General Electric A-64 advice needed.....low volume

[Kamakiri]

Got this radio in a very sorry state.....the underside of the chassis was PACKED SOLID with a rodent nest, tubes missing, etc, but the cabinet was nice, and not deteriorated, as you'd expect something like this to be.

But getting to the issue, I've recapped the set (this thing has an ungodly amount of capacitors), but it's exhibiting an odd condition.....I get low volume no matter how high I turn it up, but I can flick the radio on and off quickly and for about half a second when I flip it on, the audio comes up VERY strong and loud. I figure that it's got to be something that someone will recognize immediately and something that I might never find

[leadlike]

Have you tried injecting audio into the center tap of the volume control? If it is still weak, it is your audio amp circuit. Next would be to try a new 6a8, as aged/shorted converter tubes can create some unique situations. Of course, this is all to be done after cleaning every contact, switch, etc, etc.

[Kamakiri]

Um, how would one go about injecting audio into the center tap of the volume control?

[dieseljeep]

Originally Posted by Kamakiri

Um, how would one go about injecting audio into the center tap of the volume control?

If the 1st and 2nd audio stage is working, you should be able to inject an audio signal by using your finger. You should get a louder hum than the audio level. Either that, or use a signal source, such as an FM tuner etc. Just touch the center pin of the output cable to the center lug of the volume control pot.
That radio seems to use a different type of AVC circuit, than many of the other sets of the day.

[old_coot88]

Originally Posted by Kamakiri

...I get low volume no matter how high I turn it up, but I can flick the radio on and off quickly and for about half a second when I flip it on, the audio comes up VERY strong and loud. I figure that it's got to be something that someone will recognize immediately and something that I might never find

The sudden drop sounds like a "floating grid" condition on one of the audio stages, most likely the 1st audio stage. That's where the grid has no DC return path to ground, causing the tube to self-bias to near cutoff by space charge. Is the tube a 6Q7? If so, (with the set off) measure resistance from the grid cap connector to ground. On the highest R scale, if it reads infinity (open circuit), it would indicate a floating grid condition.
If you get some kind of resistance reading there, it would eliminate that possibility.

[Reece]

An easy injection is to touch a working soldering iron to the center lug of volume control. Should get a loud hum if audio section is OK.

[Kamakiri]

Nope, got a hum, but a very quiet one.....same as the audio level.

[leadlike]

Well, then that tells me you are looking at some issue in the audio section-somewhere after the volume control, within the 6j7/6f6 circuits. I'd try subbing those tubes, do some voltage measurements, etc, to narrow down the trouble.

[Reece]

Try your soldering iron on the grid of the output tube. If louder than the volume control lug, trouble probably in the first audio section. Detective work, Mr. Watson.

[leadlike]

kind of reminds me of an at home radio repair course I have from the 1930s: since the average radio owner did not have any test gear, they instructed you to put your finger on the grid caps of each tube to determine which stage was malfunctioning-pretty clever, and something I do on my sets when trying to chase down a gremlin or two.

[Reece]

Wonder if this is like the course leadlike was talking about?

http://antiqueradios.com/features/service.shtml

[leadlike]

It is more or less the same-a cheapo pamphlet with similar illustrations. There was also an NRI ad that showed that trick- Joe soandso was so well educated in the art of radio repair (thanks to NRI) that he can repair his friend's radios with just one finger!