Marshall MHZ40 footswitch problem

[QJerry]

Hi

internal switch is ok, footswitch is working but not changing channel and effects (LED are ok & working) - any ideas??

[pecorporation]

Are you using a 3 conductor footswitch cable? Is the cable good, all three contacts have continuity, end to end?
Next suspect would be failing solder on footswitch jack in amp.

[QJerry]

footswitch is original & fully working,input jack is ok - problem is internal (zeners are ok, voltage is changing) - main IC is switching with panel switch...

[Enzo]

Look at the footswitch jack. The tip and ring contacts connect through resistors to pins 2 and 4 of IC2. Seriously, is IC2 really called an ATMEGA8? But the sleeve contact has a cutout contact that is in use. The sleeve contact goes to Digital ground through pin 12 of the connector as expected. But the cutout contact also feeds through pin 8 of the connector and winds up talking to IC2 pin 6. Is that working at pin 6? An open R111 would leave pin 6 unable to pull itself up when that jack contact opens. And that would leave the system unable to recognize the footswitch as present.