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Remove all the phillips head screws. Then, where the power cord comes out,use a small thin screwdriver to pry that area outwards. Then pop the back off.
You may find almost anything in this. I think the power amp is on a separate board so the connections to that might have been broken. Possibility include cracks in the circuit board. First thing is to do after visual check is to check power supply voltages to verify power switch/fuse/transformer/rectifier/capacitor/regulator health. Switches are sometimes damaged... and also wires can fall of or break.
The problem is very likely a blown tube. You may be able to see which tube is dead by looking in the back of the amp with the room lights turned low and the amp on. If you can that it is one of the four 12AX7 tubes that isn't lighting up, that one should be replaced. If you can see that it is one of the KT66 tubes that isn't lighting up, buy a new matched pair and replace both at the same time.
* WARNING: UNPLUG YOUR AMP BEFORE CHANGING TUBES *
If all of the tubes are lighting up, then one or more may be bad, but not yet completely dead. Determining the problem would require more testing. If you have one spare, good 12AX7 tube around, try replacing them one at a time and see if that fixes the problem. TURN OFF AND UNPLUG YOUR AMP IN BETWEEN TUBE SWAPS. If a new preamp tube doesn't fix the problem, order a new set of KT66 power tubes and you should be back rocking out in no time.
First thing is to verify the MAIN power source is getting to the amp. It may not be the fault of the footswitch at all. Connectors and cables are the usual cause of these problems. Seating of connectors is a common problem...
I haven't played or worked on a MG100HFX but I understand amp circuits very well. The behavior you are describing sounds to me like a bad capacitor in the Treble circuit. This is a problem that would require the skills of an experienced amp tech. If you are within your warranty period you might want to return it. If not, a good local repair shop should be able to diagnose the problem.
Some of the rocker switches can be PRIED out from the outside as they have plastic "springs" at the ends... look at you new one.
IF you can pry the old one out you MAY have long enough wires to allow the old one to come out a bit and transfer to new one and push it back in.
ELSE you have to take the amp out.
USUALLY the speaker and the board it sits on remains in the cabinet.
There will be screws that hold the amp in... You kind of have to try to find which to remove to release the amp chassis. You may have to remove the speaker wires or wires to a reverb tank... MARK ALL these and restore to exact same places.
HINT: Once the screws are out, OFTEN the Tolex cabinet covering loves to catch on the chassis so you have to fight to get the chassis out... a real pain and THEN putting it back in the Tolex fights you again !!!
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