Marshall MG50 DFX makes a squealing noise on power up

It's possible that your Marshall MG30 DFX amplifier is experiencing a hardware issue. The noise you're hearing when you turn it on and off could be caused by a malfunctioning component in the amp's power supply or switching circuit. Similarly, the intermittent operation of the channel switch could be caused by a problem with the switch itself or with the wiring that connects it to the rest of the amp.
To troubleshoot these issues, you may want to try the following steps:

1. Make sure that all the cables and connections to your amp are secure and not damaged in any way. Loose or damaged cables can cause all sorts of problems, including noise and intermittent operation.
2. Check the fuse in your amp to make sure it's not blown. A blown fuse can cause the amp to make noise or to stop functioning entirely.
3. If the problem persists, you may want to consider taking your amp to a professional technician for repair. A trained technician will be able to diagnose and fix any hardware issues that may be causing the problems you're experiencing.

I hope this helps! Let me know if you have any other questions.

Several possibilities (i have experienced all):

1. 120V.AC line noise. Try moving to a new receptacle in a different room or breaker. Try a In line AC filter (like a Furman power filter/conditioner)
2. RF interference. Plug in a different core and guitar to isolate.
3. Guitar wiring. Bad ground internally or pot (volume) became ungrounded.
4. Defective amp preamp section. Pot or transistor.

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].

As a guitar player myself learn that not all speaker from some amps hold the ouput wattage. The fuse could ba a problem or the speaker is giving out.

Contrary to what's posted above, the Marshall MG series does not pass the speaker power output through the headphone jack, and inserting the headphones into the jack does not disable the speaker. The headphone amp is a separate circuit and it's input is taken before the master volume. So Jan's post is exactly right. Turn off the master volume and turn up all the other gain/volumes as high as they will go. The other issue is that the headphone jack's output does not drive low efficiency headphones well, so you need some headphones similar to Sony's MDR-V700s to hear it well. Earbuds just don't cut it.

Try carefully cleaning your cooling fan with qtips for the blades and a vacuumn. See if that helps the fan. If not it will need to be replaced. If you are experienced with amp repairs have a look for loose parts on the circuit board. If you can safely disassemble it, look at the soldered side for bad solder joints. From another poster on here with the same problem I have learned that resoldering R107 and R96 helped. Make sure they aren't burned out or have changed in value.They are in the FDD circuit. Have a look at your owners manual and it will make sense. The schematic is available here:http://www.ampix.org/displayimage.php?album=search&cat=0&pos=0 If it doesn't come up use the search function on that site. Do not attempt to service this yourself unless you are absolutely sure you won't hurt yourself or the amp.

Either the control is damaged (happens if they get hit) or it is possible the circuit board it is mounted on has broken solder OR worse, a crack near the control. The unit will require servicing. Parts are readily available.

Well if you want goo dpinch harmonics to squeal you need a decent amount of gain and good pickups. What guitar are you using, some lower model passive pickups have bad frequency response when you try to do pinch harmonics. If you have decent pickups I would say get a higher gain preamp and just run the amp on the clean channel. There a re tube preamp pedals out there that have masaive amounts of gain and they are reasonably priced. There are also so good amps out there for a decent price like the 6550 from peavey. I have a 5150 block letter and that thing can produce good pinch harmonics along with my EMGs. Good luck man, oh and there is nothing wrong with your amp, it is just time to move on. There is only so much you can do with solid state amplifiers in my opinion.

I suspect the power supply for the effects processor has failed. The effects unit will "go crazy" when power supply is too low voltage.

If the amp has loud noise, then the power amp portion is PROBABLY OK.

I had a similar problem in a Behringer amp caused by a bad electrolytic filter cap.