[DMcMahon]

Spent quite a bit of time trying to locate source of the hum (with heads still disconnected) without any success.

Photo shows the hum at max volume at the output of the TBA 820M audio amp IC (IC 601), around 600mV peak to peak. Input to amp was around 30mV, hum signal was basically the same on mains and batteries.

Mains ripple on the DC supply seems fairly high at 160mV

As far as I could, checked all ground connections at components and cables etc both my continuity (to battery -ve) and voltage measurements (to battery +ve) all checked out good. Once again carefully checked all soldered connections and tracks for breaks/cracks using strong magnifying glass but all good, I did resolder a few slightly suspect looking soldered joints.

Very sensitive to pickup from hand being close by, were the first 2 pre-amp transistors area (Q1501 & Q1502) particularly the electrolytic capacitors (CE1501, CE1502 & CE1504).

Will refit the PCB and reconnect the head wiring and recheck.