Marshall combo amp audio failures - Marshall Mg50dfx 50-Watt Combo Guitar Amplifier

Hi, I have repaired loads of these kinds of amps. The first thing to check is if the input jack socket is still soldered to the circuit board properly. These have a tendency to break.
They all have a dual opamp and a 15 watt power amp chip. Invariably it is the power amp chip that fails IF it is an electronic fault. Both chips can be bought for a few pounds.
The variations, aux input, spring reverb and this, digital effects are extremely unlikely to cause this kind of failure.
At one time anyone with a screwdriver and a soldering iron could fix these, but in the days of health and safety, find someone who knows what they are doing.
It should take half an hour and about £3 worth of bits.
All the best, Chris the Fader.

Guitar amps have preamp sections (think unpowered audio mixer) and power amp sections (Crown rack-mount amp). An excellent way to isolate audio problems if amp is still producing audio is to check the preamp out (line out). Preamp output distorted = preamp failure. Connect MP3 player to "Power Amp In"...sound distorted = power amp failure. In your case the latter is the case as your "line out" is clean. Also test each speaker for clear sound as speaker cone damage from being overpowered is fairly common. At 300w. I'll assume the failure was not from cranking the volume to loud but a shorted speaker or speaker wiring could zap the power amp in an instant.

Hello,

I've seen a bunch of Marshall Combos and a very common failure comes from broken resistor legs at pre-amp and power amp modules insides, they are the low-value/2-3W rated and the soldered legs broke so easy because of enclosure vibrations, let me know a bit more to improve help...

Internal failure... take in for repair. It is NOT DIY unless you are electronically adept and have necessary equipment to repair it.

Check for a dry/cracked solder joint where the 1/4 inch phono input jack connects to the printed circuit board. Solid state Marshalls are renowned for this.

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.

Replace the fan with an IDENTICAL part. Do this before using the unit again.

There is a possibility that the fan is controlled by temperature and the failure is actually with that electronics rather than the fan itself.

The fan will have a manufacturer's part and name on it.

Shop Digikey.com or Mouser.com for identical device.

Note that SOME amps have the fan throttled depending on temp and these fans get noisey when throttled down.
Not sure about your Marshall on. The usual fan failure is bearings get dry and coke the lubricant.

Very nice amp!

First off, is the amp getting sufficient power? An unreliable AC current from your wall can affect the performance.

Have you tried the amp with a different speaker? Checked for a loose connection from the amp to the speaker? Is it the correct ohms rating?

Second, are you experienced in electronics? The problem could lie in a faulty solder connection, a bad component, etc.

If you are not confident in opening the amp and troubleshooting it yourself, take it to a audio repairman. Marshall's are some of the easier ones to fix.....

contact marshall direct ...;they have a great spares dept