Is the 4.5 MHZ trimmer a prime suspect or should I be looking for another defective part? Tubes were all replaced with new ones. Could a drifted off value resistor cause this problem?
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:Is the 4.5 MHZ trimmer a prime suspect or should I be looking for another defective part? Tubes were all replaced with new ones. Could a drifted off value resistor cause this problem?
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Are the tuning plates shorting? Is the functional part of the band on frequency?
Johnnysan-
The functional part of the mid-band picks up stations strongly and it tracks my RF Generator. The alignment is not perfect but close.
I was having a problem with the tuning plates shorting that was affecting reception between 5 and 8 MHZ. I cleaned and bent the outer-most rotor plates ever-so-slightly and now the 5 to 8 MHZ range is excellent. I will check the tuning capacitor again under a magnifying lens and see if I can find any more rubbing and shorting.
For what its worth, there is truly 'nothing' between 3 and 4.5 MHZ - no crackle, no pops, no background hiss. It's as if the RF oscillator or converter tubes just 'cut out' and stop working in that freq. range. Then when you tune back toward 3 MHZ the set suddenly roars back to life again. And the Broadcast Band reception is excellent across the entire tuning range.
Will post back about what I find with the tuning condenser. Thanks for your advise!
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:Are the tuning plates shorting? Is the functional part of the band on frequency?
:Johnnysan-
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Tuner plates aren't touching since you receive the entire broadast band. Can't be a bad coil as it works on part of the band.
Does the oscillator quit operating where the band is dead? Measure grid #1 on the oscillator tube. It should be several volts negative. Try another oscillator tube.
Norm
:Hi Johnnysan,
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: The functional part of the mid-band picks up stations strongly and it tracks my RF Generator. The alignment is not perfect but close.
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:I was having a problem with the tuning plates shorting that was affecting reception between 5 and 8 MHZ. I cleaned and bent the outer-most rotor plates ever-so-slightly and now the 5 to 8 MHZ range is excellent. I will check the tuning capacitor again under a magnifying lens and see if I can find any more rubbing and shorting.
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:For what its worth, there is truly 'nothing' between 3 and 4.5 MHZ - no crackle, no pops, no background hiss. It's as if the RF oscillator or converter tubes just 'cut out' and stop working in that freq. range. Then when you tune back toward 3 MHZ the set suddenly roars back to life again. And the Broadcast Band reception is excellent across the entire tuning range.
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:Will post back about what I find with the tuning condenser. Thanks for your advise!
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::Are the tuning plates shorting? Is the functional part of the band on frequency?
::Johnnysan-
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First I tried another oscillator tube. The substitute tube did nothing for the dead mid-band segment but it did produce a low pitched rumbling noise across the broadcast band. I put the original front end tube back real quick!
Voltage readings are definitely off. The tube in question is a 7G7 / 1232 locktal. Set to the broadcast band, the pin 6 grid reading is only minus 1 volt (schematic shows either minus 4.5 or minus 45 volts - hard to read it clearly). The pin 3 grid measures 73.4 volts and schematic indicates it should be 128 volts. Pin 2 Plate is running 219 volts (schematic says 233 volts) but I was expecting that because I'm using a 1500 ohm power resistor to temp. substitute for the 1250 ohm speaker choke coil.
When I change the bandswitch to the High Shortwave Band the 7G7 oscillator throws a real fit if I try to read pin 6 grid minus voltage - the tube cuts out with a pop and voltage reads practically nothing.
There is a small choke coil with a mica capacitor across it and these are connected to the 7G7 Plate and couple to the Converter Tube Grid. A network of 3 resistors is also tied to these feeding the grids on the 7G7. I'm wondering if that mica cap is bad and fouling up the voltages?
:Hi Daniel
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: Tuner plates aren't touching since you receive the entire broadast band. Can't be a bad coil as it works on part of the band.
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: Does the oscillator quit operating where the band is dead? Measure grid #1 on the oscillator tube. It should be several volts negative. Try another oscillator tube.
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:Norm
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::Hi Johnnysan,
::
:: The functional part of the mid-band picks up stations strongly and it tracks my RF Generator. The alignment is not perfect but close.
::
::I was having a problem with the tuning plates shorting that was affecting reception between 5 and 8 MHZ. I cleaned and bent the outer-most rotor plates ever-so-slightly and now the 5 to 8 MHZ range is excellent. I will check the tuning capacitor again under a magnifying lens and see if I can find any more rubbing and shorting.
::
::For what its worth, there is truly 'nothing' between 3 and 4.5 MHZ - no crackle, no pops, no background hiss. It's as if the RF oscillator or converter tubes just 'cut out' and stop working in that freq. range. Then when you tune back toward 3 MHZ the set suddenly roars back to life again. And the Broadcast Band reception is excellent across the entire tuning range.
::
::Will post back about what I find with the tuning condenser. Thanks for your advise!
::
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:::Are the tuning plates shorting? Is the functional part of the band on frequency?
:::Johnnysan-
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The oscillator shouldn't be 7G7. Tubes like 7A8, 7B8, 7Q7 are usually used for the osc/mixer stage. Do you have something like one of these? Some radios used 7A4 or 14AF7 for osc. Not sure of your radio model number.
Mica caps seldom are bad but a few are found with leakage. I wouldn't expect just a dead segment of the band.
Norm
:Norm and Johnnysan,
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: First I tried another oscillator tube. The substitute tube did nothing for the dead mid-band segment but it did produce a low pitched rumbling noise across the broadcast band. I put the original front end tube back real quick!
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:Voltage readings are definitely off. The tube in question is a 7G7 / 1232 locktal. Set to the broadcast band, the pin 6 grid reading is only minus 1 volt (schematic shows either minus 4.5 or minus 45 volts - hard to read it clearly). The pin 3 grid measures 73.4 volts and schematic indicates it should be 128 volts. Pin 2 Plate is running 219 volts (schematic says 233 volts) but I was expecting that because I'm using a 1500 ohm power resistor to temp. substitute for the 1250 ohm speaker choke coil.
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:When I change the bandswitch to the High Shortwave Band the 7G7 oscillator throws a real fit if I try to read pin 6 grid minus voltage - the tube cuts out with a pop and voltage reads practically nothing.
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:There is a small choke coil with a mica capacitor across it and these are connected to the 7G7 Plate and couple to the Converter Tube Grid. A network of 3 resistors is also tied to these feeding the grids on the 7G7. I'm wondering if that mica cap is bad and fouling up the voltages?
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::Hi Daniel
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:: Tuner plates aren't touching since you receive the entire broadast band. Can't be a bad coil as it works on part of the band.
::
:: Does the oscillator quit operating where the band is dead? Measure grid #1 on the oscillator tube. It should be several volts negative. Try another oscillator tube.
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::Norm
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:::Hi Johnnysan,
:::
::: The functional part of the mid-band picks up stations strongly and it tracks my RF Generator. The alignment is not perfect but close.
:::
:::I was having a problem with the tuning plates shorting that was affecting reception between 5 and 8 MHZ. I cleaned and bent the outer-most rotor plates ever-so-slightly and now the 5 to 8 MHZ range is excellent. I will check the tuning capacitor again under a magnifying lens and see if I can find any more rubbing and shorting.
:::
:::For what its worth, there is truly 'nothing' between 3 and 4.5 MHZ - no crackle, no pops, no background hiss. It's as if the RF oscillator or converter tubes just 'cut out' and stop working in that freq. range. Then when you tune back toward 3 MHZ the set suddenly roars back to life again. And the Broadcast Band reception is excellent across the entire tuning range.
:::
:::Will post back about what I find with the tuning condenser. Thanks for your advise!
:::
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::::Are the tuning plates shorting? Is the functional part of the band on frequency?
::::Johnnysan-
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:Hi Daniel
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: Tuner plates aren't touching since you receive the entire broadast band. Can't be a bad coil as it works on part of the band.
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: Does the oscillator quit operating where the band is dead? Measure grid #1 on the oscillator tube. It should be several volts negative. Try another oscillator tube.
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:Norm
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Have a look at the Zenith 7-S-529 Schematic available here at Nostalgia. See the tube compliment. Hopefully I'm not going mad but the RF Oscillator is a 7G7 and next after that is a 7B8 Converter.
Whatever else is or isn't right about this set it is actually working quite well after recapping and replacing several drifted resistors. Now if I can just resolve the 'dead' spot in the mid-band...
Karl,
Thanks so much. I was under the mistaken impression that modern DVM's had similar characteristics to the old VTVM's. I have a spare probe and I will add a 1 megohm resistor to it and take my readings once again and post back with the results.
--Daniel
Karl Said:
:You need to use either use a VTVM or "unload" a DVM to measure the oscillator grid voltage, otherwise the oscillator may stop. If you don't have a VTVM with a resistor in the probe you can use a DVM by attaching a 1 meg resistor to the probe tip. A DVM with a 10 meg input will then read about 0.9 of the correct voltage, so you need to multiply the reading by 1.1 to get a good reading.
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::Hi Daniel
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:: Tuner plates aren't touching since you receive the entire broadast band. Can't be a bad coil as it works on part of the band.
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:: Does the oscillator quit operating where the band is dead? Measure grid #1 on the oscillator tube. It should be several volts negative. Try another oscillator tube.
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::Norm
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Thanks for the model number:
http://www.nostalgiaair.org/PagesByModel/332/M0025332.pdf
Look at 7B8 pins #3, 4 and 7. These pins make up the oscillator. Monitor voltage on pin #4, grid #1. Use a resistor in series with meter probe so it doesn't stop the oscillator. 100K will be ok using a digital meter.
Norm
:Norm,
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:Have a look at the Zenith 7-S-529 Schematic available here at Nostalgia. See the tube compliment. Hopefully I'm not going mad but the RF Oscillator is a 7G7 and next after that is a 7B8 Converter.
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:Whatever else is or isn't right about this set it is actually working quite well after recapping and replacing several drifted resistors. Now if I can just resolve the 'dead' spot in the mid-band...
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:Karl,
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:Thanks so much. I was under the mistaken impression that modern DVM's had similar characteristics to the old VTVM's. I have a spare probe and I will add a 1 megohm resistor to it and take my readings once again and post back with the results.
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:--Daniel
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:Karl Said:
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::You need to use either use a VTVM or "unload" a DVM to measure the oscillator grid voltage, otherwise the oscillator may stop. If you don't have a VTVM with a resistor in the probe you can use a DVM by attaching a 1 meg resistor to the probe tip. A DVM with a 10 meg input will then read about 0.9 of the correct voltage, so you need to multiply the reading by 1.1 to get a good reading.
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:::Hi Daniel
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::: Tuner plates aren't touching since you receive the entire broadast band. Can't be a bad coil as it works on part of the band.
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::: Does the oscillator quit operating where the band is dead? Measure grid #1 on the oscillator tube. It should be several volts negative. Try another oscillator tube.
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:::Norm
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: Look at 7B8 pins #3, 4 and 7. These pins make up the oscillator. Monitor voltage on pin #4, grid #1. Use a resistor in series with meter probe so it doesn't stop the oscillator. 100K will be ok using a digital meter.
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: Norm
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Sorry, didn't notice your post suggesting 1 meg. We agree value isn't critical as long as oscillator isn't loaded.
Norm
:What's the point of quibbling about the resistor value? The point is to unload the meter.
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:: Look at 7B8 pins #3, 4 and 7. These pins make up the oscillator. Monitor voltage on pin #4, grid #1. Use a resistor in series with meter probe so it doesn't stop the oscillator. 100K will be ok using a digital meter.
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:: Norm
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Please pardon my delay in replying. Before I explain my voltage measurements I made a very interesting discovery! I decided to purposefully tune the set to 4.5 MHZ (the dead part of the spectrum) and then check the voltage on the 7B8, Pin 4 using a one megohm probe as Norm and Karl suggested. The instant that I did so, the set came back to life at 4.5 MHZ! Lift the probe off of Pin 4 of the 7B8 and it immediately goes dead again!
Aside from the possibility of a bad solder joint, there is a 47.5K ohm resistor (schematic calls for 47K) across the 7B8 pins 4 and 7 and I'm wondering if slightly changing that resistor's value might restore the band between 3 and 4.5 MHZ? Perhaps also another 7B8 tube might be "less picky" and behave better?
But not to put the cart before the horse, here are the readings I've found:
Broadcast Band:
Pin 2 (Plate) - 236.5 volts (schematic calls for 253. Remember I've got a 1500 ohm power resistor subbing for a 1250 ohm choke)
Pin 3 - 224 volts (schematic calls for 178 - odd)
Pin 4 - negative 7.38 volts (schematic says -8.5)
High Shortwave Band:
Pin 2 (Plate) - 237.6 volts
Pin 3 - 223.3 volts
Pin 4 - negative 3.9 volts
When I tuned to 4.5 MHZ the Pin 4 Reading was negative 2.86 volts.
All above measurements taken with one meg probe and values shown obtained by using Karl's formula of multiplying by 1.1
I did not check Pin 7 of the 7B8 because schematic says it should be zero volts.
Interesting, EH?
:Karl
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: Sorry, didn't notice your post suggesting 1 meg. We agree value isn't critical as long as oscillator isn't loaded.
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:Norm
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::What's the point of quibbling about the resistor value? The point is to unload the meter.
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::: Look at 7B8 pins #3, 4 and 7. These pins make up the oscillator. Monitor voltage on pin #4, grid #1. Use a resistor in series with meter probe so it doesn't stop the oscillator. 100K will be ok using a digital meter.
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::: Norm
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The " typical problem area" concentrates at the glass seal and exends outwardly, up to ~1/8-- 1/4 of an inch from the tube envelope. Your measured voltage to pin 3 is definitely not shortchanging the oscillator "plate" of adequate voltage. Or on another oscillator aspect, if the C2 coupling capacitor (raaaaarely fails . . . since its using silver mica construction) for the oscillator might need replacement or confirm if the R3 grid resistor is currently of the right value. That info on the experience of the shifting of the 1st grid oscillator voltage, when tuning completely across the problem band would be most helpful. |
:Those Lock-in type tubes always seem to have dirty pin to socket contact. Makes for contact resistance. Clean all the pins with some lite grit sand paper. Clean all the other lock-in tube pins as well.
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I am going to give the 7B8 pins and socket a good cleaning tonight and we'll see what results.
Edd I really appreciate your explanation. The chassis I have is a Zenith 7A02 which was used in the 7-S-529
table radio and several other table and console models.
I will now write 25 times on a piece of paper:
"Always remember to carefully clean locktal tube pins"
-Daniel
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:Sir Daniel. . . . .
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:My procedure on any "wire leaded " tube pin is to take every pin and do a linear scraping of the pin with a #11 Exacto blade, eventually working ALL of the way completely AROUND each pin.
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:The " typical problem area" concentrates at the glass seal and exends outwardly, up to ~1/8-- 1/4 of an inch from the tube envelope.
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:Additionally, be sure that there is no oil--Wd-40 remnants--etc-- that might be on the socket proper, scrubbbing and flooding the socket with a degreaser assures that not being a problem.
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:Since we are dealing with the local oscillator aspect and you have monitored the activity of the local oscillator by its developed 1st grid voltage.
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:Lets go a step further and monitor that concerned band from ONE END to the OTHER END.
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:You are going to find that the developed voltage is NOT uniform across the band, but will be found to gradually diminish in level.
:The most voltage will be read at the low end of the band where the tuning condenser is completely inner meshed and then will gradually start declining in level, as you unmesh the cap in its tuning to the very high end of the band.
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:The point of interest is to start running updial and " get a feel " for the rate at which the voltage declines.
:Then you evaluate to see if there is ANY abnormality / transition in the voltage change effect, specifically in that "problem area" of 3.5--4 on the dial, of that band.
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:Matter of fact . . . I never have seen mention of EXACTLY what what your radio IS, other than some subtle suggestion in referring to using a Zenith 7S529 schematic.
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:The only sets I find using that potential tube combo are the Zenith 7S598---7S529---7S530 ---7S547 7S557---7S558---7S559---and the 7S548 . . .all involving the 7A11 chassis.
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:In that case the sub in of a 7G7 for the 1232 RF tube would be relevant to the frontal RF circuitry, with the oscillator and conversion aspect being fulfilled solely by the 7B8 tube.
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:In the 7B8 oscillator circuit the pin 4 is your 1st grid while the pin 3 grid is acutally functioning as your " plate " in the oscillator circuit.
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:Your resistor's wires added capacitance to pin 4 apparently kicked the oscillator into functioning in that problem area.
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:Your measured voltage to pin 3 is definitely not shortchanging the oscillator "plate" of adequate voltage.
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:With its voltage now being high, its drop down is dependent upon the R 5 and R 17 resistors coming from the power supply feed line. (Decreased in value(s) ?)
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:Or on another oscillator aspect, if the C2 coupling capacitor (raaaaarely fails . . . since its using silver mica construction) for the oscillator might need replacement or confirm if the R3 grid resistor is currently of the right value.
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:A little higher value of R3 . . possibly on up to 68 k . . . is a help or more picofarads of coupling for C2 . . possibly on up to 68 pf is an aid in the case, when finding sluggish oscillator action.
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:That info on the experience of the shifting of the 1st grid oscillator voltage, when tuning completely across the problem band would be most helpful.
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:73's de Edd
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::Those Lock-in type tubes always seem to have dirty pin to socket contact. Makes for contact resistance. Clean all the pins with some lite grit sand paper. Clean all the other lock-in tube pins as well.
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My first suspicion was correct - the mid-band trimmer capacitor was causing all the trouble! Before I acquired this set someone had adjusted (make that mis-adjusted) the mid-band trimmer fully clockwise which had reduced the oscillator's sensitivity to essentially nothing between 3 and 4.5 MHZ!
HOW I FIGURED THINGS OUT -
After following everyone's suggestions (and thank you all) without results I shut the set off and began poking around with the ohm meter looking for anything not right on the 7B8 oscillator circuit. There are four little trimmers on the back of the chassis to align and tune up the bands. Two of those trimmers had nearly a dead short across their terminals while the other two trimmers had much higher readings. When I put an adjustment screwdriver to the mid-band trimmer it was screwed up tight and I knew 'no way' it was set correctly! I turned the trimmer to it's mid-range, hooked up the signal generator and followed Zenith's directions for that particular trimmer.
Tonight I have strong 75 to 80 meter reception across the entire mid-band dial! Eureka! Ye-haw!
Warren and Edd encouraged me to clean the locktal tube pins and that I did along with the tube socket as well. No change.
Edd explained about the voltages on pin 4 of the oscillator tube not being a constant across the dial.
SIR EDD....
FYI, before I tracked the problem to the mis-adjusted trimmer I took these readings:
At the low end of the spectrum Pin 4 was a minus 3.63 volts. The voltage dropped in proportion to the tuning UNTIL about 3.5 MHZ. At 3.5 MHZ the reading was minus 0.23 volts. That value remained A CONSTANT ACROSS THE REMAINDER OF THE HIGH END SPECTRUM!
Those of you familiar with Star Trek Next Generation know of the Alien named "Q" who was always full of mayhem and mystery. The mid-band Trimmer Cap on the Zenith 7A02 Chassis is curiously labeled "Q" on the Schematic and Alignment Instructions. I should have seen that one coming. You might say that I missed my "Q" - LOL!
In any event, another Classic Radio back in service! Thanks everyone for all of your suggestions!