Any amp geniuses here willing to give advice?

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Rick

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I got hold of a Technics SU-8088K that I'm trying to repair.

I really like it and would love to get it working but its becoming a bigger challenge than I could imagine. I have repaired many amps and generally have a decent understanding but the circuitry in this thing is confusing me me they are using things like 2 diodes in series in single packages all over the place as well. Also, made in 1979 most of the transistors used are now obsolete, I have had to substitute best I can spending many hours immersed in data sheets.

I have the preamp section working perfectly now, its just the power amp, biasing seems to be an issue, after shorting out and wasting 8 X MJ15003/4 I have learned one lesson at least to power up though a 100W light bulb.

Below is one channel, bias is way too high, all damaged components have been replaced I switch on and it goes almost straight into thermal runaway, could be oscillation but I cant see anything on the scope.

I'm looking at how to modify and lower the bias.


 
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