Just an update on progress or lack thereof.
Lifted R48 and hum remained.
Then lifted R50 (off pin 3 of V2) and hum stopped. I reconnected R50 and hum came back. So thought I’d narrowed it down to something off pin 3 V2.
Then carefully traced out the path off V2 to identify the components: R50, R51, R52, R53, R54 and R55, C28, C29 and FETs Q8 and Q9. All resistors and caps tested ok. No shorts or open lines and resistors with spec. Not an expert at testing FETs - is there any way to test in circuit? If it’s not the FETs, then stumped as all components off pin 3 of V2 seem ok.
As an aside - when I was doing some continuity testing to ground I discovered the middle pin of channel C “High” control had intermittent ground continuity ie the test would beep rhythmically (rather than a solid tone). This terminal also connects to the one of the outer pins of the channel C and B gain controls and these show the same behaviour and on the continuity test (not the ground pin side of gain control. Unsure if this is normal or could be related. I note the “high” pot is connected to pin 3 of V2 via the “pull bright” switch on that pot. Probably just incidental.
Enough for today. Will try some more tomorrow