feedback noise
 

What does it mean when a tube radio makes a squealing noise similar to microphone feedback when the mic gets too close to the speaker? My Zeinith table top radio does this once in a while. It goes away either on its own, or if I monkey with the knobs a little. Sometimes, taking it out of the AFC mode does the trick.

I'm not an expert troubleshooter, by any means, but I do know that those kinds of intermittent problems are the most difficult to isolate.

You didn't say much about the radio itself. Is it still running on all of its original components? My experience has been that any radio more than several decades old is living on borrowed time, as far as its capacitors are concerned. Failed or failing bypass caps in the RF and IF stages are notorious for causing oscillation problems. If this radio still has any of its original paper or electrolytic caps, you would be time and money ahead to replace those before going any further. This can either solve or prevent a whole slew of problems.

Also, check for loose or ungrounded RF or tube shields, broken or intermittant ground connections, or wiring that may have been improperly dressed during previous servicing. Less likely would be a tube with an open supressor grid. This won't always show up on a tester, so it's best to substitute with another tube.

Just a few ideas for you. Someone else may have more direct experience with this particular problem.

Good luck!

Thanks for the ideas. I know nothing about these things, and have not removed the back, so I really can't answer any of those questions. Even if I removed the back I doubt I would know what I was looking at. However, it is intermittent. For example tonight, I played it for 4 hours or so and it played beautifully, like new. Almost like it knew if it didn't shape up I might start messing around in there. NObody wants that!:rolleyes:

Oh, is this that Zenith you just got on Ebay? I would say that if the problem is that infrequent, and doesn't bother you so much, just let it go. Wholesale recapping of radios just because of their age is a controversial practice, though it's usually the first thing I consider if the radio shows signs of more serious malfunction. Just keep threatening to tear into 'er, and she should stay pretty well in line. ;)

Paula

Perhaps a microphonic tube in the audio stages? Try swapping them out. Replacments should be easy and cheap to come by.

Yep, it's that Zenith. I'm starting to suspect gremlins. My Marantz 4300 refuses to play this morning. Never happened before.

I agree with Paula and Markw - this is a good place to start. I would like to add that it is not healthy for the output tube when oscillation mode kicks in.

It's not healthy for anybody! If it does it more, I'll take the back off and electrocute myself. Seriously, the radio is not valuable, so I'll see what I can see. Don't you people sleep?

Improper lead dress can cause feedback problems: one time I was having feedback when the volume was turned up and it was caused by a coupling capacitor into the grid of the 12AV6 lying too near a plate lead...re-orienting the parts cured the feedback.