[jerryhawthorne]

Radiola, your turning into an idiot That should been the first you changed. All electrolytics! That is the first one!
Do that and let us know. Who knows how long ago it was changed. Could be your problem but you always want new fresh caps before and after the "field coil". See what that does. Still watching and we will get it right with smarter people than me.

Jerry

[morzh]

Electrolytics have notoriously short life, it is in their nature, so if it is any more than 10 years old it is probably a good candidate for being replaced. Not that you problems are necessarily are due to it, but in order to eliminate all possible and unnecessary questions I'd change every single cap except possibly the mica ones (and even they sometimes do pose problems).

[codefox1]

Gentlemen, let's be kind to one another. Sure all the wax and lyticic caps need to be replaced, but trouble after 10 minutes playing very often points to a carbon resistor drifting over the rainbow. Tube troubles usually show up sooner than that, but not each and every time. Bad silver micas usually crap out much sooner than 10 minutes. Ping around gently with a dry chopstick to rule out mechanicals as in tube sockets. Lots of other ways to locate point of failure, but why not start here.

[jerryhawthorne]

Codefox, I did put a smiley after my first comment. I thought all caps had been replaced. Yes, something is heating up but nice to have a solid base of good caps, especially the electrolytic ones. Just surprised that one had not been replaced. No offence to the OP and I apologize if taken as such.
Let us continue to help him. I will try to be more mellow. The comment was probably inappropriate.
Jerry

[radiola]

Hi, guys. First let me say that it was not an electrolytic cap that was not replaced. They, I replace before anything. It is a blue paper cap which I let go because it looked kind of new. I've yet to replace it because I've been busy, but should get it taken care of tomorrow. I'm particularly interested in codefox1's comment regarding resistors. If the cap doesn't change things, I'll change a few resistors that look ragged, even though they did pass the 20% test. Thanks for all suggestions. I'll keep in touch.

[radiola]

Well, well. I changed the cap in question. On the schematic, it is #39, which is a .004 cap. It seems on the schematic to be coming from the plate of the 7c6 tube (second detector), to the cathode of the 7B5 tube (output tube). I'm no expert, and don't know what that cap does (perhaps someone can tell me), but when I changed it, the set passed the hour bench test. Now for the restringing of the tuner cord (everyone's favorite task). Thanks, guys.
NOTE: Someone first stated to me that when you switch out a field coil for a power resistor, it is necessary to increase a certain capacitor. If that's true, can you let me know which one to change? Possibly an .05 cap?

[jerryhawthorne]

Radiiola, as long as you do not have objectionable hum, I wouldn't touch the new electrolytics. If you do have hum try taking the second one up about 50% in value. Still noticable hum, take the first electrolytic cap up less, no more than maybe 20% in value.
Congrats on getting it back and running.
Jerry

[radiola]

Thanks, Jerry. And I assume it will be safe with the power resistors. For some reason the Joseph J. Carr book does not suggest using resistors, except in unison with a choke. Thanks !!!

[jerryhawthorne]

Radiola, again, assuming no objectionable hum all is well. It might be interesting to see what the B+ is after the dropping resistor just to see how closely your resistor matched in reducing the B+ to the original field coil if you have any voltage levels indicated in the schematic. You should be set to go. Most all of the later tube radios like the AA5s never used a choke and had permanent magnet speakers, so no field coil. Generally they had higher value electrolytic caps to make up for the loss in filtering and most important, keep the cost of manufacturing down.
Bear in mind, I'm not the sharpest on radio repair but have a couple radios running speaker replacements using the resistor method and all have worked nicely for years.

Jerry

[Ron Ramirez]

I changed the cap in question. On the schematic, it is #39, which is a .004 cap. It seems on the schematic to be coming from the plate of the 7c6 tube (second detector), to the cathode of the 7B5 tube (output tube). I'm no expert, and don't know what that cap does (perhaps someone can tell me), but when I changed it, the set passed the hour bench test.

This serves as a coupling capacitor between the plate of the 2nd det.-1st audio tube and the control grid of the audio output tube. If shorted, it would pull B+ down, resulting in poor or no audio.

Just another reason why every old paper and electrolytic capacitor must be replaced if you intend to use your old radios, even if the cap in question looks like it was replaced in the last 30-40 years.

[radiola]

Thanks, Ron, for the explanation. Much appreciated.

[radiola]

Someone gave me a tip, that the cap in question is a 1000 volt cap, and would have to be changed again, eventually. I can't see that on my fuzzy schematic. Any tips?

[codefox1]

Agree with Ron, the cap from the detecter can be a show stopper, all it is supposed to do is filter out IF component, but when it gets leaky or shorts, little or no sound. Also agree that wholesale replacement of paper and electrolytic capacitors should always be done.

There are two capacitors connected to the plate of the 41 output tube, pin 2. One is to suppress oscillation and goes to ground, the other is to the tone control to suppress treble. These must be high voltage types as there are transients present. I would rather see 1000 volt types used here, although many would just stick with 630 volt types, but nothing less will do. If you haven't changed them out, now's the time. Symptoms are similar when they start to go, consequences much more severe as in a blown output transformer!

Agree that the scan of the schematic for this set on the other forum is terrible, but you can get a clear good set of detailed information very reasonably from this forum moderator Chuck Schwark here:

http://www.philcorepairbench.com/schematics.htm

[Ron Ramirez]

Someone gave me a tip, that the cap in question is a 1000 volt cap, and would have to be changed again, eventually.

No, you won't have to change it again unless you get in there with a hot soldering iron and touch the iron to the new cap.

In other words, cap 39 does not have to have a 1000 volt rating. The most voltage it is going to see is around 50 VDC, give or take a few volts.

[morzh]

1. Ron is correct, the C39 will see tens of volts, not even close to a hundred.
2. No matter what the voltahe is, a correctly rated cap will serve you forever, with the notable exception of electrolytics which are destroyed by mostly heating due to ambient temp. plus ripple current times ESR heat losses. Otherwise a modern ceramic or film cap will live very long if not forever.