On Friday, March 17, 2017 at 5:47:07 PM UTC-4, wrote:
On Friday, March 17, 2017 at 3:54:14 PM UTC-4, David Farber wrote:
wrote:
Isolate the problem first. Does the distortion change with a change
in volume? I would guess that there is a transistor in the front end
that is starting to fail because of heat. It doesn't take much.

You have an advantage here, one channel works as expected. Take some
voltage measurements and compare left to right channels. Use a scope
and see where the distortion starts.

IC431 (4580) might be a suspect. Follow the audio signal in both
channels.

Let us know how you make out.

Dan

Hi Dan,

I powered it on again and after about 15 minutes, both channels were
distorted. The signal going in and out of IC431 is fine. Adjusting the
volume does not affect the distortion. The top half of the wave is clipped.
I did notice that both dual op-amps, IC431 and IC503 are inline 4580's. I
swapped them to see if that made a difference but the distortion stayed the
same after warm up at IC503. Inverting input pins 2 and 6 were distorted
while noninverting pins 3 and 5 were not distorted.

Thanks for your reply.
--

David Farber
Los Osos, CA

Have you tried preheating the assy and see if the distortion appears more quickly or even immediately? A good percentage of timed issues are also thermal ones.

Actually, use freeze spray. spray one IC at a time and give it a few seconds to react. You should be able to isolate the offending IC that way. Works for transistors too.