Here’s a simple problem that I came across on a Marshall JCM2000 DSL amplifier: The Reverb works fine but when plugging in a footswitch to the Reverb jack, you cannot turn it on or off. It could also be the other way around (won’t turn on, stuck “off”).
The reverb’s switching circuitry is actually pretty simple. It is controlled by TR1 transistor on the reverb board that changes state depending on whether the footswitch is depressed or not. You can check whether its working or not by checking for current flow on pin 1 of IC2 (M5201). When R14 has current going through it, reverb channel B of the M5201 is active, and when no current is present, channel A is active.
The schematic for the reverb board can be found here:
The M5201 datasheet
The problem on this amp was in fact TR1, an MPSA13 darlington transistor that is easily available from mouser.com.
Here’s the board with the new TR1 installed:
With everything connected back up, the reverb footswitch control now works again and all is well 🙂