Handy Troubleshooting Stuff

Sometimes when you're working on an amp you get a sudden volume drop as you advance the volume past a certain point. It could be oscillation above the auditory range, but how do you sort this out?

The answer is the garden variety AM transistor radio, an example of which is depicted here. Oscillation splatters frequencies all over the place, including the AM range of 535 khz to 1700 khz more or less, and the amplitude modulation radio system is more sensitive to interference.

Simply tune the radio to a place between 600 and 700 khz where there are no stations broadcasting, turn the volume up, and then wave your magic wand over the chassis you have advanced in volume until it oscillates. You'll hear a distinct hiss, and this can also be used to track down other sources of RF noise in the house like noisy computer power supplies. Some of the boys and girls who make cheap power supplies probably figure that nobody listens to am radio anymore, which is an attitude common among people who think the world is their anime oyster and nothing else matters but their own navels and their own disorder.

After all, if you don't like truth make up your own, right? You'd be surprised at how many of these folks think that Germany won world war 2. But I digress.

You've got to be careful here because oscillation imposes crazy loads on the circuit and you can find yourself in red plate country. If you're doing this it's wise to have some method of measuring bias current connected because if it spikes that's also a clue that you're into oscillation above the audible range.

Thanks to irishsammy, where ever he may be for the image that follows.

Here's a red plated tube in case you were wondering.