Replace the switch. You gave no make/model number for your amp but if it has a jack for foot pedal operation of clean/dist function then try that. (on most amps) it will switch in spite of a bad front panel switch.
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Need more info. What kind of foot switch and what are you hooking it up to. For guitar they are used for switching distortion to clean on 1 switch, one for reverb, one for chorus, and possibly 1 for bypass. All depending on the amp you use, and what effect the amp has built in it. Some amps only have clean and destortion which make the other channels on the footswitch useless. Hope this helps.
I have worked on a number of these amps and found the most common cause of intermittent channel switching is dry joints on the foot switch jack or the contacts becoming contaminated. In either case you will require a tech to look at your problem as the main board will require being lifted out of the metal chassis for inspection. Because of the ribbon type cables between main board and the board that supports all the valves I recommend professional help . Not a DIY job
As far as I can tell from this; you could have either one of two problems. 1. Your guitars output or amps input could be loose. This happened to me with my bass. All you have to do to fix that is get into the electronics (usually accesed behind plates with screws), find where the cord goes and pinch the two clips together to make the fit tighter 2. There could be a problem with your power; which could possibly originate from the socket itself, a loose attachement of the power cable to the amp, or a loose cable inside the amp. Id reccomend plugging some other electronics into the wall and seeing if their power dims, if they dont then plug in and unplug the part of the cord that attaches to your amp and if that doesnt work you'll need to take i in to an expert to look at the internals of it.
This is because your clean channel only runs through the pre-amp section, requiring an increased volume level to match that of the volume of the drive section which is overdriven by the pre-amp circuit. my suggestion is to use lower volume level on the clean channel of the amp. To get any volume boost you may need to match the two channels you could use a signal/volume booster or overdrive pedal between your guitar and amp input, switching it off and back on when you switch back and forth between channels.
DO NOT USE a guitar pedal in effects loop as it has the WRONG signal levels.
Also, when you boost the gain preceeding a distortion setting in an amp, the amp will just limit out and it will appear that the effects in the amp aren't working... but what is happening is you wind up with too much input signal and the "flat topping" distortion occurs right at the inout of the amp so no further can occur when you select a distortion in the amp.
The switching is done electronically by an analog gate chip. On IC9, check if the signal on pin 8 changes with switching between clean and OD... This is the actual signal that changes the gate. If you don't see a change there of say 5 volts, then the problem is in the circuits that drive this gate. If the signal is there, then the problem is in the clean channel audio op amps or the components around them. Let me know the results of the test.
initially you want to think bad solder joint, but it think its a bad mosfet or rectifier or diode. it would be hard for an untrained person to identify your problem let alone an experienced tech. it may be time to move on to another amp. in my shop it would be at least 65 bucks for the repair. so where did the "heat"/energy go from that point and what parts were affected. there may be a domino affect. burn one part, not usually. yostamplifier.com
Here is the schematic:http://www.ampix.org/albums/userpics/10003/MArshall_MG100DFX.pdf There are a few possibilities.The jack for the footswitch for channel changing JS1 might be faulty. Zener diodes 1 and/or 2 might be faulty. Switch 2 may be defective. TR1 might be defective. IC 4 or 5 might be defective.Start with the channel switching circuitry testing in the order I gave you. Check the actual clean channel circuitry to see if you actually have a clean channel anymore. Before you do any of this make sure you know enough about troubleshooting and have at least a good VOM and are familiar with safety procedures. The filter capacitors in this amp store enough charge to kill you. A cheat to partially discharge them without damaging the amp is to unplug it from the wall while it is hooked up to the speaker cab you are using while it is on at a very low volume with a guitar cord plugged in. Make sure you use exact replacement parts if you tackle this. They don't have to be from Marshall, they just have to be exact electronic equivalents from a quality manufacturer installed correctly. I would recommend that if you have no experience or are at all unsure of the repair to take it to a Marshall dealer to have it done. Please rate me if this helps.
There are 2 channels, Which means there are 2 sets of knobs ... One for clean and one for crunch. Start out with a low master volume and set up the crunch, Then the clen and switch between the 2 and make adjustments until they match volume wise ... Then turn the master up.