Marshall Mg100dfx 100-Watt Combo Guitar Amplifier - Answered Questions & Fixed issues

I may be missing something but i cant see what amp you have? Is it a valve or solid state marshall?

Please, take it to a tech in your area, or look for a good one to ship it to. An 2007 amp is not old at all.

You'll find one here- Amp Archives Marshall Schematics Layouts MG Schematics

It was Manufactured between 1981 - 1982 So it's about 33-34 years old...

It must go to a repair shop. There's always a reason that a fuse blows.

Buzzing could be normal for this amp, especially if you have the gain cranked way up (or the volume way up) see if the buzz goes away when you turn the gain/volume down. If not, there is a problem that needs to be properly diagnosed by the amp tech at your local music store. The crackling is also unfortunately rather normal for an amp that's a little older. Oxidation builds up inside the knobs and makes that noise as you turn them. If it only happens when you change settings I wouldn't worry about it, but if it gets bad, a tech guy can clean it out for you.

This looks like an overhaeting problem at fiirst glance. This shouldn't happen however. Have you had it long before it started failing? Doe it happen every time you play it, and also at low volume? If so, all I can think of is having it checked out by a technician.

Well i know ive destroyed a guitar amp with a bass guitar before :-p this is why they make bass amps and guitar amps instead of just amps

I am very new here and don't know how to answer a question to a specific problem so I will write about the most common problems that I have had over the decades doing guitar amp repairs. If you find that your amp is extremely low in volume and then suddenly comes back to normal- check out the effects return [or power amp in] jack. Especially in Marshall amps-these corrode and your signal dies. If there is loud noise such as hissing and crackling with no guitar patch cord in the input jack- the input jack is likely at fault. They normally have switching contacts that ground part of the input circuit when the 1/4 inch male plug is disconnected. When this in is corroded or too weak to make normal contact, you can get these symptoms. If your amp is a solid state Marshall or Fender and there is a random buzzing sound with no guitar signal connected- hit the side of the amp and listen for the buzz to go away or become louder-if it does, this usually means that the connections are loose at the two large filter capacitors. In one or more Marshall amps, they use a filter circuit in the low voltage power supple for the preamp tubes. If you encounter a dead Marshall[the model escapes me now] remove the circuit board and resolder the four connections at the bottom of the bridge rectifier. If you have any other amplifier repair questions you can email me direct at [email protected].

Using a standard 1/4 inch stereo plug....Ground (sleeve) to tip for one switch and ground to ring for the other.

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For a test in troubleshooting, see if level from the headphone jack fades or not when this happens and get back to us.

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.

corrosion at the effects return jack. either jumper the jack to the effects send or spray it and work a plug in and out..or replace the jack

I would try hitting the side of the amp before turning it on. There may be a loose piece of solder or other conductive particle stuck somewhere. Bang the cabinet rather hard with your fist. If that doesn't help it might be a loose solder connection at a resistor or capacitor but a tech should check that out.

It sounds like a bad IC to me. Also, it could be a bad solder connection at that channel's volume pot or the pot itself.

I am guessing that an IC is at fault. You don't really mean something like "no sound when switching to channel two" do you? If that's the case it could be a defect in a pot such as the volume pot. I have worked on hundreds of Marshall amps over the years. You can email me at [email protected] anytime for help if you want.

Something has probably failed or become a bad joint. If you turn up the gains and volume to max is there any hiss or hum from the speaker? If not there will be a problem in the power amp or power supply. Check fuses at back. The speaker may be open circuit too, that is easy to check with a multimeter. If you can't get far yourself, you'll need a repairman.

Solid state amps are not that reliable these days, there are so many things that can fail due to all the effects that people demand to be built in. Look at how many people are reporting problems with ss amps here.

probolly all you did was run a head into another amp and just caused your marshall to overheat/overpower and blew your output transistor. i you drop the chassis out and look at the chip on the heatsink is it blown? also check all internal and external fuses.

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