[Tubesforme]

I fully rebuilt a model 20 chassis. I played it outside of the cabinet for several hours over a period of several days. I only had a wire antenna of about 15' and the radio played great. I installed it inside of the cabinet and it played very well for over a week. then suddenly, one day the volume dropped way down to barely audible. I discovered that the first 24 RF tube was defective. This brought the volume back to normal. then I began to notice that the volume would drop lower on its own and sometimes when a light or an appliance would come on the volume would return to normal. These are all on separate electrical circuits. I attached wires to the major B+ points and was going to monitor them, with the chassis in the cabinet. However whenever I would touch the meter probe to any wire, the volume would come back to normal. Easy right? Bad filters or rectifier? So I rebuilt the filter capacitor can with all new components and replaced the rectifier. (All tubes have also been changed.) Still the volume will drop slightly lower and then on its own come back full again. I took the chassis out of the cabinet and attached the signal tracer probe to the input grid of the first AF tube and placed the chassis bottom side down to build up the heat. The radio played for four days, no problems and it also played through the tracer without a single problem. I reinstalled the radio in the cabinet and the problem returned. You can still hear the radio playing when the volume drops, but it's annoying, when it comes back up on its own. Any ideas or is this something that happens to a TRF receiver? The radio has been tried on a number of different stations with the same results. Sorry guys this is my first experience with a TRF set.

[KCMike]

  I can't give you detailed checks to do but have you tried poking all the connections and various parts with a non conductive stick? You might just have a bad solder joint or loose connection somewhere.

[Radioroslyn]

<Easy right?
Not so much else you would have found it.
Perhaps another way to troubleshoot may help. I'd use a piece of small gauge insulated solid wire and wrap it around the grid pin of the 27. reinstall the tube and use that to monitor the signal level while the set is in the cabinet.
As an aside I can tell you that the things I've found on model 20's that have been problematic are rf coils, audio driver transformer, and output transformer.
Something else to look for is bad connections. The 660 I'm working on has a bad connection at the ant fastenstock clip though rivet and wire that goes the ant coil. Same issues volume would go up and down at will. Also check connection to chassis ground

Happy hunting
Terry

[Steve Davis]

Check all the ground connections where lugs are riveted to the chassis. Also, check the screws on the tuning condenser that hold the stator assemblies in place. Loosen each screw, one at a time and retighten.

Steve 

[morzh]

One more thing to think of: if it is good outside the cabinet and bad inside, and it is gradual, the issue could be thermal.
Also, see if it persists at a lower voltage, say you lower it to 110V with a variac.
And yes, poking helps.

[TV MAN]

If your radio has a multi tap wire wound resistors, sub them. Had the same thing on my 70 which gave me fits. That is what I finally traced the problem to, and it checked fine with my meter. It looked rough instead of shinny and smooth.