Marshall mg30dfx Repair - Gain Not Working
I have always wanted a Marshall amp and I found an MG30dfx at a garage sale for $20, so I thought I would try one out. The only caveat, according to the previous owner, was the speaker would cut in and out. I am pretty handy with electronics, though I have not worked on amps before, but I thought for $20 why not give repairing it a try. I got it home and opened it up and found that the solder connection for the speaker wire had broke loose. I soldered this back up and the speaker no longer cut out.
I started messing with the knobs and found that all of the inputs, push buttons and knobs work except for the gain. The gain knob felt kind of loose and the pot turned without much resistance, so I decided to replace the pot. After replacing the pot I still have the same issue. What it will do is when you move it from 0 to 1 it will make a sound out of the speaker, kind of like when your first turn on the amp, but not quite as harsh. After this first sound there is nothing, it just sounds like it is still on the clean channel but not as loud. I thought that maybe the channel selector was not working, but the light comes on for the overdrive channel and all of the equalizer knobs work in overdrive so I believe that it is sending me to the overdrive channel.
I pulled the wiring schematic online and noticed a red and green light in series with the gain pot, so I opened this up and looked at these when I turned the knob. I found that at 0 and in the overdrive channel neither of the lights are on, but when I go from 0 to 1 the green light will come on and I will get the pop sound I mentioned earlier out of the speaker. I would assume that since this light comes on I am getting some kind of response out of the gain pot, but I still do not get the overdrive sound out of the speaker. I looked at the circuit board and could not find any evidence of anything getting burnt up, or any loose solder connections so I am at a loss with this.
I know this is a low end Marshall amp, but I really like the sound that comes out of it on the clean channel and I do not think I could find anything better for $20. Also, I would like to learn a little more about repairing amps in general so this will be a good tool to do this without destroying one of my more expensive amps.
Does anyone have any suggestions on where to go from here?