Zenith dead

9/24/2009 7:07:45 AMJohn

My 7S633 is just dead. I can inject a signal into the grid of the 6P5 and get sound so seems like that is working from there on. When I put a scope on the grid of the 7A7 there is lots of signal there and I can then hear some station sound but not loud. When I remove the scope lead from the 7A7 grid there is no more station sound at all. Putting the scope on the 7B6 pins 5 and 6 I see no signal. Also, scope shows plenty of signal on the 7A7 plate. What could be wrong?

9/24/2009 11:53:18 AMDoug Criner

Have you checked all the tube-socket voltages and compared them to the schematic?
Doug

9/24/2009 12:38:40 PMBill G.

:My 7S633 is just dead. I can inject a signal into the grid of the 6P5 and get sound so seems like that is working from there on. When I put a scope on the grid of the 7A7 there is lots of signal there and I can then hear some station sound but not loud. When I remove the scope lead from the 7A7 grid there is no more station sound at all. Putting the scope on the 7B6 pins 5 and 6 I see no signal. Also, scope shows plenty of signal on the 7A7 plate. What could be wrong?
:
Hi John,
It seems you know where to take the readings.
You stated, "Putting the scope on the 7B6 pins 5 and 6 I see no signal. Also, scope shows plenty of signal on the 7A7 plate." The big thing between is the output IF can.
It could be leaky, out of tune or open. First check the voltage at the 7B6 detector plates. That should be negative. If it is positive, then current is leaking across the IF transformer. If that is OK, check the continuity of the secondary, 10 to 40 ohms. Yopu have continuity on the primary, else you wouldn't have signal on the 7A7 plate.
If you have continuity and good voltage, it may just be out of tune.

Best Regards,

Bill Grimm

9/24/2009 1:22:31 PMRadiodoc

Bill G.,

Please don't think I am trying to be picky, but I think he will measure around 47K between the IF secondary lead/tterminals. I believe there is a 47K Ohm resistor in series with the secondary winding. Just didn't want him to think the IF was bad because he measured several thousands Ohms resistance.

Radiodoc
*****************

::My 7S633 is just dead. I can inject a signal into the grid of the 6P5 and get sound so seems like that is working from there on. When I put a scope on the grid of the 7A7 there is lots of signal there and I can then hear some station sound but not loud. When I remove the scope lead from the 7A7 grid there is no more station sound at all. Putting the scope on the 7B6 pins 5 and 6 I see no signal. Also, scope shows plenty of signal on the 7A7 plate. What could be wrong?
::
:Hi John,
: It seems you know where to take the readings.
: You stated, "Putting the scope on the 7B6 pins 5 and 6 I see no signal. Also, scope shows plenty of signal on the 7A7 plate." The big thing between is the output IF can.
: It could be leaky, out of tune or open. First check the voltage at the 7B6 detector plates. That should be negative. If it is positive, then current is leaking across the IF transformer. If that is OK, check the continuity of the secondary, 10 to 40 ohms. Yopu have continuity on the primary, else you wouldn't have signal on the 7A7 plate.
: If you have continuity and good voltage, it may just be out of tune.
:
:Best Regards,
:
:Bill Grimm
:

9/24/2009 2:18:20 PMJohn

Before leaving for work this morning I measured from the plates, pins 5 &6, of the 7B6 to ground and it was only 80 ohms so someting is screwy there but will recheck tonight.

:Bill G.,
:
:Please don't think I am trying to be picky, but I think he will measure around 47K between the IF secondary lead/tterminals. I believe there is a 47K Ohm resistor in series with the secondary winding. Just didn't want him to think the IF was bad because he measured several thousands Ohms resistance.
:
:Radiodoc
:*****************
:
:
:::My 7S633 is just dead. I can inject a signal into the grid of the 6P5 and get sound so seems like that is working from there on. When I put a scope on the grid of the 7A7 there is lots of signal there and I can then hear some station sound but not loud. When I remove the scope lead from the 7A7 grid there is no more station sound at all. Putting the scope on the 7B6 pins 5 and 6 I see no signal. Also, scope shows plenty of signal on the 7A7 plate. What could be wrong?
:::
::Hi John,
:: It seems you know where to take the readings.
:: You stated, "Putting the scope on the 7B6 pins 5 and 6 I see no signal. Also, scope shows plenty of signal on the 7A7 plate." The big thing between is the output IF can.
:: It could be leaky, out of tune or open. First check the voltage at the 7B6 detector plates. That should be negative. If it is positive, then current is leaking across the IF transformer. If that is OK, check the continuity of the secondary, 10 to 40 ohms. Yopu have continuity on the primary, else you wouldn't have signal on the 7A7 plate.
:: If you have continuity and good voltage, it may just be out of tune.
::
::Best Regards,
::
::Bill Grimm
::
:

9/24/2009 2:48:08 PMRadiodoc

John,

Looking at the schematic, from the diode plates to ground should measure around 500K or so Ohms to chassis/ground. I note that the secondary lead of the IF transformer may go thru a shield of sorts perhaps a braid or a spring looking shield. If the old brittle rubber wire was in the radio, there may be a partial short inside of the shield. Also there may be a problem with the fixed caps in I believe the base of the transformer.

Radiodoc
*****************

:Before leaving for work this morning I measured from the plates, pins 5 &6, of the 7B6 to ground and it was only 80 ohms so someting is screwy there but will recheck tonight.
:
::Bill G.,
::
::Please don't think I am trying to be picky, but I think he will measure around 47K between the IF secondary lead/tterminals. I believe there is a 47K Ohm resistor in series with the secondary winding. Just didn't want him to think the IF was bad because he measured several thousands Ohms resistance.
::
::Radiodoc
::*****************
::
::
::::My 7S633 is just dead. I can inject a signal into the grid of the 6P5 and get sound so seems like that is working from there on. When I put a scope on the grid of the 7A7 there is lots of signal there and I can then hear some station sound but not loud. When I remove the scope lead from the 7A7 grid there is no more station sound at all. Putting the scope on the 7B6 pins 5 and 6 I see no signal. Also, scope shows plenty of signal on the 7A7 plate. What could be wrong?
::::
:::Hi John,
::: It seems you know where to take the readings.
::: You stated, "Putting the scope on the 7B6 pins 5 and 6 I see no signal. Also, scope shows plenty of signal on the 7A7 plate." The big thing between is the output IF can.
::: It could be leaky, out of tune or open. First check the voltage at the 7B6 detector plates. That should be negative. If it is positive, then current is leaking across the IF transformer. If that is OK, check the continuity of the secondary, 10 to 40 ohms. Yopu have continuity on the primary, else you wouldn't have signal on the 7A7 plate.
::: If you have continuity and good voltage, it may just be out of tune.
:::
:::Best Regards,
:::
:::Bill Grimm
:::
::
:

9/24/2009 5:25:20 PMEdd

Sir John. . . .

Just filling in to what has already mentioned:

If you have scoped a healthy RF . . IF signal being viewed at [A], along with your confirmation that an AF signal into [B] will drive the speaker healthily.

Then , my friend, you has done went and got's yourself the . . . . . "Dreaded, missing of [A] to [B] Syndrome."

Good to hear that there is no problem with the final DC coupled AF output circuitry, that boggles some peoples mind.

First electro / physical thing to check, would be the integrity of the wiring thru the shielded loop that I have marked up as [G].
One certainly would not want the high side of the IF output to not be getting thru to the det diodes.

The markup [C] is the origin point of your receive audio and it routes on down wards as shown and upwards to the 1st grid of the 7B6 at [D].

Note my markup of the tone control cluster switch in [FUSCIA] and check to see that nothing /nor/ parts are abnormal in that section, nor is there a mechanical problem with switch contacts or mechanical positioning to end up being between a contact function.

Now back to the observation of RF on the plate of the IF 7A7 via low capacitance scope coupling . . .and I doubt that you are going to be having a high bandwith sampling scope, such that you could be capable of watching each individual wavelet of a 455 signal.

But instead, you would be watching at ~1 MS hoz sweep timebase setting, where you merely be getting a compressed vertical amplitude display of the RF "grass" on the screen.

Now the question is, did you take observation of a couple of your strongest received stations, and confirm that there was the off station presence of a mere baseline, thus displaying an insignificant level of I.F. signal, whereas, when you tuned onto the stations, you did see the appreciable presence of signal amplitude,

An alternative would have to been having placed the scope vertical input in its DC coupling mode and monitored my [ F ] reference on the AVC buss and noticing an appreciable DC negative downwards swing of the baseline as a station was tuned into. Tuning off of the staton would result in its return upwards in a more positive direction .

If so, I would now suggest the use of 1 (or 2) jumper clips along with a .01 ufd cap and couple [C] to [B] and see if tning in of stations now result in some minor degree of audio.

That would be your injecting a quite high I.F. RF level into the detector and it should respond to that level of signal.

If so, then move to coupling the .01 between [C] and [D] and, hopefully, hold onto your socks.

If no results so far, then get your self a 47----220 pf silver mica or disc ceramic capacitor and connect between [A] and the two 7B6 detector diode plates that you see just past my [G] marking feeding into the 7B6.

If still no luck, lets make a quick and easy reassesment of your audio section test, but which you
seemed to have only evaluated the 6P5 onwards to the speaker.

Lets get back with the .01 and get a healthy ~18V P/P of a 60~ signal from any of the hot filament connections . .e.g. . . .[H]

Couple that 60~ signal via the .01 to [B] and a quick "match striking" touch should give one healthy HUMMMMM response .

If that works, assuredly it will, then you can inject from [H] to [C] and should the volume be at ite lowest setting and its then adjustment upwards should control the progressive HUMMM level.

If it doesn't, then you might attempt a quick touch from [H] to [D], but the signal will saturate and go into square wave clipping at the level limited by the plate load resistor . . .R14 . . .and will sound QUITE raspy.
That's about all but the possibility of a 7B6 problem, but the sig in -- sig out testing should confirm the triode amp section.

One could only confirm the tubes internal diodes aspect, by the temp subbing in of a solid state detector diode from the det plates connection to ground,

That would be with the subbing diodes anode going to the plates and its cathode going to ground.

Since you handle scopes your orientation is probably more towards SS circuitry, so a common 1N914---1N4148 are as common to have, as is sand to the beach.

Now,not being for RF use, but even one of the common 1N 4XXXX series power diodes would do in a pinch.

That would somewhat confirm, without having to go out and locate another 7B6.

(No longer a 7-Eleven
item at their old tube tester kiosk.)

Last thing would be passing us, the tube voltages around the 7B6 circuitry.

Standing by . . .

73's de Edd

Zenith Schema Thumbnail:

9/25/2009 5:30:01 AMmon . . .parte deux . . .(79490:79467)

Sir John. . . .

ADDENDA:

Just now caught your . . .parte deux . . .additional comeback, with the info on the secondary of the IF transformer.
That seems to be the problem source if being only ~80 ohms in respect to ground referencing.

That is a bit abnormal, also considering the actual physical separation offered between the primary and secondary coils, and the only other consideration being that the most to be expected, peak voltage wise, is the decades of voltage of RF on the secondary coil winding.

That then asks you too look at the two external connections that go outwards from that secondary, one being the circuitry tied onto the top side of the secondary winding ending up at the RF diode detector plates. A strange route to be finding an 80 ohm fault on, a short, maybe, yes .
The more likely route is the lower side of the secondary winding with its initially encountering the 3 elements of a "tweet" filter and the first element being in the form of a 100 pf mica / (ceramic?) RF bypass condenser and then the 47K of isolation provided by the filter resistor element and then the final 100 pf HF AF bypass cap element. Then, there is the insignificantly high resistance provided by the 470 K diode load resistor.

That makes the otherwise least likely to be suspected, first 100 pf capacitor , needing to get a look over . That being even though it is never subjected to more than mini decades of peak RF voltage.

That's in counterbelief of the , typical, higher breakdown voltage spec of mica capacitors.

Another aspect . . but with my being unable to examine the trimmer base area construction . . would be the possibility of
silver deposition upon mica sheet being used for the 100 pf capacitors and a lateral migration of a contaminated silver trail.
BUT ~ 80 ohms is an EXTREMELY low value for that type of situation.

I guess that after the initial inspection of the diode plate tie in . . . that a then disassembly of the I.F. transformer and evaluation of its innards might then reveal all.

Standing by . . .

73's de Edd

9/25/2009 6:57:21 AMJohn(79491:79490)

Hi EDD,
Well last night after work I used another meter to measure from 5and6 of the 7B6 to ground and it showed 102 ohms. Said the heck with it until this morning I started to undo the two nuts under the chassis that hold the IF can. Got half way thru undoing them and then thought I should first remove the tube and check again to see if I had the low resistance to ground in case the tube was the culprit. Tube previously checked good tho. With the tube out there was no low resistance to ground. Put the tube back and no low resistance to ground and the radio played. Retightened the nuts on the IF can and radio still plays. Not sure what was causing the low measurement to ground. Maybe 7B6 is intermittent!

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:Sir John. . . .
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:ADDENDA:
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:Just now caught your . . .parte deux . . .additional comeback, with the info on the secondary of the IF transformer.
:That seems to be the problem source if being only ~80 ohms in respect to ground referencing.
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:That is a bit abnormal, also considering the actual physical separation offered between the primary and secondary coils, and the only other consideration being that the most to be expected, peak voltage wise, is the decades of voltage of RF on the secondary coil winding.
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:That then asks you too look at the two external connections that go outwards from that secondary, one being the circuitry tied onto the top side of the secondary winding ending up at the RF diode detector plates. A strange route to be finding an 80 ohm fault on, a short, maybe, yes .
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:The more likely route is the lower side of the secondary winding with its initially encountering the 3 elements of a "tweet" filter and the first element being in the form of a 100 pf mica / (ceramic?) RF bypass condenser and then the 47K of isolation provided by the filter resistor element and then the final 100 pf HF AF bypass cap element. Then, there is the insignificantly high resistance provided by the 470 K diode load resistor.
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:That makes the otherwise least likely to be suspected, first 100 pf capacitor , needing to get a look over . That being even though it is never subjected to more than mini decades of peak RF voltage.
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:That's in counterbelief of the , typical, higher breakdown voltage spec of mica capacitors.
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:Another aspect . . but with my being unable to examine the trimmer base area construction . . would be the possibility of
:silver deposition upon mica sheet being used for the 100 pf capacitors and a lateral migration of a contaminated silver trail.
:BUT ~ 80 ohms is an EXTREMELY low value for that type of situation.
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:I guess that after the initial inspection of the diode plate tie in . . . that a then disassembly of the I.F. transformer and evaluation of its innards might then reveal all.
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:Standing by . . .
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:73's de Edd
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9/25/2009 3:40:21 PMBill G.(79514:79491)

Hi John,
More likely than the tube is its socket at being a problem. Loctals and their sockets are well known for having problems with corrosion.
Best Regards,
Bill Grimm