SA660, 1969, High frequency oscillation

[diverse]

Blows out the Zobel shunt, 10 ohm resistors at both channel outputs.
Is there a known fix for this intermittent oscillation problem.
Circuit breakers act and reset
The CR pair in the overall feedback of the power amp has a time constand of 0.5 microseconds ? - alter that plus adding hf caps on all DC rails ?

[Earl K]

Hi Diverse,

Welcome !

FYI; this forum focuses on ( almost exclusively ) discussion of speaker systems & transducers. I'm not aware of any regular posters that will walk you through a thorough "online", test & repair procedure.

- On the other hand ; if you were to register at diyaudio (http://www.diyaudio.com/) and ask your question there / you may get a more detailed response .
- The diyaudio forum has some extremely talented electronic tech types that might deem your "case" to be worth getting into , up to their elbows .
- See if you can get "Anatech" interested in your repair needs .



:)

[GordonW]

What parts have been changed out in the amp?

If the capacitors in the signal path have not been changed, it may be time. Those, especially any compensation caps, can be problematic... if they lose effective value (which can happen as they dry out), then the amp may lose its stability, due to changes in the gain/phase relationships (ie, too much gain at very high frequency, where the amp goes into 180 degree phase shift in the feedback loop)...

Also, if any transistors have been changed... make SURE that higher-gain versions have NOT been installed. Too much gain is the enemy of stability, in many cases. Unless you're willing to change ALL the transistors for higher-bandwidth devices and re-calibrate the amp for them (to get the proper phase/gain relationships for stability), then you need to make SURE you don't change ANYTHING that affects the open-loop gain or the amount of feedback...

There may be other problems, but that's what leaps to mind as the most likely causes...

Regards,
Gordon.