New guy here, scratching my head with radio problem.

Hello there,
I've been messing around with radios off and on for about 10 years. I'll admit that I'm definitely more of a amateur, thus not as well informed as most of you all are. Anyhow, I have this mysterious problem with a radio I've been restoring. Its a Hoffman set from the 40's with typical all american 5 layout. The unit uses a transformer for primary power. When I brought the set home, all of the tubes lit but there was no audible sound. Only a very quiet hum, which sounded more like feedback from the transformer.

Since then I have replaced all capacitors and tested all the tubes. All tubes tested very good. Once done re-capping, I tested the set out. Again there is no sound. Everything lights up. All wiring appears intact. All connections to the speaker and speaker transformer appear intact as well. When I mean quite, I mean nothing. No static or anything. So I am scratching my head on this one and hoping some of you might have an idea of some things I could try to remedy the problem. My only guess- and this is a big guess, is that perhaps the volume control is faulty.

Thanks in advance!

Welcome Bob. AA5 and power transformer is contradicting. AA5's are series strung filaments using no power transformer. That aside, you might touch the center tab on the volume control and see if you get a loud noise or growl from the speaker. If not, you need to check the components in the audio amplifier stage. Speaker, Output transformer windings, resistors in that stage. If you do get the loud noise out, audio amp. stage is working. Do you have a model # so we can look up a schematic and see the circuit?

This may sound too easy, but are you sure the tubes are in the correct sockets? I've seen it before, where someone had switched a tube around.

Gene

Apologies, its actually a 6 tube unit. The model is a A300 Hoffman. Now, when I touch the center tab of the Vol. control, I assume I would also be touching the chassis? Thanks in advance.

Yes, I actually did check that as well. They are all correct.

I'm also going to check the resistors in the circuit as there was one that appeared to be slightly damaged- burned.

Here's a pic of the underside. I also realized that I missed one of the capacitors. Its a .001, 600 volt dry capacitor in a metal wrapping. It connects to several pins of the 6X5 tube as well as on of the transformer leads. I wonder if this is the culprit.

You did a nice job though... looks a lot better than the RCA I just did.

Check and make sure its not the silver/mica caps bad in the IFs! Dont know if it has them or not.Sprman : <

Well, just got back from soldering in the last cap. Still nothing. There is a very slight hum that is definitely coming from the speaker. The switch is also the tone control. Vol is separate. I think the next step might be to see if the schematic pasted to the bottom mentions the speaker transformer power output. I'm sort of guessing at this point.

As far as compliments on the job, well it was a heck of a mess soldering some of those in there. It was a real rat's nest. The new ones sure take up a lot less room so it looks nice and tidy!

Me being an amateur and all, when you mention mica caps in IF's, what do you mean by IF? Sorry for my ignorance.

With the volume control turned up, you should get a hum from the speaker when you touch the grid of the 6SQ7, or the center terminal of the volume control. Have you checked any DC voltages on the tubes?

Schematic is here:
http://www.nostalgiaair.org/PagesByModel/181/M0009181.pdf

Tim KA3JRT

Haven't checked the DC of the tubes yet. Again, professing to my ignorance, when touching the center terminal of the volume knob, what is the procedure? Sorry for the dumb question.

Marcus and Levy's book on servicing radios suggests touching it with tip of soldering iron. Most just touch it with a finger.