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Back to the Drawing Board - PAradiogeek - 05-07-2014

I thought I was done with the restoration of my Model 19, but now that I've been listening to it for a couple of days, I have a minor problemIcon_mad

The problem is a combination of tone and volume dependent. The tone switch has four positions. When the tone control is turned to the far right (with the most bass), it takes almost no volume to hear a kind of a "rattle" or a feedback in the audio. With the tone turned one setting to the left of that, the sound is much better,. You can listen to the set at almost normal volume, but if there is much bass in the sound you start getting the same effect. The two leftmost tone positions with the most treble seem to not induce (or mask) this issue.

Other findings:
1) There is a small tear in the speaker that I never noticed when I had everything out of the cabinet. The tear is kind of "V" shaped, less than .5 inches on a side. I'm not sure if I tore it when I was putting it back, but I don't think so. I will repair the speaker in any case.

2) If I place my finger on the top grid cap of the 36 oscillator tube (with the tone control rotated all the way to the right), the sound cleans up considerably. This tube is missing it's shield. I am in the process of trying to locate one for this tube, but no luck so far, maybe I'll hit paydirt in Kutztown on FridayIcon_smile.

3) I tried the chassis/speaker out before I put it all back in the cabinet. I didn't notice this, but it is quite possible that I simply missed it if I had the tone control set to one of the two leftmost positions.

Is this microphonics?

Could the speaker tear cause this? I kind of don't think so because the rattle goes away with less bass, and also goes away when I touch the osc grid.

Would a shield on the 36 tube help this?

Any suggestions on what to look for?

Thanks!
Jon

RE: Back to the Drawing Board - skyscraper - 05-07-2014

1. Fix the speaker
2. Reflow the solder on the cap of the 75 tube. Mine had a 'thunder' problem, this fixed it.
3. Recap the tone control if you haven't already.

Not sure about microphonics. Try to find a shield for the 36, but you could try kludging something with aluminum foil just to quickly see if that is the problem.

RE: Back to the Drawing Board - codefox1 - 05-08-2014

Agree with all. Quickly patch in a good speaker for differential diagnosis if rattle continues after you fix the speaker (most now use fabric glue to fix a small tear.) A very polite tap with a chopstick will reveal (audio) microphonics in tubes and components, and clean controls and switches with Deoxit, or whatever, if you haven't already.

RE: Back to the Drawing Board - PAradiogeek - 05-08-2014

Skyscraper, Codefox,

Thanks for the replies! I will definitely try resoldering the 75 cap. I will also try light tapping on the tubes and see if that induces the noise. I already recapped and cleaned the tone control and other switches when I had it on the bench the first time.

Something is going on with the radio. Performance is degrading noticeably since yesterday. I get no stations at all below about 850 or 900 on AM. Shortwave still seems to work though that is harder to tell because there isn't much on that band. I think something is going on (failing) with the oscillator. I will take it all apart (cry) and check things out over the weekend. I'm heading to K'town tomorrow.

Thanks again guys!
Jon