Solid State Guitar Amp Forum | DIY Guitar Amplifiers

I have 2 of these amp heads which both don't have working FXs. The 5 volt regulator is working fine and the FX board has 5V at the connector (no schematic for the FX board available?). I connected a footswitch but I can only change channels but not enable FX....

I was going to trouble shoot the footswitch (FS_c) connection components and the function of IC2 (DG212 MOS switch) but the reverb is not working as well and this signal originates within the FX board and is not switched by IC2. So it looks like the issue is the FX board and not the control circuitry.

Do I have 2 dead FX boards - really? Is there way to trouble shoot the FX board - reload the config bit stream for the DSP??

edit - now that I look at the schematic again - IC3 may also be the issue. I will look at the signal or lack of it at + terminal of C8/7.

I'm quite certain that even Marshall themselves don't even build or troubleshoot the DFX themselves, they buy it as a working OEM supplied module.

Factory repair means full board replacement (which to them must cost 10 bucks or less).

So I found the issue with these amps - if you bring up the mains voltage with a VARIAC the DSP appears to either not get the configuration from memory or these is an issue with supply sequence. Plug both amps into 120 VAC and the FX works fine. Odd.

Not all that surprising with a Single Board Computer, which is what the Fx module is.  The supply(s) aren't within spec when the power up reset times out.  You could try bringing it up slowly, then manually triggering the power up reset, and I expect it will fire up okay.