In article ,
Cursitor Doom wrote:

On Sat, 30 Aug 2014 18:44:50 +0200, Cursitor Doom
wrote:

Correction! It's the scope loading on the output that's making the
amplifier unstable. Apologies...

Such is life. My amplifiers oscillate but my oscillators don't.

You can measure your generator's output impedance with a variable
resistor. Measure open circuit voltage. Then connect the variable
resistor and adjust for one half the voltage. Measure the resistor,
which should be the same as your output impedance.

Your distortion might be related to the instability. An oscilloscope
should have negligible effect on the extremely low output impedance of
an audio power amplifier. The amplifier might be rated for an 8 Ohm
speaker, but must have much lower output impedance to keep Voltage
constant for a varying load impedance over the audio frequency range.

The first things I would check are the electrolytic capacitors.
Assuming it is an older unit, several of the capacitors might have
developed high equivalent series resistance (ESR). That includes power
supply filters, audio bypass capacitors as well as a capacitor in the
output stage that blocks DC from getting to your speakers.

An ESR meter is handy to have if you do much repair work. I have the
AnaTek Blue ESR meter, built from a kit.

Fred