Check the power supplies, look with an oscilloscope for AC component content. Also check the supply of the electron tube circuit, if you have a spare electron tube, replace it for a test. Check the soldering of the potentiometers and jack sockets, there may be a ground fault.
The error can also be caused by old capacitors with a loss of capacity.
See the schematic diagram:
Hello Dave!
Check the power supplies, look with an oscilloscope for AC component content. Also check the supply of the electron tube circuit, if you have a spare electron tube, replace it for a test. Check the soldering of the potentiometers and jack sockets, there may be a ground fault.
The error can also be caused by old capacitors with a loss of capacity.
See the schematic diagram:
https://elektrotanya.com/marshall_avt150_avt275.pdf/download.html
https://elektrotanya.com/marshall_avt50_avt100_avt150_avt150h_avt275_mod...
Good luck with the repair.
Regards: Tony