This is my first boatanchor. I've done the usual recap and am working on alignment. I think I can handle the crystal filter alignment. My problem is with the RF alignment. I cannot get the top end of the BC band to come in right. A 1200 KC signal comes in at 1500, 1400 not at all. Loosening the oscillator trimmer all the way gets 1400 close to where it should be but that causes weak reception.
Anyone ever run in to this? I've never had this problem on house radios.
Thanks for any help.
Bud
Sir Bud: Maybe that do jes' be because most all of the cheeper units that you have encountered so far were minimally inclusive of In your sets situation, they are using what is called a padder condenser (A22) for making tracking compensation in the local oscillator circuit. With, your tuning condensers osc section having that (A22) . . .padder. . .in series with it, along with a 2-22 mmfd trimmer cap that is shunting across the L7 oscillator coil. In your sets situation, they are using what is called a padder condenser (A22) for making tracking compensation in the local oscillator circuit. With, your tuning condensers osc section having that (A22) . . .padder. . .in series with it, along with a 2-22 mmfd trimmer cap that is shunting across the L7 oscillator coil.
Oh yes, but WHAT a boatanchor !
the adjustment capabilities of the osc circuitry parameters, choosing to use a fixed osc coil value.
More involved receivers circuitry will include the capability of slightly skewing the value of the inductive value of the oscillator coil.
That means that you will then be able to trim in the capability of the local osc frequency produced and the RF- Mixer tuning sections
with an optimal differential 455 khz I.F. frequency being produced across MORE of the total tuning spectrum of the band involved.
HOW-SOME-EVER, in your situation, you will find a variant in the manner of accomplishing that on the BCB, seems like you are
encountering the oscillator circuit using the SAME physical size and capacitive value of tuning condenser being used for the BCB
local osc circuit, as is used in the RF and Mixer sections.
Now, let's think this out . . . . .vewy, vewy carefuwwy. . .a la Elmer P. Fudd. . . . , if that tuning condenser is placed in its max open
position there will be little capacitance between the cap vanes of the rotor and stator sections, typically in the low decades of mmfds. PLUS any trimmer capacitance which is shunting across the circuitry, SHOULD the tuning condenser have an onboard mounted trimmer capacitor built into it, or aside somewhere.
The end effect is that if the tuning cap is max open that little capacitance value presented, along with the companion inductance of
BCB osc coil is needed to resonate with the osc circuit be producing a frequency of ~ 1740 + 455 khz=2195khz.
Thefore if the local osc is producing 2195, and the RF section(s) tuning condenser is tuned to 1745 khz, then the mixer output
circuitry to the 1st IF transformer is receiving a down conversion to a 455 IF signal to be passed on down the IF strip.
Now what that is going to do, is make corrective compensation by the fact that two condensers in series are of LESS value than each of the series values. Whereas, two condensers in parallel make an additive value, but THAT latter situation is not what is needed here. The 410 mmfd value is needing reduction down into the ~328 mmfd value.
This now gets us to the situation here, where if the condensers vanes were fully meshed, that would be accomplishing the lower 540 khz tuning for the set.
Now we need to refer back to the 2195 osc signal that was earlier obtained and plugging in a likely value of . . .say 25 mmfd having been provided by the tuning cap in its wide open vaned state, that computes to a value of ~ 210 uh of osc coil (L7) inductance needed to create that 2195 khz osc frequency.
With that fixed inductance now in mind, we work backwards and see how much capacitance ( with those tuning vanes completely meshed ) will be needed in order to create a desired 1095 khz ( 455 + 540 ) local oscillator frequency for the reception of the lowest received frequency of 540 on the BCB. That then computes out to be in the order of ~328 mmfd for the required capacitance value, to be producing that frequency.
But. . . but. . . . .BUT, BUT, BUT. . . . the osc tuning cap section will be offering WAAAAY too much capacitance and that frequency created would be waaaaay down at about ~485 khz.
That, then, is where the ( A22 ) padder comes into play, if you would then trim it into a value of ~320----550 mmfd, somewhere therein, the series combination then becomes ~328 mmfd and creates that desired 1095 local osc freq at ~max tuning condenser meshing.
On some sets, there is a combination of a fixed mica cap for the most of the capacitance for that “A22” value, with a shunting parallel variable cap doing the trimming in.
It takes a lot of multiple leaves in a mica compression trimmer capacitors construction in order to achieve those higher capacitive values.
End synopsis:
“A 1200 KC signal comes in at 1500, 1400 not at all”
With the frequency calibration error that you have specified, the value of A20 /A22 is/ are set improperly, if you will slowly adjust in more capacitance, the received 1200 station should then “walk” on downwards from the scales 1500 logging and towards the correct logging of 1200.
This is easiest accomplished via the main tuning condenser being rocked on and off the station while slowly adjusting (A20 / A22) and then the direction and degree of correction is readily ascertained.
With great tuning errors, one also needs to go back and confirm optimization of the companion R.F. amp and Mixer sectons of that tuning condenser in maintaining an optimum 455 difference signal strength level.
When all is successfully accomplished, there will be good set sensitivity and tolerable dial scale calibration across the BCB.
Basically, you are alternatively trimming in padder condenser (A22) to get the lower dial scale calibration correct in the ~600 khz area and the setting of trimmer (A20) to get the correction at the high end of the dial at ~1400.
Taking note that the (A22)adjustment can appreciably compress or stretch out your overall tuning error result.
RF FRONTAL THUMBNAIL:
Sir Bud:
And WHAT a boatanchor, it is !
Maybe that do jes' be because most all of the cheeper units that you have encountered so far were minimally inclusive of
the adjustment capabilities of the osc circuitry parameters, choosing to use a fixed osc coil value.
More involved receivers circuitry will include the capability of slightly skewing the value of the inductive value of the oscillator coil.
That means that you will then be able to trim in the capability of the local osc frequency produced and the RF- Mixer tuning sections
with an optimal differential 455 khz I.F. frequency being produced across MORE of the total tuning spectrum of the band involved.
HOW-SOME-EVER, in your situation, you will find a variant in the manner of accomplishing that on the BCB, seems like you are
encountering the oscillator circuit using the SAME physical size and capacitive value of tuning condenser being used for the BCB
local osc circuit, as is used in the RF and Mixer sections.
Now, let's think this out . . . . .vewy, vewy carefuwwy. . .a la Elmer P. Fudd. . . . , if that tuning condenser is placed in its max open
position there will be little capacitance between the cap vanes of the rotor and stator sections, typically in the low decades of mmfds. PLUS any trimmer capacitance which is shunting across the circuitry, SHOULD the tuning condenser have an onboard mounted trimmer capacitor built into it, or aside somewhere.
The end effect is that if the tuning cap is max open that little capacitance value presented, along with the companion inductance of
BCB osc coil is needed to resonate with the osc circuit be producing a frequency of ~ 1740 + 455 khz=2195khz.
Thefore if the local osc is producing 2195, and the RF section(s) tuning condenser is tuned to 1745 khz, then the mixer output
circuitry to the 1st IF transformer is receiving a down conversion to a 455 IF signal to be passed on down the IF strip.
Now what that is going to do, is make corrective compensation by the fact that two condensers in series are of LESS value than each of the series values. Whereas, two condensers in parallel make an additive value, but THAT latter situation is not what is needed here. The 410 mmfd value is needing reduction down into the ~328 mmfd value.
This now gets us to the situation here, where if the condensers vanes were fully meshed, that would be accomplishing the lower 540 khz tuning for the set.
Now we need to refer back to the 2195 osc signal that was earlier obtained and plugging in a likely value of . . .say 25 mmfd having been provided by the tuning cap in its wide open vaned state, that computes to a value of ~ 210 uh of osc coil (L7) inductance needed to create that 2195 khz osc frequency.
With that fixed inductance now in mind, we work backwards and see how much capacitance ( with those tuning vanes completely meshed ) will be needed in order to create a desired 1095 khz ( 455 + 540 ) local oscillator frequency for the reception of the lowest received frequency of 540 on the BCB. That then computes out to be in the order of ~328 mmfd for the required capacitance value, to be producing that frequency.
But. . . but. . . . .BUT, BUT, BUT. . . . the osc tuning cap section will be offering WAAAAY too much capacitance and that frequency created would be waaaaay down at about ~485 khz.
That, then, is where the ( A22 ) padder comes into play, if you would then trim it into a value of ~320----550 mmfd, somewhere therein, the series combination then becomes ~328 mmfd and creates that desired 1095 local osc freq at ~max tuning condenser meshing.
On some sets, there is a combination of a fixed mica cap for the most of the capacitance for that “A22” value, with a shunting parallel variable cap doing the trimming in.
It takes a lot of multiple leaves in a mica compression trimmer capacitors construction in order to achieve those higher capacitive values.
End synopsis:
“A 1200 KC signal comes in at 1500, 1400 not at all”
With the frequency calibration error that you have specified, the value of A20 /A22 is/ are set improperly, if you will slowly adjust in more capacitance, the received 1200 station should then “walk” on downwards from the scales 1500 logging and towards the correct logging of 1200.
This is easiest accomplished via the main tuning condenser being rocked on and off the station while slowly adjusting (A20 / A22) and then the direction and degree of correction is readily ascertained.
With great tuning errors, one also needs to go back and confirm optimization of the companion R.F. amp and Mixer sectons of that tuning condenser in maintaining an optimum 455 difference signal strength level.
When all is successfully accomplished, there will be good set sensitivity and tolerable dial scale calibration across the BCB.
Basically, you are alternatively trimming in padder condenser (A22) to get the lower dial scale calibration correct in the ~600 khz area and the setting of trimmer (A20) to get the correction at the high end of the dial at ~1400.
Taking note that the (A22)adjustment can appreciably compress or stretch out your overall tuning error result.
RF FRONTAL THUMBNAIL:
73's de Edd
If I align this radio when the ambient temperature is say 60 degrees(the normal winter temp in my radio room), come summer,the dail calibration is off by 20kc at the upper end of the dial.
This model has a ceramic temperature compensating capacitor(negative temp. coefficient) of the "semi-fixed" type in Parallel with the oscillator section of the tuning cap.Interestingly,this was done away with on later models such as the s-40b.I had an s-40b and the dial calibration was quite good at any room temp. after normal warm-up.
There is an adjusting nut on this capacitor that looks like it has been tampered with.Should I try adjusting this capacitor?Is it possible it is adding too much capacitance as the ambient temp. rises and throwing calibration out?Can I do like I would like to do and just clip it out and throw it away?The s-40b's circuit is pretty much identical and seems to live just fine without it.Would appreciate your thoughts(and anyone else's for that matter).Thank you,Mike.
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