Jim,
I reckon there are two important steps in sorting a problem of this sort.
1. Get some sort of spectrum analyser, or even a scanner, and find out what
frequency the oscillation goes off at.
2. Try to put a scalar analyser through the amp, firstly when its cold, and
look for frequencies where there's a peak in response. You will probably
find one around the frequency you measure in 1. You can then look for
changes in circuit or layout that kill the peak. Once the peak is gone, you
can repeat the test with the amp active.
Ok, not everyone has a swept system, but you can do it with a very basic
spectrum analyser and signal generator. You might have to spend a little
time and beer money finding one to borrow, but it's a quicker and better
route in the end.
Steve
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