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38-2 Audio Yes Signal No - Printable Version

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38-2 Audio Yes Signal No - dfields69 - 01-07-2020

I am working on a 1938 38-2. The recap and resistors are done. The tubes are all good. I have audio. Filament voltage to all tubes reading 6.5-6.6 volts. However, I have not taken the RF unit bottom shield back off to check there. I think it may be an oscillation problem. I wanted to ask the more learned folks here if you all know of any common problems I may be able to look for as well to add to my trouble shooting list? Thanks!

RE: 38-2 Audio Yes Signal No - radiodale - 01-07-2020

Check to see if the oscillator is working. Put a scope on the grid or put another radio close and listen for the tone at 470kHz above your tuned frequency.

RE: 38-2 Audio Yes Signal No - dfields69 - 01-08-2020

Thanks!

RE: 38-2 Audio Yes Signal No - morzh - 01-08-2020

Check all the RF coils for continuity.

RE: 38-2 Audio Yes Signal No - RodB - 01-08-2020

If you can feed a modulatted 470 khz IF signal in to the antenna and hear the aurio from the speaker then the local oscillator has a problem.

Rod

RE: 38-2 Audio Yes Signal No - dfields69 - 01-08-2020

I am taking it over to a friend's tomorrow to troubleshoot it and we will do that. Many thanks.

RE: 38-2 Audio Yes Signal No - radiodale - 01-09-2020

We'll be awaiting the results.

RE: 38-2 Audio Yes Signal No - Nathan Slingerland - 01-10-2020

Watching this. Lurking

RE: 38-2 Audio Yes Signal No - dfields69 - 01-10-2020

So here's the result. Craziness! LOL. we used a signal tracer and found that the radio was not getting signal to the IF section. All coils were good. We found tons of loose tube socket pin sockets and as were the case with these quite a few were brittle and cracked off partially. We were able to fashion good connections with the remaining material. Right now, my friend has it. He got the IF to work but says it is still not producing signal (no stations through the speaker)? He's going to work on it again tomorrow. The center tap of the volume control now provides feedback when touched. Any ideas? We checked resistors, caps, coils, tubes (checked to make sure the 6J5 G's were not microphonic) found two block caps I accidentally shorted by leaning cap wires together and did not notice. The set is a run no. 3 and has the tuned permeability IF's?  I am, as well, looking for a dial surround for it that has the original paint on it. Someone stripped it down to the brass back in the 70's or earlier due to some Bicentennial color scheme tabs inserted in the pre-set ring. Any ideas what might be killing the signal?

RE: 38-2 Audio Yes Signal No - radiodale - 01-11-2020

Did you check the oscillator?

RE: 38-2 Audio Yes Signal No - dfields69 - 01-11-2020

Yes it is working. I'm heading over there now. We're tracing wires at this point. They made a change on the 38-2 to add the new IF's on the run that I have, run 3. It oscillates, The IF is working, the RF is working, the tuning condenser i snot shorting at any point on any band. The FR and IF just aren't working in tandem and pushing signal through to the audio section is my guess at this point?

RE: 38-2 Audio Yes Signal No - dfields69 - 01-14-2020

The radio is now working and electronically finished. One of the coils in the RF section was hooked up wrong. As well, I found out you cannot sub a 6Q7 for the 6R7 on this set. It plays poorly with a washed out, poor sound quality. The tube sockets on this radio were very fragile. Thanks for all of your help!