Let's talk about oscillation

[electra225]

Oscillation, at least unwanted oscillation, is the worst fault to troubleshoot for me. I'm sure I'm not alone. Squealing, putt-putting, motorboating, whatever you want to call it. To me, Zenith radios immediately pre-WW2 are the king of the squealers. Zenith used the output filter cap for cathode bypass duty rather than use a separate capacitor. Zenith radios get infested with the "green crud", corrosion that invades coils and controls, even the tube sockets. When the insulation falls off the rubber wiring, that upsets lead dress and that can make the stupid things squeal like a pig.

I have a Zenith radio that I finally traced the oscillation to poor alignment. Oscillation can occur in almost any stage of the radio, and can be caused by almost any component in the radio. Something is backfeeding. The problem is figuring out what and where. I usually start by splitting the circuit then figuring out where the squeal ISN'T. But many times I still chase my tail.

We have a lot of talent on this forum. I know you guys have all run into unwanted oscillation at times. How did you troubleshoot it? I can generally isolate the section that is squealing, then I can isolate the stage that is oscillating. Where I run into trouble is finding out where the oscillation is starting, what component is causing it. Then, when I am dealing with oscillators that won't start or won't run, that can present a challenge for me. Oscillator coils are something I struggle with, particularly something like the ones Zenith used on their multi-band chassis, where there are maybe a dozen connections on the coil. Then following the connections on the function switch on these chassis is another challenge. There has to be a simple way. My stepdad could just take his bone rod and "give it a right smart rap" and the squealing would stop. It was so easy for him, he couldn't fully explain what he did to me. Perhaps there is a certain sound to the oscillation that would give a clue as to where it starts. Or maybe the stage that is oscillating is a clue to the origin. Let's hear your suggestions and your challenges when you deal with oscillation.

[TC Chris]

I let this one fester for a while. I have few test instruments (no 'scope) so my approach is to poke around some, and then maybe disable a section that seems to be the prime suspect. Is it in AF or RF? Then I experiment. I have a Philco 46-350, the old AC-DC portable with the little tambour door over the dial. It was oscillating. Quite by chance, I discovered that shielding one of the tubes can't recall which) made it stop. So I installed a tube shield connected to ground. Now I wonder if the tube had a bad connection in one of the pins, or if another component was out of spec.

The online info about the cool GE transistor portable P-780 (I have A and D examples) suggests that lead dress is very important, so take photo before disassembling. That might also be good advice generally, in case location might cause feedback.

Chris Campbell

[electra225]

The "quite by accident" part is what usually fixes oscillations for me. I generally can isolate the oscillating stage, but I have trouble narrowing down the cause. Your idea of shielding is a good one, hadn't thought of that. Thanks, Chris.