Marshall MG100HDFX no Clean but Crunch is ok.

[Memphis]

I have a MG100HDFX on the bench. The clean channel is not working but when the crunch button is pushed in it appears to work ok. Same with the OD channel nothing on OD1 but OD2 seems to work ok. I plugged the guitar into the CD in jack and it plays clean and loud. jumping the effects lop makes no difference. Any direction is appreciated.

[52 Bill]

IC2 controls clean/overdrive switching as well as od gain, so I guess that would be a good place to start. Check the control voltages going to the chip to see that they are working. If they are, then the IC is suspect.

[Memphis]

Ok so i don't see IC2 on the schematic or the PCB. Any thoughts ?

[Enzo]

Top center, see OVERDRIVE GAIN control VR5? Rught directly above the control is a section of IC2, the DG212 IC. Another two sections of it are in or near the upper right. More towards the bottom left, next to the EFFECTS LEVEL control is another section. Lower right on power supply page shows its power connections, note that its logic runs on +5, but it also has +/-15v for the analog part.


I have no idea where it might be on the board, but it is a 16-pin DIP, and I doubt there are many of those.


I don't see how your symptom could occur. The crunch is just the clean channel with a little extra cap switched in and some clipper diodes in another stage. FOr your problem, the C110/R89 branch must be open.

And the only difference between OD1 and OD2 is swirched in extra cap. For that to make no sound in OD1, but OK in OD2, a number of things would have to be open.

IC2 is a multisection analog switch. Two sections on the right alternate - when one is on the other is off - to select the clean or the OD channel. WHen clean is on the OD is cut off. And for noise reasons, when the clean is on, and extra section of IC2 shuts down the gain stage of the OD chanel.