Carlsbro hornet 45

Good afternoon
I am working on carlsbro 45 hornet : it generates self oscillating at ita output, that has caused speaker distruction. The phenomena is present, typically for some combination of gain/tone position ( generally gain and treeble at maximum)
Anybody has faced this problem?
I was not able to find schematics: it uses rc4136 opamp ( 3).
Thank You all for any suggestion
Saverio
 

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