chasing an amp fault in this solid state Marshall

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Yes c10. Typo.


Further testing measuring pin5 of ic 4 and get clean and boost channels without switching. So I can adjust level changes by adjusting gain on clean and boost.

?????? That is totally wrong. Switching should allow only ONE pathway through ONE set of volume controls and tone stacks at any given time. Switch to clean, boost controls have no effect and vice versa.
Pin7 of ic 4 yields -35(od1) and -18o(od2) Dani respectively,

This is consistent with previous measurements and the level drop at R22.🤔
 
5201 should only allow one of it's opamps to pass signal at any time. by your measurements prob does seem to be in the vicinity of ic4. ic4A has unity gain. ic4B has gain of 2.4. Check to see that you're getting such gains. Also look at switching pin1...does it swing all the way from On voltage to OFF voltage? A partial swing could be the problem. If swing is correct then switch inside ic4 is likely broken.
 
As far as I remember - you will NOT see Pin1 change very much voltage vise when you change between the two halves (that has caught me a few times).
 
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