupled and is erratic coupled to cathode of converter.!@#$#%$. Tuning condenser is good and since problem occurs at settings above 1200KC, plates are at minimum capacitance (near to full open)so shorting is not a problem (plus I've checked for this with meter). However, I do get some howling when dial is set to these upper freqs...does sound like wire dress, but poking around, moving wires does not change anything and I can't find any grounding. Wait!!! I have found a bypass cap that appears to be in the wrong place (C2) (cap looked original when I re-capped so I just replaced it) but the schematic shows C2 in a different position - off of antenna coil. I'll move it and see what happens. Maybe C2 was replaced years ago in the wrong spot!
Marc
:Neil...
:Signal generator is directly coupled to antenna post thru 200pf cap, so should not be getting any stray signals. Alas, my freq counter will not register LO freq loose coupled and is erratic coupled to cathode of converter.!@#$#%$. Tuning condenser is good and since problem occurs at settings above 1200KC, plates are at minimum capacitance (near to full open)so shorting is not a problem (plus I've checked for this with meter). However, I do get some howling when dial is set to these upper freqs...does sound like wire dress, but poking around, moving wires does not change anything and I can't find any grounding. Wait!!! I have found a bypass cap that appears to be in the wrong place (C2) (cap looked original when I re-capped so I just replaced it) but the schematic shows C2 in a different position - off of antenna coil. I'll move it and see what happens. Maybe C2 was replaced years ago in the wrong spot!
:Marc
:
:Neil...
:Signal generator is directly coupled to antenna post thru 200pf cap, so should not be getting any stray signals. Alas, my freq counter will not register LO freq loose coupled and is erratic coupled to cathode of converter.!@#$#%$. Tuning condenser is good and since problem occurs at settings above 1200KC, plates are at minimum capacitance (near to full open)so shorting is not a problem (plus I've checked for this with meter). However, I do get some howling when dial is set to these upper freqs...does sound like wire dress, but poking around, moving wires does not change anything and I can't find any grounding. Wait!!! I have found a bypass cap that appears to be in the wrong place (C2) (cap looked original when I re-capped so I just replaced it) but the schematic shows C2 in a different position - off of antenna coil. I'll move it and see what happens. Maybe C2 was replaced years ago in the wrong spot!
:Marc
:
It's an 8-S-563 and it gets no reception on short wave. The same phenomina occurs however, namely the dial is silent until you get to the high third, then it begins to howl and chirp intermittently - that's it. Sure ought to have something to do with the tuning condenser, but that thing is clean as a whistle and I can' find a short anywhere (and I've had several radios with this problem, usually shows up at the low end of the dial where the plates are meshed more completely). Took it out again and put new grommets on the anchors - no luck. Tomorrow I'll reistall C2 where it appears on the schematic and let you know what happened. I've got another set of NOS tubes coming just to make sure they aren't the problem....and it continues!@#$%^&!
Marc
Sir Marc . . . .
Reading over the conversation between "youse guys" . . a . . la Noo Yawk Citee . . or Joisey Citee . . . so far.
If you inspect the C1 tuning condenser VEWY carefuwwy . . .a . .la Elmer Fudd . . . . you will see that it has
same sized rotor plate clusters being used for the variable(s) antenna tuning function and the oscillator tuning function .
And since you are now talking about BCB operation, it will be needing effectively less capacitance to be used by the
tuning of the OSCILLATOR section of C1 .
You will be needing the resultant oscillator spectrum to be encompassing from ~955 Kc swinging on upward
to ~ 2205 Kc. in the coverage of the BCB.
Since the effective capacitance present on the C1 oscillator section is FAR too great for attaining that range, it is then
electrically dependent upon the adjunct utilization of an adjustable capacitance being in series to ground with it. . . .aka . .
a padder condenser.
E.g. . . .if the variable capacitor was 15-420 pf thru its adjustment range, then if a 450 pf cap was placed in series with it, the effective adjustment range of the unit would then become from ~12-220 pf . . ..HEY ! . . .that's getting in the order of the value needed . .for those higher osc frequencies, so if one then adjusts that ~450 series value, it can be trimmed in to the exact resultant value needed.
On your set, that is being fulfilled with "padder" condenser "J". If you willl now check it, I think that you will find it so grossly misadjusted . . too much capacitance . . . such that it has skewed the sets reception by compressing it
waaaaay down towards and even below the 550 spectrum.
Corrective action:
Tune onto the first station that you can receive downwards from 1200 . .where I think that you said that you lost all upwards from that point .
You then tune just off from it , higher in freq, and slightly walk "J" adjustment until you are "on" that station again and
then do the same repeatedly until you have "skewed" the WHOLE BCB spectrum out of that shifted and "compressed"
state and the stations then start spreading apart and start falling into place and approaching proper dial scale alignment.
All is being accomplished by the use of that "J" padder adjustment.
In the end, the C1 ANTENNA section tuning trimmer might need peaking, to optimize the ~1400-1500 kc sensitivity, and
overall tracking.
Standing by for feedback . . . .
73's de Edd
Thanks for the very informative test procedure. Sorry this is late but I just returned from having a toupee fitted as I just finished pulling out the last hairs on my head!! Here's what happened after following your advice. Sure enough, after opening and opening and opening "J", I was able to get the freqs above 1200KC to come in. Problem is the lower 2/3 of the dial were so far out of track I couldn't get them back, even using "G" (antenna trimmer)and in fact I'd lost everything below about 800KC again. Forget "F" (BC Osc trimmer)as it is so sensitive that tuning it sends everything out of whack instantly. I reset "J" so that I have good tracking and reception below 1200KC all the way to 550KC. "J" now reads about 470pf which is in the range you specified. I can get an intermittent signal on a station at 1230KC but it breaks up badly. I noticed this sometimes happens on other stations. Ended up checking all speaker & antenna plug connections as well as tube sockets AGAIN - all seems tight. One last thing. The only connection that I can 'wiggle' and get popping noises from is the grid lead to IF amp tube (6K7G) so I pulled the 1st IF can cover and replaced the lead, even though it looked okay. Could it be a problem with the control grid in the tube? It tests good, but it wouldn't be the first time a tube tested good but a replacement fixed the problem (I have a NOS tube coming). OR, maybe I will have to live with 2/3 of a dial!
........standing by, Marc
One more thing. Every once in awhile, on the higher freqs, the output starts motorboating, heterodyning. I can clear it up immediately by placing my finger on the grid cap of the 6K7 (IF). Volume decreases slightly but motorboating stops. I can't read the schematic very well, but I think it says voltage at the 6K7 grid cap should be -45 (-4.5??) and I'm reading about -2.7v. Do you make anything of this? Thanks, Marc
Sir Marc . . . .
Great on your progress so far. . . . but the alignments further finalization is now dependent upon an interrelated "Ying and Yang " type of situation .
Specifically . . .Re your :
I will expound upon those series of statements a bit: Now, remember back to our resolving the requirements that the sets local oscillator would need to be having the tuning condensers plates swung fully open to be tuning to the high end of the BCB and the resultant capacitive value being connected across the fixed inductive value of the oscillator coil. The pair will be creating the oscillator resonant circuit setting the high end NOW you go back to your situation of the 420 pf of value that the tuning condenser has in its fully meshed positon and the fact that value is WAAAAYYY more capacitance than you can use for the creation of a 955 Kc signal from the oscillator to then be hetrerodyned within the mixer and create the desired 455 IF signal. So with that info now perceived, you need to work with the adjustment of the " F " to establish your very high end of the BCB BUT, keeping in your mind also, that each ad justment will slightly counter react with the other adjustment, but also having evaluated all of the the "how it works" info above and now know that the adjustment of the "J " padder has maximum effect in establishing the very low end of the dial scale correlation while the " F " trimmer sets the very high end of the dial scale correlation. Seems like I remember a freq counter being mentioned ? If you should be so lucky as to have a full function one with dedicated adjustable preamp and triggering capabilities . . . . . . .VERSUS . . . . .the Mickey M . . .O. . .U . . .S. . .E. . ..incorporation of a He'd go off, rolling away the unit, with the intense concentration of need of correcting a fallacy akin to the trans-siderial commutation and declination of a Martian lunar cycle.
Forget "F" (BC Osc trimmer)as it is so sensitive that tuning it sends everything out of whack instantly.
Now, that " F " trimmer adjustment is used for the " Custom Tailoring " of the MINIMUM capacitance value of your C1 tuning condenser oscillator section specs /capacitance adjustment range .
Whereas , as you have just discovered, that the " J " . . .series padder trimmer . . .is being used to " Custom Tailor " the MAXIMUM value of C1's oscillator section.
You have that capacitors cluster of stator plates sitting there in their "island", being all rigidly, mechanically mounted and insulated away from the condensers frame proper. That will result in the probable effective capacitance, in reference to the distant framework, only having resulted in creating about 10 pf of capacitive value. Then, if you have the rotor clusters plates swung to their maximum unmeshed position the resultant gap between the two clusters edges of plates will result in only the addition of but an extra ~7 pf or so.
Thus, you come up with an effective capacitance value of ~17 pf with the tuning condensers oscillator section in its full unmeshed plate positoning.
One the other end of the spectrum, with the tuning condenser in its fully meshed position, you would find the closely intermeshing of all of the plates having resulted in the presence of two popular values of capacitance value that tuning condensers are usually offered as.
One value is ~365 and the other ~420 pf, (540 sez, don't forget me !) and on your particular set, I am thinking it to be closer to the 420 pf value.
~2205 Kc signal required to heterodyne with an incoming received signal of 1750 Kc to result in an outputted 455 Kc IF signal.
Now here is where another variable comes in, the utilization of a trimmer capacitor most commonly built right onto the end of the stator plate cluster and creating capacitance between its single leaf, separated with a mica insulator, and compressing up against the condenser framework.
This would let the minimum capacitive value of the condenser in its almost fully opened open conditon , be "fine tuned" for frequency of operation. So figure that an initial value of ~17 pf from the tuning condenser proper and then the addition of ~7 pf from the built trimmer, if adjusted to its leaf max separated position, creates an effective paralled capacitance value of ~22 pf.
In its other extreme, if that osc trimmer is max compressed , expect that being in the order of ~15 pf , thereby creating a combined capacitive pair value of ~32 pf.
That is quite a capacitance adjustment swing capability at the high end of the BCB, in having an initial value of only ~22 pf but being able to add up to another ~15 pf of capacitive value with the trimmer being in its max compressed state.
BUT T T T .. there is a like additional aspect, and that would be the adding of that SAME adjusted value of trimmer capacitance to the tuning condenser in its FULLY meshed capacitance position, when you are tuning for the reception of a received signal down at the low end of the BCB at 540 Kc.
Putting it in a perspective and comparative situation, 15 pf of capacitance change added to an initial 420 pf value would be minimal as compared to only be working with ~22 pf of capacitance available, and then adding up to a full 15 pf of value.
So you can see that there would be an additional skewing of the frequency down at the low end of the BCB when adjusting that osc trimmer capacitance value, but mainly that " F " trimmer is associative with its adjustment being utilized for the adjustment of the oscillator up at the high end of the BCB.
BUT, you have already found that the utilization of the series added adjustable padder condenser "J " will permit you to precisely trim in that max capacitance value of the tuning condenser.
dial scale alignment correlation in the 1400 KC area, and then check back down at the low end of the BCB to see how the ~600 Kc alignment to the dial scale is setting. Down there, it can be tweaked with the " J " padder condenser adjustment.
added "frequency counter" function to a common DVM with its limited 'lebenteen" other "SUPER" add on functions, even including the addition of a stainless steel probe set for analyzing the acidity index of tiger urine when consulting an accompanying cross correlation chart.
You could loosely, capacitively couple into the osc circuit to monitor its freq output and check 600 Kc dial scale setting as having
an ~1055 Kc outputted signal and then swing up to 1400 Kc on the dial scale and check for an osc outputted signal of ~1855 Kc.
No Additional Charge . . . Add on extra:
Now if you have a freq counter with less than optimal front end conditoning and processing , a technique which has served me admirably for multi-decades is the use of a Tektronix scope which has a Gated output on the rear for A or B /or/both Vert channels. (This even goes back to the days double strapped carrying . . . ."BLEEP". . . I didn't mean carrying, I meant manhandling handles . . .on those HUGE old 535, 45, 65 etc behemoths).
The idea is for the utilization of the vert amp section of the scope to be coupled to the circuiitry under evaluation and
the vert sensitivity advanced to the point that the presence of the RF signal amplitude is starting to fill up the screen.
That will initially be needing the Hoz sweep timebase being set to the order of a .05 us sweep rate. Now, even at that speed , don't expect to be viewing . .one little wavelet . . .two little wavelets . .three little wavelets . . .of that frequency.
Instead, just a massed /compressed horizontal display of all of the frequencies sines, and the vert deflection level signifying
their voltage amplitude.
They are not even to be seen at 10X mag, but possible, if the scope has B delayed sweep and its resolving capability is utilized
BUT expect the face display trace brighness to be diminished to a mere "candle light" level.
Also, the scope NEEDS to be in conventional sweep mode anyway, and at that speed the monitoring of a 950 kc signal wil result on the scope triggering and producing less than a full sine across the display, in the rear Gated output, there will be the outputting of up to a 6.6 V positive polarity pulse created by the presence of the scope triggering on that incoming sine wavelets presence. Now if a frequency counter coupled into that Gate connector doesn't respond flawlessly to that pristine of, and level of signal . . .you might as well chunk that counter and shed your socks and utilize a finger and toe count of the signals frequency.
The only caveat is that the Hoz sweep timebase be set fast enough such that the trace only responds to one sine. . . . (or less than a single sine). If it does retrigger during a single display trace, there will be the outputting of a second gate signal on the rise and triggering of the second start of the sine on the scope screen. ( This is very demonstrable if one uses a lower frequency, such as an ~ 1KC signal being applied to the scope probe from your CAL output square wave fron the units front terminal.)
( I used to razzzz any "newbie" of the calibration staff at NASA with my sometime retort of " This scopes cal is in error . . .its outputting at 1002 cycles ! " )
If the scope retriggers on a sweep trace, such that another sine starts, one merely needs to realize that the freq count would be doubled such that you have to mentally halve your "frequency count" number. (Or divide by three if three wavelets get thru on a single trace display, etc)
This extends the frequency capability a wee bit as, ~2 Mc is the upper limit in only having a .05 us Hoz timebase fastest sweep speed capability on most units.
END
As per your spurious oscillation situation, check tube shields and presence of good grounding. Also if the screen grids of the RF tubes are being decoupled less than optimally, a feedback oscillation might make its presence known. To confirm that situation
I will take a short leaded .01 ufd disk CERAMIC capacitor and hold in place to shunt across from screen grid to nearby acessible chassis ground . If the oscillation / squeal disappers or even shifts , I will know that stage needed more efficient RF bypassing, and then move on to other RF stages for doing the same test.
On SOME select few sets, even too sharp of a peaked IF alignment, can cause a problematic spurious oscillation onset.
As per that RF tubes grid voltage . .definitely not -45 . . but -2.7 is more in the range . . AS . .even that value will vary in accordance to any AVC voltage involved.
Standing by for feedback . . . .
73's de Edd
:
:
:
:
:
:Sir Marc . . . .
:
:
:
:
:
:Great on your progress so far. . . . but the alignments further finalization is now dependent upon an interrelated "Ying and Yang " type of situation .
:
:
:Specifically . . .Re your :
:
:
:
:
:Forget "F" (BC Osc trimmer)as it is so sensitive that tuning it sends everything out of whack instantly.
:
:
:
:Now, that " F " trimmer adjustment is used for the " Custom Tailoring " of the MINIMUM capacitance value of your C1 tuning condenser oscillator section specs /capacitance adjustment range .
:
:
:
:
:Whereas , as you have just discovered, that the " J " . . .series padder trimmer . . .is being used to " Custom Tailor " the MAXIMUM value of C1's oscillator section.
:
:
:
:I will expound upon those series of statements a bit:
:
:
:You have that capacitors cluster of stator plates sitting there in their "island", being all rigidly, mechanically mounted and insulated away from the condensers frame proper. That will result in the probable effective capacitance, in reference to the distant framework, only having resulted in creating about 10 pf of capacitive value. Then, if you have the rotor clusters plates swung to their maximum unmeshed position the resultant gap between the two clusters edges of plates will result in only the addition of but an extra ~7 pf or so.
:
:
:Thus, you come up with an effective capacitance value of ~17 pf with the tuning condensers oscillator section in its full unmeshed plate positoning.
:
:
:One the other end of the spectrum, with the tuning condenser in its fully meshed position, you would find the closely intermeshing of all of the plates having resulted in the presence of two popular values of capacitance value that tuning condensers are usually offered as.
:
:
:One value is ~365 and the other ~420 pf, (540 sez, don't forget me !) and on your particular set, I am thinking it to be closer to the 420 pf value.
:
:
:
:Now, remember back to our resolving the requirements that the sets local oscillator would need to be having the tuning condensers plates swung fully open to be tuning to the high end of the BCB and the resultant capacitive value being connected across the fixed inductive value of the oscillator coil. The pair will be creating the oscillator resonant circuit setting the high end
:~2205 Kc signal required to heterodyne with an incoming received signal of 1750 Kc to result in an outputted 455 Kc IF signal.
:Now here is where another variable comes in, the utilization of a trimmer capacitor most commonly built right onto the end of the stator plate cluster and creating capacitance between its single leaf, separated with a mica insulator, and compressing up against the condenser framework.
:
:
:This would let the minimum capacitive value of the condenser in its almost fully opened open conditon , be "fine tuned" for frequency of operation. So figure that an initial value of ~17 pf from the tuning condenser proper and then the addition of ~7 pf from the built trimmer, if adjusted to its leaf max separated position, creates an effective paralled capacitance value of ~22 pf.
:In its other extreme, if that osc trimmer is max compressed , expect that being in the order of ~15 pf , thereby creating a combined capacitive pair value of ~32 pf.
:
:
:That is quite a capacitance adjustment swing capability at the high end of the BCB, in having an initial value of only ~22 pf but being able to add up to another ~15 pf of capacitive value with the trimmer being in its max compressed state.
:
:
:BUT T T T .. there is a like additional aspect, and that would be the adding of that SAME adjusted value of trimmer capacitance to the tuning condenser in its FULLY meshed capacitance position, when you are tuning for the reception of a received signal down at the low end of the BCB at 540 Kc.
:
:
:Putting it in a perspective and comparative situation, 15 pf of capacitance change added to an initial 420 pf value would be minimal as compared to only be working with ~22 pf of capacitance available, and then adding up to a full 15 pf of value.
:
:
:So you can see that there would be an additional skewing of the frequency down at the low end of the BCB when adjusting that osc trimmer capacitance value, but mainly that " F " trimmer is associative with its adjustment being utilized for the adjustment of the oscillator up at the high end of the BCB.
:
:
:
:NOW you go back to your situation of the 420 pf of value that the tuning condenser has in its fully meshed positon and the fact that value is WAAAAYYY more capacitance than you can use for the creation of a 955 Kc signal from the oscillator to then be hetrerodyned within the mixer and create the desired 455 IF signal.
:
:
:BUT, you have already found that the utilization of the series added adjustable padder condenser "J " will permit you to precisely trim in that max capacitance value of the tuning condenser.
:
:
:
:So with that info now perceived, you need to work with the adjustment of the " F " to establish your very high end of the BCB
:dial scale alignment correlation in the 1400 KC area, and then check back down at the low end of the BCB to see how the ~600 Kc alignment to the dial scale is setting. Down there, it can be tweaked with the " J " padder condenser adjustment.
:
:
:
:BUT, keeping in your mind also, that each ad justment will slightly counter react with the other adjustment, but also having evaluated all of the the "how it works" info above and now know that the adjustment of the "J " padder has maximum effect in establishing the very low end of the dial scale correlation while the " F " trimmer sets the very high end of the dial scale correlation.
:
:
:
:Seems like I remember a freq counter being mentioned ? If you should be so lucky as to have a full function one with dedicated adjustable preamp and triggering capabilities . . . . . . .VERSUS . . . . .the Mickey M . . .O. . .U . . .S. . .E. . ..incorporation of a
:added "frequency counter" function to a common DVM with its limited 'lebenteen" other "SUPER" add on functions, even including the addition of a stainless steel probe set for analyzing the acidity index of tiger urine when consulting an accompanying cross correlation chart.
:
:
:You could loosely, capacitively couple into the osc circuit to monitor its freq output and check 600 Kc dial scale setting as having
:an ~1055 Kc outputted signal and then swing up to 1400 Kc on the dial scale and check for an osc outputted signal of ~1855 Kc.
:
:
:
:No Additional Charge . . . Add on extra:
:
:
:
:Now if you have a freq counter with less than optimal front end conditoning and processing , a technique which has served me admirably for multi-decades is the use of a Tektronix scope which has a Gated output on the rear for A or B /or/both Vert channels. (This even goes back to the days double strapped carrying . . . ."BLEEP". . . I didn't mean carrying, I meant manhandling handles . . .on those HUGE old 535, 45, 65 etc behemoths).
:
:
:The idea is for the utilization of the vert amp section of the scope to be coupled to the circuiitry under evaluation and
:the vert sensitivity advanced to the point that the presence of the RF signal amplitude is starting to fill up the screen.
:
:
:That will initially be needing the Hoz sweep timebase being set to the order of a .05 us sweep rate. Now, even at that speed , don't expect to be viewing . .one little wavelet . . .two little wavelets . .three little wavelets . . .of that frequency.
:
:
:Instead, just a massed /compressed horizontal display of all of the frequencies sines, and the vert deflection level signifying
:their voltage amplitude.
:
:
:They are not even to be seen at 10X mag, but possible, if the scope has B delayed sweep and its resolving capability is utilized
:BUT expect the face display trace brighness to be diminished to a mere "candle light" level.
:
:
:Also, the scope NEEDS to be in conventional sweep mode anyway, and at that speed the monitoring of a 950 kc signal wil result on the scope triggering and producing less than a full sine across the display, in the rear Gated output, there will be the outputting of up to a 6.6 V positive polarity pulse created by the presence of the scope triggering on that incoming sine wavelets presence. Now if a frequency counter coupled into that Gate connector doesn't respond flawlessly to that pristine of, and level of signal . . .you might as well chunk that counter and shed your socks and utilize a finger and toe count of the signals frequency.
:
:
:The only caveat is that the Hoz sweep timebase be set fast enough such that the trace only responds to one sine. . . . (or less than a single sine). If it does retrigger during a single display trace, there will be the outputting of a second gate signal on the rise and triggering of the second start of the sine on the scope screen. ( This is very demonstrable if one uses a lower frequency, such as an ~ 1KC signal being applied to the scope probe from your CAL output square wave fron the units front terminal.)
:
:
:( I used to razzzz any "newbie" of the calibration staff at NASA with my sometime retort of " This scopes cal is in error . . .its outputting at 1002 cycles ! " )
:
: He'd go off, rolling away the unit, with the intense concentration of need of correcting a fallacy akin to the trans-siderial commutation and declination of a Martian lunar cycle.
:
:
:If the scope retriggers on a sweep trace, such that another sine starts, one merely needs to realize that the freq count would be doubled such that you have to mentally halve your "frequency count" number. (Or divide by three if three wavelets get thru on a single trace display, etc)
:
:
:This extends the frequency capability a wee bit as, ~2 Mc is the upper limit in only having a .05 us Hoz timebase fastest sweep speed capability on most units.
:
:
:END
:
:
:
:As per your spurious oscillation situation, check tube shields and presence of good grounding. Also if the screen grids of the RF tubes are being decoupled less than optimally, a feedback oscillation might make its presence known. To confirm that situation
:I will take a short leaded .01 ufd disk CERAMIC capacitor and hold in place to shunt across from screen grid to nearby acessible chassis ground . If the oscillation / squeal disappers or even shifts , I will know that stage needed more efficient RF bypassing, and then move on to other RF stages for doing the same test.
:
:
:On SOME select few sets, even too sharp of a peaked IF alignment, can cause a problematic spurious oscillation onset.
:
:
:As per that RF tubes grid voltage . .definitely not -45 . . but -2.7 is more in the range . . AS . .even that value will vary in accordance to any AVC voltage involved.
:
:
:
:
:
:
:
:
:
:
:
:Standing by for feedback . . . .
:
:73's de Edd
:
:
:
:
:
:
I understood most of what you said, but i'm not sure that what I thought you said is what you really meant...Just trying to be funny. Actually I'm hanging by, very much interested in this problem...Neal
::
::
::
::
::
::Sir Marc . . . .
::
::
::
::
::
::Great on your progress so far. . . . but the alignments further finalization is now dependent upon an interrelated "Ying and Yang " type of situation .
::
::
::Specifically . . .Re your :
::
::
::
::
::Forget "F" (BC Osc trimmer)as it is so sensitive that tuning it sends everything out of whack instantly.
::
::
::
::Now, that " F " trimmer adjustment is used for the " Custom Tailoring " of the MINIMUM capacitance value of your C1 tuning condenser oscillator section specs /capacitance adjustment range .
::
::
::
::
::Whereas , as you have just discovered, that the " J " . . .series padder trimmer . . .is being used to " Custom Tailor " the MAXIMUM value of C1's oscillator section.
::
::
::
::I will expound upon those series of statements a bit:
::
::
::You have that capacitors cluster of stator plates sitting there in their "island", being all rigidly, mechanically mounted and insulated away from the condensers frame proper. That will result in the probable effective capacitance, in reference to the distant framework, only having resulted in creating about 10 pf of capacitive value. Then, if you have the rotor clusters plates swung to their maximum unmeshed position the resultant gap between the two clusters edges of plates will result in only the addition of but an extra ~7 pf or so.
::
::
::Thus, you come up with an effective capacitance value of ~17 pf with the tuning condensers oscillator section in its full unmeshed plate positoning.
::
::
::One the other end of the spectrum, with the tuning condenser in its fully meshed position, you would find the closely intermeshing of all of the plates having resulted in the presence of two popular values of capacitance value that tuning condensers are usually offered as.
::
::
::One value is ~365 and the other ~420 pf, (540 sez, don't forget me !) and on your particular set, I am thinking it to be closer to the 420 pf value.
::
::
::
::Now, remember back to our resolving the requirements that the sets local oscillator would need to be having the tuning condensers plates swung fully open to be tuning to the high end of the BCB and the resultant capacitive value being connected across the fixed inductive value of the oscillator coil. The pair will be creating the oscillator resonant circuit setting the high end
::~2205 Kc signal required to heterodyne with an incoming received signal of 1750 Kc to result in an outputted 455 Kc IF signal.
::Now here is where another variable comes in, the utilization of a trimmer capacitor most commonly built right onto the end of the stator plate cluster and creating capacitance between its single leaf, separated with a mica insulator, and compressing up against the condenser framework.
::
::
::This would let the minimum capacitive value of the condenser in its almost fully opened open conditon , be "fine tuned" for frequency of operation. So figure that an initial value of ~17 pf from the tuning condenser proper and then the addition of ~7 pf from the built trimmer, if adjusted to its leaf max separated position, creates an effective paralled capacitance value of ~22 pf.
::In its other extreme, if that osc trimmer is max compressed , expect that being in the order of ~15 pf , thereby creating a combined capacitive pair value of ~32 pf.
::
::
::That is quite a capacitance adjustment swing capability at the high end of the BCB, in having an initial value of only ~22 pf but being able to add up to another ~15 pf of capacitive value with the trimmer being in its max compressed state.
::
::
::BUT T T T .. there is a like additional aspect, and that would be the adding of that SAME adjusted value of trimmer capacitance to the tuning condenser in its FULLY meshed capacitance position, when you are tuning for the reception of a received signal down at the low end of the BCB at 540 Kc.
::
::
::Putting it in a perspective and comparative situation, 15 pf of capacitance change added to an initial 420 pf value would be minimal as compared to only be working with ~22 pf of capacitance available, and then adding up to a full 15 pf of value.
::
::
::So you can see that there would be an additional skewing of the frequency down at the low end of the BCB when adjusting that osc trimmer capacitance value, but mainly that " F " trimmer is associative with its adjustment being utilized for the adjustment of the oscillator up at the high end of the BCB.
::
::
::
::NOW you go back to your situation of the 420 pf of value that the tuning condenser has in its fully meshed positon and the fact that value is WAAAAYYY more capacitance than you can use for the creation of a 955 Kc signal from the oscillator to then be hetrerodyned within the mixer and create the desired 455 IF signal.
::
::
::BUT, you have already found that the utilization of the series added adjustable padder condenser "J " will permit you to precisely trim in that max capacitance value of the tuning condenser.
::
::
::
::So with that info now perceived, you need to work with the adjustment of the " F " to establish your very high end of the BCB
::dial scale alignment correlation in the 1400 KC area, and then check back down at the low end of the BCB to see how the ~600 Kc alignment to the dial scale is setting. Down there, it can be tweaked with the " J " padder condenser adjustment.
::
::
::
::BUT, keeping in your mind also, that each ad justment will slightly counter react with the other adjustment, but also having evaluated all of the the "how it works" info above and now know that the adjustment of the "J " padder has maximum effect in establishing the very low end of the dial scale correlation while the " F " trimmer sets the very high end of the dial scale correlation.
::
::
::
::Seems like I remember a freq counter being mentioned ? If you should be so lucky as to have a full function one with dedicated adjustable preamp and triggering capabilities . . . . . . .VERSUS . . . . .the Mickey M . . .O. . .U . . .S. . .E. . ..incorporation of a
::added "frequency counter" function to a common DVM with its limited 'lebenteen" other "SUPER" add on functions, even including the addition of a stainless steel probe set for analyzing the acidity index of tiger urine when consulting an accompanying cross correlation chart.
::
::
::You could loosely, capacitively couple into the osc circuit to monitor its freq output and check 600 Kc dial scale setting as having
::an ~1055 Kc outputted signal and then swing up to 1400 Kc on the dial scale and check for an osc outputted signal of ~1855 Kc.
::
::
::
::No Additional Charge . . . Add on extra:
::
::
::
::Now if you have a freq counter with less than optimal front end conditoning and processing , a technique which has served me admirably for multi-decades is the use of a Tektronix scope which has a Gated output on the rear for A or B /or/both Vert channels. (This even goes back to the days double strapped carrying . . . ."BLEEP". . . I didn't mean carrying, I meant manhandling handles . . .on those HUGE old 535, 45, 65 etc behemoths).
::
::
::The idea is for the utilization of the vert amp section of the scope to be coupled to the circuiitry under evaluation and
::the vert sensitivity advanced to the point that the presence of the RF signal amplitude is starting to fill up the screen.
::
::
::That will initially be needing the Hoz sweep timebase being set to the order of a .05 us sweep rate. Now, even at that speed , don't expect to be viewing . .one little wavelet . . .two little wavelets . .three little wavelets . . .of that frequency.
::
::
::Instead, just a massed /compressed horizontal display of all of the frequencies sines, and the vert deflection level signifying
::their voltage amplitude.
::
::
::They are not even to be seen at 10X mag, but possible, if the scope has B delayed sweep and its resolving capability is utilized
::BUT expect the face display trace brighness to be diminished to a mere "candle light" level.
::
::
::Also, the scope NEEDS to be in conventional sweep mode anyway, and at that speed the monitoring of a 950 kc signal wil result on the scope triggering and producing less than a full sine across the display, in the rear Gated output, there will be the outputting of up to a 6.6 V positive polarity pulse created by the presence of the scope triggering on that incoming sine wavelets presence. Now if a frequency counter coupled into that Gate connector doesn't respond flawlessly to that pristine of, and level of signal . . .you might as well chunk that counter and shed your socks and utilize a finger and toe count of the signals frequency.
::
::
::The only caveat is that the Hoz sweep timebase be set fast enough such that the trace only responds to one sine. . . . (or less than a single sine). If it does retrigger during a single display trace, there will be the outputting of a second gate signal on the rise and triggering of the second start of the sine on the scope screen. ( This is very demonstrable if one uses a lower frequency, such as an ~ 1KC signal being applied to the scope probe from your CAL output square wave fron the units front terminal.)
::
::
::( I used to razzzz any "newbie" of the calibration staff at NASA with my sometime retort of " This scopes cal is in error . . .its outputting at 1002 cycles ! " )
::
:: He'd go off, rolling away the unit, with the intense concentration of need of correcting a fallacy akin to the trans-siderial commutation and declination of a Martian lunar cycle.
::
::
::If the scope retriggers on a sweep trace, such that another sine starts, one merely needs to realize that the freq count would be doubled such that you have to mentally halve your "frequency count" number. (Or divide by three if three wavelets get thru on a single trace display, etc)
::
::
::This extends the frequency capability a wee bit as, ~2 Mc is the upper limit in only having a .05 us Hoz timebase fastest sweep speed capability on most units.
::
::
::END
::
::
::
::As per your spurious oscillation situation, check tube shields and presence of good grounding. Also if the screen grids of the RF tubes are being decoupled less than optimally, a feedback oscillation might make its presence known. To confirm that situation
::I will take a short leaded .01 ufd disk CERAMIC capacitor and hold in place to shunt across from screen grid to nearby acessible chassis ground . If the oscillation / squeal disappers or even shifts , I will know that stage needed more efficient RF bypassing, and then move on to other RF stages for doing the same test.
::
::
::On SOME select few sets, even too sharp of a peaked IF alignment, can cause a problematic spurious oscillation onset.
::
::
::As per that RF tubes grid voltage . .definitely not -45 . . but -2.7 is more in the range . . AS . .even that value will vary in accordance to any AVC voltage involved.
::
::
::
::
::
::
::
::
::
::
::
::Standing by for feedback . . . .
::
::73's de Edd
::
::
::
::
::
::
:
Motorboating is one of many problems that can afflict radio transmitters and similar devices. Radio transmitters are vulnerable to unwanted feedback; one possible symptom of unwanted feedback are rapid changes in power output, which repeats about 20 to 20,000 times per second; this is called motorboating because when the radio transmission is received, it resembles the sound of a motorboat engine.
Contents [hide]
1 Technical description
2 Theory
3 Example
4 External links
[edit] Technical description
Motorboating is a term for a self-inflicted EMC problem where Radio frequency (RF) from a device leaves through one route and then re-enters the unit through another conduit where it causes an increase in the level of the RF generated by the unit. When this RF output climbs above a given level a catastrophic change occurs which causes the RF power output to drop rapidly. This causes a series of audio frequency pulses to be generated, in an AM system these can be heard clearly as a sound which is like the engine of a motorboat.
In general, as the intended RF power output of the device is increased the possibility of motorboating is increased. If motorboating is occurring in a system then the rate of the low frequency oscillation will increase.
[edit] Theory
If an electronic device is considered as a black box whose output is related to a series of inputs by a mathematical function, then if one of the inputs to the device is leaking RF into the system then the output will be a function of the RF input.
If the RF leaks into the device which generates the RF it can alter the RF power output, it might cause an increase in the RF power level, which in turn leads to an increase of the effect which causes the effect increasing the RF output to increase. This, if unchecked, will lead to a greater and greater increase in power output.
Assuming that the device does not fail due to this power surge, then it is likely that the system will reach a state where it is either insensitive to any further increase in RF input (then the power increase will cease), or a sudden catastrophic change will occur to state where the RF power output is smaller. The second of these cases can lead to an oscillation.
[edit] Example
Imagine a 27 MHz CB radioset connected to an external 12 volt DC supply, if the decoupling capacitors which link the supply wires to the RF ground (case) are missing then it is possible for RF to enter the radio set through the supply wires. This then causes the motorboating to occur.
::EDD,
:I understood most of what you said, but i'm not sure that what I thought you said is what you really meant...Just trying to be funny. Actually I'm hanging by, very much interested in this problem...Neal
:::
:::
:::
:::
:::
:::Sir Marc . . . .
:::
:::
:::
:::
:::
:::Great on your progress so far. . . . but the alignments further finalization is now dependent upon an interrelated "Ying and Yang " type of situation .
:::
:::
:::Specifically . . .Re your :
:::
:::
:::
:::
:::Forget "F" (BC Osc trimmer)as it is so sensitive that tuning it sends everything out of whack instantly.
:::
:::
:::
:::Now, that " F " trimmer adjustment is used for the " Custom Tailoring " of the MINIMUM capacitance value of your C1 tuning condenser oscillator section specs /capacitance adjustment range .
:::
:::
:::
:::
:::Whereas , as you have just discovered, that the " J " . . .series padder trimmer . . .is being used to " Custom Tailor " the MAXIMUM value of C1's oscillator section.
:::
:::
:::
:::I will expound upon those series of statements a bit:
:::
:::
:::You have that capacitors cluster of stator plates sitting there in their "island", being all rigidly, mechanically mounted and insulated away from the condensers frame proper. That will result in the probable effective capacitance, in reference to the distant framework, only having resulted in creating about 10 pf of capacitive value. Then, if you have the rotor clusters plates swung to their maximum unmeshed position the resultant gap between the two clusters edges of plates will result in only the addition of but an extra ~7 pf or so.
:::
:::
:::Thus, you come up with an effective capacitance value of ~17 pf with the tuning condensers oscillator section in its full unmeshed plate positoning.
:::
:::
:::One the other end of the spectrum, with the tuning condenser in its fully meshed position, you would find the closely intermeshing of all of the plates having resulted in the presence of two popular values of capacitance value that tuning condensers are usually offered as.
:::
:::
:::One value is ~365 and the other ~420 pf, (540 sez, don't forget me !) and on your particular set, I am thinking it to be closer to the 420 pf value.
:::
:::
:::
:::Now, remember back to our resolving the requirements that the sets local oscillator would need to be having the tuning condensers plates swung fully open to be tuning to the high end of the BCB and the resultant capacitive value being connected across the fixed inductive value of the oscillator coil. The pair will be creating the oscillator resonant circuit setting the high end
:::~2205 Kc signal required to heterodyne with an incoming received signal of 1750 Kc to result in an outputted 455 Kc IF signal.
:::Now here is where another variable comes in, the utilization of a trimmer capacitor most commonly built right onto the end of the stator plate cluster and creating capacitance between its single leaf, separated with a mica insulator, and compressing up against the condenser framework.
:::
:::
:::This would let the minimum capacitive value of the condenser in its almost fully opened open conditon , be "fine tuned" for frequency of operation. So figure that an initial value of ~17 pf from the tuning condenser proper and then the addition of ~7 pf from the built trimmer, if adjusted to its leaf max separated position, creates an effective paralled capacitance value of ~22 pf.
:::In its other extreme, if that osc trimmer is max compressed , expect that being in the order of ~15 pf , thereby creating a combined capacitive pair value of ~32 pf.
:::
:::
:::That is quite a capacitance adjustment swing capability at the high end of the BCB, in having an initial value of only ~22 pf but being able to add up to another ~15 pf of capacitive value with the trimmer being in its max compressed state.
:::
:::
:::BUT T T T .. there is a like additional aspect, and that would be the adding of that SAME adjusted value of trimmer capacitance to the tuning condenser in its FULLY meshed capacitance position, when you are tuning for the reception of a received signal down at the low end of the BCB at 540 Kc.
:::
:::
:::Putting it in a perspective and comparative situation, 15 pf of capacitance change added to an initial 420 pf value would be minimal as compared to only be working with ~22 pf of capacitance available, and then adding up to a full 15 pf of value.
:::
:::
:::So you can see that there would be an additional skewing of the frequency down at the low end of the BCB when adjusting that osc trimmer capacitance value, but mainly that " F " trimmer is associative with its adjustment being utilized for the adjustment of the oscillator up at the high end of the BCB.
:::
:::
:::
:::NOW you go back to your situation of the 420 pf of value that the tuning condenser has in its fully meshed positon and the fact that value is WAAAAYYY more capacitance than you can use for the creation of a 955 Kc signal from the oscillator to then be hetrerodyned within the mixer and create the desired 455 IF signal.
:::
:::
:::BUT, you have already found that the utilization of the series added adjustable padder condenser "J " will permit you to precisely trim in that max capacitance value of the tuning condenser.
:::
:::
:::
:::So with that info now perceived, you need to work with the adjustment of the " F " to establish your very high end of the BCB
:::dial scale alignment correlation in the 1400 KC area, and then check back down at the low end of the BCB to see how the ~600 Kc alignment to the dial scale is setting. Down there, it can be tweaked with the " J " padder condenser adjustment.
:::
:::
:::
:::BUT, keeping in your mind also, that each ad justment will slightly counter react with the other adjustment, but also having evaluated all of the the "how it works" info above and now know that the adjustment of the "J " padder has maximum effect in establishing the very low end of the dial scale correlation while the " F " trimmer sets the very high end of the dial scale correlation.
:::
:::
:::
:::Seems like I remember a freq counter being mentioned ? If you should be so lucky as to have a full function one with dedicated adjustable preamp and triggering capabilities . . . . . . .VERSUS . . . . .the Mickey M . . .O. . .U . . .S. . .E. . ..incorporation of a
:::added "frequency counter" function to a common DVM with its limited 'lebenteen" other "SUPER" add on functions, even including the addition of a stainless steel probe set for analyzing the acidity index of tiger urine when consulting an accompanying cross correlation chart.
:::
:::
:::You could loosely, capacitively couple into the osc circuit to monitor its freq output and check 600 Kc dial scale setting as having
:::an ~1055 Kc outputted signal and then swing up to 1400 Kc on the dial scale and check for an osc outputted signal of ~1855 Kc.
:::
:::
:::
:::No Additional Charge . . . Add on extra:
:::
:::
:::
:::Now if you have a freq counter with less than optimal front end conditoning and processing , a technique which has served me admirably for multi-decades is the use of a Tektronix scope which has a Gated output on the rear for A or B /or/both Vert channels. (This even goes back to the days double strapped carrying . . . ."BLEEP". . . I didn't mean carrying, I meant manhandling handles . . .on those HUGE old 535, 45, 65 etc behemoths).
:::
:::
:::The idea is for the utilization of the vert amp section of the scope to be coupled to the circuiitry under evaluation and
:::the vert sensitivity advanced to the point that the presence of the RF signal amplitude is starting to fill up the screen.
:::
:::
:::That will initially be needing the Hoz sweep timebase being set to the order of a .05 us sweep rate. Now, even at that speed , don't expect to be viewing . .one little wavelet . . .two little wavelets . .three little wavelets . . .of that frequency.
:::
:::
:::Instead, just a massed /compressed horizontal display of all of the frequencies sines, and the vert deflection level signifying
:::their voltage amplitude.
:::
:::
:::They are not even to be seen at 10X mag, but possible, if the scope has B delayed sweep and its resolving capability is utilized
:::BUT expect the face display trace brighness to be diminished to a mere "candle light" level.
:::
:::
:::Also, the scope NEEDS to be in conventional sweep mode anyway, and at that speed the monitoring of a 950 kc signal wil result on the scope triggering and producing less than a full sine across the display, in the rear Gated output, there will be the outputting of up to a 6.6 V positive polarity pulse created by the presence of the scope triggering on that incoming sine wavelets presence. Now if a frequency counter coupled into that Gate connector doesn't respond flawlessly to that pristine of, and level of signal . . .you might as well chunk that counter and shed your socks and utilize a finger and toe count of the signals frequency.
:::
:::
:::The only caveat is that the Hoz sweep timebase be set fast enough such that the trace only responds to one sine. . . . (or less than a single sine). If it does retrigger during a single display trace, there will be the outputting of a second gate signal on the rise and triggering of the second start of the sine on the scope screen. ( This is very demonstrable if one uses a lower frequency, such as an ~ 1KC signal being applied to the scope probe from your CAL output square wave fron the units front terminal.)
:::
:::
:::( I used to razzzz any "newbie" of the calibration staff at NASA with my sometime retort of " This scopes cal is in error . . .its outputting at 1002 cycles ! " )
:::
::: He'd go off, rolling away the unit, with the intense concentration of need of correcting a fallacy akin to the trans-siderial commutation and declination of a Martian lunar cycle.
:::
:::
:::If the scope retriggers on a sweep trace, such that another sine starts, one merely needs to realize that the freq count would be doubled such that you have to mentally halve your "frequency count" number. (Or divide by three if three wavelets get thru on a single trace display, etc)
:::
:::
:::This extends the frequency capability a wee bit as, ~2 Mc is the upper limit in only having a .05 us Hoz timebase fastest sweep speed capability on most units.
:::
:::
:::END
:::
:::
:::
:::As per your spurious oscillation situation, check tube shields and presence of good grounding. Also if the screen grids of the RF tubes are being decoupled less than optimally, a feedback oscillation might make its presence known. To confirm that situation
:::I will take a short leaded .01 ufd disk CERAMIC capacitor and hold in place to shunt across from screen grid to nearby acessible chassis ground . If the oscillation / squeal disappers or even shifts , I will know that stage needed more efficient RF bypassing, and then move on to other RF stages for doing the same test.
:::
:::
:::On SOME select few sets, even too sharp of a peaked IF alignment, can cause a problematic spurious oscillation onset.
:::
:::
:::As per that RF tubes grid voltage . .definitely not -45 . . but -2.7 is more in the range . . AS . .even that value will vary in accordance to any AVC voltage involved.
:::
:::
:::
:::
:::
:::
:::
:::
:::
:::
:::
:::Standing by for feedback . . . .
:::
:::73's de Edd
:::
:::
:::
:::
:::
:::
::
:
:Motorboating is one of many problems that can afflict radio transmitters and similar devices. Radio transmitters are vulnerable to unwanted feedback; one possible symptom of unwanted feedback are rapid changes in power output, which repeats about 20 to 20,000 times per second; this is called motorboating because when the radio transmission is received, it resembles the sound of a motorboat engine.
Technical description
:Motorboating is a term for a self-inflicted EMC problem where Radio frequency (RF) from a device leaves through one route and then re-enters the unit through another conduit where it causes an increase in the level of the RF generated by the unit. When this RF output climbs above a given level a catastrophic change occurs which causes the RF power output to drop rapidly. This causes a series of audio frequency pulses to be generated, in an AM system these can be heard clearly as a sound which is like the engine of a motorboat.
:
:In general, as the intended RF power output of the device is increased the possibility of motorboating is increased. If motorboating is occurring in a system then the rate of the low frequency oscillation will increase.
:
:Theory
:If an electronic device is considered as a black box whose output is related to a series of inputs by a mathematical function, then if one of the inputs to the device is leaking RF into the system then the output will be a function of the RF input.
:
:If the RF leaks into the device which generates the RF it can alter the RF power output, it might cause an increase in the RF power level, which in turn leads to an increase of the effect which causes the effect increasing the RF output to increase. This, if unchecked, will lead to a greater and greater increase in power output.
:
:Assuming that the device does not fail due to this power surge, then it is likely that the system will reach a state where it is either insensitive to any further increase in RF input (then the power increase will cease), or a sudden catastrophic change will occur to state where the RF power output is smaller. The second of these cases can lead to an oscillation.
:
: Example
:Imagine a 27 MHz CB radioset connected to an external 12 volt DC supply, if the decoupling capacitors which link the supply wires to the RF ground (case) are missing then it is possible for RF to enter the radio set through the supply wires. This then causes the motorboating to occur...Neal
:
:
:
:::EDD,
::I understood most of what you said, but i'm not sure that what I thought you said is what you really meant...Just trying to be funny. Actually I'm hanging by, very much interested in this problem...Neal
::::
::::
::::
::::
::::
::::Sir Marc . . . .
::::
::::
::::
::::
::::
::::Great on your progress so far. . . . but the alignments further finalization is now dependent upon an interrelated "Ying and Yang " type of situation .
::::
::::
::::Specifically . . .Re your :
::::
::::
::::
::::
::::Forget "F" (BC Osc trimmer)as it is so sensitive that tuning it sends everything out of whack instantly.
::::
::::
::::
::::Now, that " F " trimmer adjustment is used for the " Custom Tailoring " of the MINIMUM capacitance value of your C1 tuning condenser oscillator section specs /capacitance adjustment range .
::::
::::
::::
::::
::::Whereas , as you have just discovered, that the " J " . . .series padder trimmer . . .is being used to " Custom Tailor " the MAXIMUM value of C1's oscillator section.
::::
::::
::::
::::I will expound upon those series of statements a bit:
::::
::::
::::You have that capacitors cluster of stator plates sitting there in their "island", being all rigidly, mechanically mounted and insulated away from the condensers frame proper. That will result in the probable effective capacitance, in reference to the distant framework, only having resulted in creating about 10 pf of capacitive value. Then, if you have the rotor clusters plates swung to their maximum unmeshed position the resultant gap between the two clusters edges of plates will result in only the addition of but an extra ~7 pf or so.
::::
::::
::::Thus, you come up with an effective capacitance value of ~17 pf with the tuning condensers oscillator section in its full unmeshed plate positoning.
::::
::::
::::One the other end of the spectrum, with the tuning condenser in its fully meshed position, you would find the closely intermeshing of all of the plates having resulted in the presence of two popular values of capacitance value that tuning condensers are usually offered as.
::::
::::
::::One value is ~365 and the other ~420 pf, (540 sez, don't forget me !) and on your particular set, I am thinking it to be closer to the 420 pf value.
::::
::::
::::
::::Now, remember back to our resolving the requirements that the sets local oscillator would need to be having the tuning condensers plates swung fully open to be tuning to the high end of the BCB and the resultant capacitive value being connected across the fixed inductive value of the oscillator coil. The pair will be creating the oscillator resonant circuit setting the high end
::::~2205 Kc signal required to heterodyne with an incoming received signal of 1750 Kc to result in an outputted 455 Kc IF signal.
::::Now here is where another variable comes in, the utilization of a trimmer capacitor most commonly built right onto the end of the stator plate cluster and creating capacitance between its single leaf, separated with a mica insulator, and compressing up against the condenser framework.
::::
::::
::::This would let the minimum capacitive value of the condenser in its almost fully opened open conditon , be "fine tuned" for frequency of operation. So figure that an initial value of ~17 pf from the tuning condenser proper and then the addition of ~7 pf from the built trimmer, if adjusted to its leaf max separated position, creates an effective paralled capacitance value of ~22 pf.
::::In its other extreme, if that osc trimmer is max compressed , expect that being in the order of ~15 pf , thereby creating a combined capacitive pair value of ~32 pf.
::::
::::
::::That is quite a capacitance adjustment swing capability at the high end of the BCB, in having an initial value of only ~22 pf but being able to add up to another ~15 pf of capacitive value with the trimmer being in its max compressed state.
::::
::::
::::BUT T T T .. there is a like additional aspect, and that would be the adding of that SAME adjusted value of trimmer capacitance to the tuning condenser in its FULLY meshed capacitance position, when you are tuning for the reception of a received signal down at the low end of the BCB at 540 Kc.
::::
::::
::::Putting it in a perspective and comparative situation, 15 pf of capacitance change added to an initial 420 pf value would be minimal as compared to only be working with ~22 pf of capacitance available, and then adding up to a full 15 pf of value.
::::
::::
::::So you can see that there would be an additional skewing of the frequency down at the low end of the BCB when adjusting that osc trimmer capacitance value, but mainly that " F " trimmer is associative with its adjustment being utilized for the adjustment of the oscillator up at the high end of the BCB.
::::
::::
::::
::::NOW you go back to your situation of the 420 pf of value that the tuning condenser has in its fully meshed positon and the fact that value is WAAAAYYY more capacitance than you can use for the creation of a 955 Kc signal from the oscillator to then be hetrerodyned within the mixer and create the desired 455 IF signal.
::::
::::
::::BUT, you have already found that the utilization of the series added adjustable padder condenser "J " will permit you to precisely trim in that max capacitance value of the tuning condenser.
::::
::::
::::
::::So with that info now perceived, you need to work with the adjustment of the " F " to establish your very high end of the BCB
::::dial scale alignment correlation in the 1400 KC area, and then check back down at the low end of the BCB to see how the ~600 Kc alignment to the dial scale is setting. Down there, it can be tweaked with the " J " padder condenser adjustment.
::::
::::
::::
::::BUT, keeping in your mind also, that each ad justment will slightly counter react with the other adjustment, but also having evaluated all of the the "how it works" info above and now know that the adjustment of the "J " padder has maximum effect in establishing the very low end of the dial scale correlation while the " F " trimmer sets the very high end of the dial scale correlation.
::::
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::::
::::Seems like I remember a freq counter being mentioned ? If you should be so lucky as to have a full function one with dedicated adjustable preamp and triggering capabilities . . . . . . .VERSUS . . . . .the Mickey M . . .O. . .U . . .S. . .E. . ..incorporation of a
::::added "frequency counter" function to a common DVM with its limited 'lebenteen" other "SUPER" add on functions, even including the addition of a stainless steel probe set for analyzing the acidity index of tiger urine when consulting an accompanying cross correlation chart.
::::
::::
::::You could loosely, capacitively couple into the osc circuit to monitor its freq output and check 600 Kc dial scale setting as having
::::an ~1055 Kc outputted signal and then swing up to 1400 Kc on the dial scale and check for an osc outputted signal of ~1855 Kc.
::::
::::
::::
::::No Additional Charge . . . Add on extra:
::::
::::
::::
::::Now if you have a freq counter with less than optimal front end conditoning and processing , a technique which has served me admirably for multi-decades is the use of a Tektronix scope which has a Gated output on the rear for A or B /or/both Vert channels. (This even goes back to the days double strapped carrying . . . ."BLEEP". . . I didn't mean carrying, I meant manhandling handles . . .on those HUGE old 535, 45, 65 etc behemoths).
::::
::::
::::The idea is for the utilization of the vert amp section of the scope to be coupled to the circuiitry under evaluation and
::::the vert sensitivity advanced to the point that the presence of the RF signal amplitude is starting to fill up the screen.
::::
::::
::::That will initially be needing the Hoz sweep timebase being set to the order of a .05 us sweep rate. Now, even at that speed , don't expect to be viewing . .one little wavelet . . .two little wavelets . .three little wavelets . . .of that frequency.
::::
::::
::::Instead, just a massed /compressed horizontal display of all of the frequencies sines, and the vert deflection level signifying
::::their voltage amplitude.
::::
::::
::::They are not even to be seen at 10X mag, but possible, if the scope has B delayed sweep and its resolving capability is utilized
::::BUT expect the face display trace brighness to be diminished to a mere "candle light" level.
::::
::::
::::Also, the scope NEEDS to be in conventional sweep mode anyway, and at that speed the monitoring of a 950 kc signal wil result on the scope triggering and producing less than a full sine across the display, in the rear Gated output, there will be the outputting of up to a 6.6 V positive polarity pulse created by the presence of the scope triggering on that incoming sine wavelets presence. Now if a frequency counter coupled into that Gate connector doesn't respond flawlessly to that pristine of, and level of signal . . .you might as well chunk that counter and shed your socks and utilize a finger and toe count of the signals frequency.
::::
::::
::::The only caveat is that the Hoz sweep timebase be set fast enough such that the trace only responds to one sine. . . . (or less than a single sine). If it does retrigger during a single display trace, there will be the outputting of a second gate signal on the rise and triggering of the second start of the sine on the scope screen. ( This is very demonstrable if one uses a lower frequency, such as an ~ 1KC signal being applied to the scope probe from your CAL output square wave fron the units front terminal.)
::::
::::
::::( I used to razzzz any "newbie" of the calibration staff at NASA with my sometime retort of " This scopes cal is in error . . .its outputting at 1002 cycles ! " )
::::
:::: He'd go off, rolling away the unit, with the intense concentration of need of correcting a fallacy akin to the trans-siderial commutation and declination of a Martian lunar cycle.
::::
::::
::::If the scope retriggers on a sweep trace, such that another sine starts, one merely needs to realize that the freq count would be doubled such that you have to mentally halve your "frequency count" number. (Or divide by three if three wavelets get thru on a single trace display, etc)
::::
::::
::::This extends the frequency capability a wee bit as, ~2 Mc is the upper limit in only having a .05 us Hoz timebase fastest sweep speed capability on most units.
::::
::::
::::END
::::
::::
::::
::::As per your spurious oscillation situation, check tube shields and presence of good grounding. Also if the screen grids of the RF tubes are being decoupled less than optimally, a feedback oscillation might make its presence known. To confirm that situation
::::I will take a short leaded .01 ufd disk CERAMIC capacitor and hold in place to shunt across from screen grid to nearby acessible chassis ground . If the oscillation / squeal disappers or even shifts , I will know that stage needed more efficient RF bypassing, and then move on to other RF stages for doing the same test.
::::
::::
::::On SOME select few sets, even too sharp of a peaked IF alignment, can cause a problematic spurious oscillation onset.
::::
::::
::::As per that RF tubes grid voltage . .definitely not -45 . . but -2.7 is more in the range . . AS . .even that value will vary in accordance to any AVC voltage involved.
::::
::::
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::::
::::
::::
::::
::::
::::
::::
::::
::::Standing by for feedback . . . .
::::
::::73's de Edd
::::
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:
I'm digesting what you wrote, but before I get started, a couple of comments and questions:
COMMENTS:
1. On the 8A02 chassis, there are no trimmers on the variable condenser stator plates (osc or ant).
2. Tube shields and bases have been cleaned and tightened but not sure if there is an RF leak there.
QUESTIONS:
1. I do have a freq counter (PROTEK) w/trigger but preamp (if it exists) must be weak because it takes a strong signal (50% modulation) from my generator to get a good reading. The question is, How do I "loosely, capacitatively couple it into the osc circuit."? I normally have just connected it to a Hazetine loop, but can't get enough signal this way to activate the gate. I tried connecting it directly to the cathode of the converter through a .01mf cap but the readings were all over the place and wouldn't stabilize.
2. I've heard about IF alignment sometimes being too 'sharp', but how do I detune it? Just back off the trimmers in each IF xfrmr the same number of volts as read on the output meter?
Now I'll get started on the rest...think I'll disconnect "F" and check capacitance range, just to make sure it's OK.
Thanks Edd for the tutorial, I'd pay money to study with you.
Marc
Sir Marc . . . .
from my generator to get a good reading. the unit didn't know which signal to respond to . . . the basic RF or the applied 400 /or / 1000 ~ modulation element. The Tech schematic sheet for the 8-S-531...548...563, says that : On isolating that problem of spurious oscillation, I would tune to the dial portion where it is continually making its worst antics. gripping fingers on wiring and components in the area of the sets RF tubes and see if there is any tonal shifting affectation or a complete loss of the spurious osc condition. One area to skip over would be the local oscillator components, as they would be inherently touchy anyway. adjust the slotted trimmer cap on an IF off position about 1/8 turn, both directions, and if it is at fault, it should be noticably affecting the situation. Then move the trimmer back to its initial position and then move on to another trimmer and in the end that should point out the most sensitive area affected.
I do have a freq counter (PROTEK) w/trigger but preamp (if it exists) must be weak because it takes a strong signal (50% modulation)
One would not want the modulation being turned on while trying to take a read on the generators basic RF sine wave . . .no wonder . . .
Trimmers F,K and N are mounted on strip 22-850, therefore, it is then logical for the cluster to be mounted somewhat close to the
oscillator coils.
Sooooo, it looks like they did mount trimmer " F " off board from C1 main tuning condenser, as well as its antenna trimmer
companion [ G ].
Initially, also check out the integrity and grounding of the shielded 6A8, 6K7 plate leads and the
shielded grid lead of the 6J5 detector.
And then you put on either one of Mammasans rubber gloves or slip your hand within a thick,heavy duty plastic bag and start placing
One would expect that to occur in the problem area.
And, as for the procedure of the IF transformers , use the same worst case on getting the unit sounding off and then sequentially
73's de Edd
1. I have very good reception and dial tracking @ 1100KC and below. ["F" Osc trimmer reading is 26.4pf]
2. I have INTERMITTENT reception of STRONG stations at 1180 and 1230KC. Here's how that happens...I WHACK the 6F5, 1st AF tube around a little and it sporadicly receives these stations. So I rechecked the 6F5 and found it has very weak emissions. (nos tube coming - in fact, ordered a whole new set of tubes, just in case). Note here that the grid cap lead shield only comes to within an inch of the grid cap. Connection of lead to cap is good.
3. All is still totally quiet above 1230KC and I should have a very strong station @ 1510KC. (remember, I got these high freqs to tune in when I screwed "J" almost all the way in, but the rest of the BCB was all out of whack with no reception below 900Kc and no received stations were where they should be on the dial.)
4. Tried again to connect my freq counter to Osc circuit thru .1mf cap at pin 5 (Osc grid) of 6A8, but only get very erratic readings which appeared to be around the dial freq, not dial + IF freq. Sooooo, I got my digital Sony Walkman and began to listen to Osc freqs using it. That works and all Osc freqs were very close up to 1100KC on the dial at which point they began to get very weak and disappeared totally above that. For some reason, my Osc seems to be shutting down at between 1555KC and 1655KC (1100KC and 1200KC on the dial).
5. Grounding integrity of tube shields and lead shields from 6A8, 6K7 and 6J5 appear and test good.
6. Detuning IF xfmrs has no effect.
*************
As I said before, I recapped the whole radio, testing each new cap before replacement and have checked each cap connection a dozen times. I did replace one resistor (R6 dogbone) which read twice its rated value. Should I begin looking at replacing other resistors? Some are a bit more than 20% above rated values. This is my 77th radio restoration and I'm humbled by how much I still don't know!
Marc
Well Edd, here's where I'm at:
:1. I have very good reception and dial tracking @ 1100KC and below. ["F" Osc trimmer reading is 26.4pf]
:2. I have INTERMITTENT reception of STRONG stations at 1180 and 1230KC. Here's how that happens...I WHACK the 6F5, 1st AF tube around a little and it sporadicly receives these stations. So I rechecked the 6F5 and found it has very weak emissions. (nos tube coming - in fact, ordered a whole new set of tubes, just in case). Note here that the grid cap lead shield only comes to within an inch of the grid cap. Connection of lead to cap is good.
:3. All is still totally quiet above 1230KC and I should have a very strong station @ 1510KC. (remember, I got these high freqs to tune in when I screwed "J" almost all the way in, but the rest of the BCB was all out of whack with no reception below 900Kc and no received stations were where they should be on the dial.)
:4. Tried again to connect my freq counter to Osc circuit thru .1mf cap at pin 5 (Osc grid) of 6A8, but only get very erratic readings which appeared to be around the dial freq, not dial + IF freq. Sooooo, I got my digital Sony Walkman and began to listen to Osc freqs using it. That works and all Osc freqs were very close up to 1100KC on the dial at which point they began to get very weak and disappeared totally above that. For some reason, my Osc seems to be shutting down at between 1555KC and 1655KC (1100KC and 1200KC on the dial).
:5. Grounding integrity of tube shields and lead shields from 6A8, 6K7 and 6J5 appear and test good.
:6. Detuning IF xfmrs has no effect.
:*************
:As I said before, I recapped the whole radio, testing each new cap before replacement and have checked each cap connection a dozen times. I did replace one resistor (R6 dogbone) which read twice its rated value. Should I begin looking at replacing other resistors? Some are a bit more than 20% above rated values. This is my 77th radio restoration and I'm humbled by how much I still don't know!
:Marc
:
The new tubes came today so I got right on it. I didn't replace all tubes just the 6A8 (converter), 6K7 (IF amp), 6J5 (detector), and 6F5 (1st audio). Original 6J5 and 6F5 were weak but should have been OK, orig 6K7 checked good but was sensitive to vibration, orig 6A8 checked good. Anyway, I can now tune the complete BCB.
Neil, the squirrelly 6K7 seemed to cause the motorboating but only intermeittently - probably microphonics set up an increasing oscillation, maybe a bad grid.
Edd, Can you come up with possible answers at to which tube or tubes may have been at fault and why??? I sure apprecialte all the help Edd and I learned a lot from you on this one. Conversley, I don't have a lot of faith in my tube tester anymore - like I said before it's happened before that a tube tests good but when replaced, the problem goes away.
Regards, Marc
:Edd & Neil,
:The new tubes came today so I got right on it. I didn't replace all tubes just the 6A8 (converter), 6K7 (IF amp), 6J5 (detector), and 6F5 (1st audio). Original 6J5 and 6F5 were weak but should have been OK, orig 6K7 checked good but was sensitive to vibration, orig 6A8 checked good. Anyway, I can now tune the complete BCB.
:
: Neil, the squirrelly 6K7 seemed to cause the motorboating but only intermeittently - probably microphonics set up an increasing oscillation, maybe a bad grid.
:
: Edd, Can you come up with possible answers at to which tube or tubes may have been at fault and why??? I sure apprecialte all the help Edd and I learned a lot from you on this one. Conversley, I don't have a lot of faith in my tube tester anymore - like I said before it's happened before that a tube tests good but when replaced, the problem goes away.
:Regards, Marc
:
:Edd & Neil,
:The new tubes came today so I got right on it. I didn't replace all tubes just the 6A8 (converter), 6K7 (IF amp), 6J5 (detector), and 6F5 (1st audio). Original 6J5 and 6F5 were weak but should have been OK, orig 6K7 checked good but was sensitive to vibration, orig 6A8 checked good. Anyway, I can now tune the complete BCB.
:
: Neil, the squirrelly 6K7 seemed to cause the motorboating but only intermeittently - probably microphonics set up an increasing oscillation, maybe a bad grid.
:
: Edd, Can you come up with possible answers at to which tube or tubes may have been at fault and why??? I sure apprecialte all the help Edd and I learned a lot from you on this one. Conversley, I don't have a lot of faith in my tube tester anymore - like I said before it's happened before that a tube tests good but when replaced, the problem goes away.
:Regards, Marc
:
Marc
Sir Marc . . . .
Toobies . . . .or not toobies . . that is the question . . . . ?
I see that the problem is now solved . . congrats !
I am just going to also include my last prepared responses thoughts along with its ADDENDA below.
That .1 ufd value of coupling is waaaay excessive , try in the order of a 47 pf or less. ADDENDA . . .after the fact . . .now would be the time to try the above procedure on the 6A8 and log in the voltages at low band- mid band- high band . . .and THEN . . you put the old 6A8 back in the circuit and try the measurements again and see if they don't reflect a disparity on high band. With the old 6A8 dropping out at high AM band, looks like it wouldn't have stood a chance at SW frequencies. I thought that I had included a working schematic within either of my prior posts . . .'twasn't so. Nothing to do with the oscillator proper dropping out or diminishing in level appreciably, but the improper adjustment of the BCB ant trimmer cap [G] could hurt your hi band sensitivity should it not be set properly.
Tried again to connect my freq counter to Osc circuit thru .1mf cap at pin 5 (Osc grid) of 6A8, but only get very erratic readings .
Monitor, with a VTVM or the equally low loading characteristics of a common DVM, between the first grid of the 6A8 and its cathode , with the ground lead going to the cathode. Actually, the DVM would be a better choice in this situation, due to the
potentially stand alone isolative characteristics of a battery powered instrument.
Start at the low end of the BCB and notice the negative grid voltage being developed by the oscillator, also note that it is normal for it to make a gradual voltage level transition when tuning from one extreme of the BCB to its other end.
What would be of interest now, is if there happens to be a decided and noticable sharp shift in that level, when you tune into the currently experienced "problem " area of reception on the higher end of the band ?
I am sure that in the back of your mind, there was that query of how come I was able to receive 1510, etc hi band statiopns back when I was starting working on the unit.
Now assuredley, that must have been because you had the BCB osc padder compressed down so much that you were creating the lower possible "twin" local oscillator frequency that could also heterodyne within the 6A8 and create a 455Kc IF signal.
Specifically, your normally expected 1965 Kc osc signal heterodyning with a 1510 station signal will yield a difference 455 Kc signal for the IF strip.
In the padders compressed, high capacitance state, it can also create a lower 1055 Kc osc signal which can also heterodyne with an incoming 1510 Kc received signal to ALSO produce a difference 455 Kc signal for the IF strip.
Now included is a nice copy of an otherwise harder to find schema, however, with it being resplendently devoid of any "M" resistor values
In its initial consulting, it seems that your prior referencing to the R9 resistor is for the B+ supply to the 6A8, and if multiple K's
up from that 1K, could cramp the 6A8's operational capabilities.
Now, in respect to the oscillator , it is adequate in the inter-inductive coupling within in the oscillator coil with its proven performance on low band portion, but now, how about lifting that C5 capacitor at the 1st grid of the 6A8 and subbing in a 47 pf value. Also confirm that the R1
(47k) grid resistor has not gone down in value, as a second effort it might not hurt to sub in a 68K value in that slot.
Then you run thru the monitoring again of the developed voltage on the 1st grid to cathode of the mixer.
Also a wild possibility should this set have had every adjustment "tweeeeeked " on it before you received it, would be the
[Fuscia line enclosed box] [E] which along with its companion inductor 9 is a series trap, that should create an attenuation at 455 signal frequency. Now I can't inspect for the physical possibility of that multi leaf trimmers MINIMUM capacity possible, but if it is opened up the , traps response will then walk on upwards into the BCB and attenuate RF level way down.
But, as I said, it would be requiring minimal capacitance to be able to shift on upwards to the hi end of the BCB.
Standing by for feedback . . . .
73's de Edd
Zenith 8A02 Schematic:  |
I don't know where to start. Thought all was fixed, and I do get good BCB reception & tracking BUT...LO seems to cut out when I go to SWB. I know this because I used a 47pf cap, as you suggested, and voila, my freq counter works great off 6A8 osc grid. After readjusting "f" and "j" using freq counter, I went to SWB and osc freq went to 0! I can get an osc reading (intermittently) on Pol band but can not adjust "n" high enough to give dial+455.
Back to your suggestion re: neg voltages across mixer cathode and osc grid. Wierd...voltages increase (negatively) from 550kc to 800kc, then begin to decrease (negatively) all the way to 1600kc. Voltages:
550kc = -4.8v; 600kc = -6.0v; 650kc = -6.9v; 700kc = -7.7v; 750kc = -8.2v; 800kc = -8.6v; 900kc = -8.5v; 1000kc = -8.1v; 1100kc = -7.0v; 1200kc = -6.3v; 1300kc = -5.1v; 1400kc = -3.8v; 1500jc = -3.0v; 1600kc = -1.5v. Additionally there is a lot of flucuation (+/- .7v) mid band and very upper end.
I thought I'd mentioned it before but found I hadn't so here is a change I made early on to get rid of the howling. C2 was originally connected on the other side of R3 from C4 and grounded. The schematic shows C2 connected to antenna coil trimmer "Q" to ground. Original connection was definitely factory, not a later fix. What's the deal here??? Also, I have now noted that there is a difference between the Rider and Beitman schematics. Namely, in the antenna circuit with Beitman (your schematic) there is a C8 hooked between "Z" antenna lug and the ant loop AND trimmer "G" is directly grounded on the other side of the loop loading coil. On Rider's schematic (AND ON MY RADIO) there is no C8 in the loop circuit and C2 is not directly grounded but is connected to loop circuit. On my radio, C2 is grounded when the band switch is moved to police and SW bands.
I can't get over the feeling there is still a problem with the osc circuits but at least I have BCB where I want it. What do you think?
Sir Marc . . . .
Your monitored oscillator 1st grid to cathode voltages seem in line with the norm, until you hit high band portion.
It will be intresting to see if that characteristic remains in the Police band after its alignment optimization . . . . as follows.
I am a little hand tied in evaluating the antenna sections top, left, cornermost segments switching actions, wihout my being able to make ohmmeter evaluaton of the contact transitions on different bands.
On some schemas of Red Ryders or Zenith they give a clarified view of the different switching contact positions in the different band modes.
Now, as this schematic is showing, that C2 is a VERY important function, in the respect of being the AVC filter capacitor.
It needs to have enough voltage- capacitance retention /time constant of the AVC voltage fed into the RF stage via R9, so that there is a gradual transiton in frontal RF gain upon application of a corrective AVC voltage to that RF stage. If not present, or of a low capacitive value, that AVC voltage would be responded to immediately and the resultant fast bobbling up and down of the gain and onset of a feedback or "Botor-moating" effect.
As per your mentioning of a presence of a decoupling capacitor on the cold side of the 6A8's R3, I can see no detrimental effect of an additional RF decoupling capacitor being used at that position also.
Your next alignment step would be the Pole-leeeece'ius band, with the use of the " K " osc trimmer for setting in the high freq portion of the band for dial scale alignment and the optimal RF sensitivity at the high end of that band with the " M " ant trimmer. With the typical deadness of that band of any received signals, excluding second harmonics of strong local BCB stations, this definitely might require the signal produced by a signal generator.
On my radio, C2 is grounded when the band switch is moved to police and SW bands.
If that is the case, then the RF stage . . and IF stage . . . is then being run at full RF gain mode, with no correction being made, with the developed AVC voltage of the set.
ALSO, if you are still having frequency counter coupling to the 6A8 via that 47 pf cap, you might completely pull that off, as it may now be loading down and throttling osc action at the now higher RF frequencies involved, if it is eventually dumping into a 50 ohm load in the freq counter front end.
Standing by for feedback . . . .
73's de Edd
This is it for awhile. I've got to put this aside before go nuts - wife says I've got to start getting other things done!
I misspoke in my last. It wasn't C2 that is grounded when I switch to SW & Pol bands, it is "G". Heres where I stand now:
1. I tried adding another C2 from R3 to gnd as was originally installed: No change. There was, however definitely NO C2 on the AVC line from R9, so that helped.
2. began alignment again. Noticed that when I hooked DVM to AVC at R9, volume was much louder?? Some kind of interaction with meter? Anyway, BCB aligns although I must use high attenuation levels on my signal generator to get a signal through at high BC freqs...osc still weak at high BC freqs.
3. Police band passes no signal at high freqs (above 3.0 MC) from generator. I can find areas of the pol band where I can get a signal through but it is intermittent and very little corretlation with dial. "N" does attenuate signal and movers it up & down the dial but just a little. It pops on and off like there is a loose connection, but I can't find it no matter what I jiggle.
4. On SWB, I can get no signal through at high freqs (above 10MC). I can get a signal, again intermittently, at around 10MC and here's something interesting. The radio will pick up WWV at 10MC faintly as long as the signal generator is connected, once I disconnect it nothing! The generator seems to act as part of the antenna as I can faintly hear other stations on the low end of SW with it connected. Again I get the signal generator signal intermittently, even loudly (max attenuation on generator) but then it fades or just quits. After fooling with the variable condenser for awhile, I might get it again???
Think I'll get to work on the cabinet where I can shut down my brain and try again when the chassis is ready to install. Thanks again for all your counsel. I'll be back on the Forum when I'm ready to install.
Marc
Still no sleep...I'll just follow along and mostly just keep my posts to myself...don't give up...it's got to be some little thing you've overlooked like SWB switch contacts. That's just an example, I know you would have verified that already...something basic...Neal
:Edd,
:I don't know where to start. Thought all was fixed, and I do get good BCB reception & tracking BUT...LO seems to cut out when I go to SWB. I know this because I used a 47pf cap, as you suggested, and voila, my freq counter works great off 6A8 osc grid. After readjusting "f" and "j" using freq counter, I went to SWB and osc freq went to 0! I can get an osc reading (intermittently) on Pol band but can not adjust "n" high enough to give dial+455.
:
:Back to your suggestion re: neg voltages across mixer cathode and osc grid. Wierd...voltages increase (negatively) from 550kc to 800kc, then begin to decrease (negatively) all the way to 1600kc. Voltages:
:550kc = -4.8v; 600kc = -6.0v; 650kc = -6.9v; 700kc = -7.7v; 750kc = -8.2v; 800kc = -8.6v; 900kc = -8.5v; 1000kc = -8.1v; 1100kc = -7.0v; 1200kc = -6.3v; 1300kc = -5.1v; 1400kc = -3.8v; 1500jc = -3.0v; 1600kc = -1.5v. Additionally there is a lot of flucuation (+/- .7v) mid band and very upper end.
:
:I thought I'd mentioned it before but found I hadn't so here is a change I made early on to get rid of the howling. C2 was originally connected on the other side of R3 from C4 and grounded. The schematic shows C2 connected to antenna coil trimmer "Q" to ground. Original connection was definitely factory, not a later fix. What's the deal here??? Also, I have now noted that there is a difference between the Rider and Beitman schematics. Namely, in the antenna circuit with Beitman (your schematic) there is a C8 hooked between "Z" antenna lug and the ant loop AND trimmer "G" is directly grounded on the other side of the loop loading coil. On Rider's schematic (AND ON MY RADIO) there is no C8 in the loop circuit and C2 is not directly grounded but is connected to loop circuit. On my radio, C2 is grounded when the band switch is moved to police and SW bands.
:
:I can't get over the feeling there is still a problem with the osc circuits but at least I have BCB where I want it. What do you think?
: