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Marshall mg30dfx Repair - Gain Not Working

Started by agwood75, October 09, 2015, 03:33:14 PM

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agwood75

I found an MG30dfx at a garage sale for $20. 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?

DrGonz78

Measure for DC voltage pins 1 and 2 of that IC2 chip. Basically the green and red led's are there as clipping diodes to create the distortion effect. What is supposed to happen is that when you turn up the volume of the gain pot it allows guitar signal to be clipped. The lights will not light up all the way unless you apply a very strong signal and as the signal decays the lights trickle off again. So if the Green LED is lighting up that is a clue there is some voltage turning it on and it is not guitar signal. Check the opamp pins.

The schematic is already uploaded on here...
http://www.ssguitar.com/index.php?topic=3414.msg25317#msg25317
"A person who never made a mistake never tried anything new." -Albert Einstein

Hackinblack

with these amps the clipping LED's light up when the overdrive is past 12 o'clock;in fact if you,look closely at the front panel,where the gap around the 'drive' switch is, you can see the red one slowly glowing inside as the gain or (input signal) rises.
nice amps these;both channels sound good...well worth repairing.
if it turns out the op-amp is poached cut it's legs off and pull the legs out from the TOPSIDE of the PCB,one at a time,while heating the solder joint with your iron;these PCB's on these Indian-made amps are not as tough as the fibreglass ones in more up-range marshalls,it's easy to pull traces off the board.