I tested Q4 as a FET switch. Drain and source were always open, even after charging the gate. But I would imagine it was partially working in circuit as you stated.
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Quote from: g1 on June 08, 2023, 06:45:42 PMMaybe it is Q4 that is responsible for Q5 source drop-out.
Looks like a limiter circuit consisting of Q4,Q5, U8 and U12.
Also, U8 and U12 are the only things running off 15VX (via R82), so maybe one of those IC's could also be responsible for the +15V rail issue. Do either feel hot?
(edit: that would be the noise gate circuit that tassieviking mentioned, not a limiter)
Quote from: g1 on June 09, 2023, 10:24:56 PMIs there much difference in voltage across R45 when ch.2 is selected compared to ch.1?
What is the voltage across R86 (to tuner) ?
Quote from: g1 on June 08, 2023, 12:31:58 PMDid you verify the resistance of R42 and check it's solder joints?