Marshall Mg50dfx 50-Watt Combo Guitar Amplifier - Answered Questions & Fixed issues

Disconnect the power. Disconnect the speaker cable from the rear of the chassis. Remove the 4 screws from the top of the cabinet. Not the handle... Wiggle the chassis up and down to disconnect the gaskets and slide it out the back. Pretty easy

Bad transistors.

You should see four screws on the top (not the ones that hold the corner protectors) - these are large Phillips screw tops. Remove the screw and put the amp on its side and remove the chassis from the back (there may be a back behind the speakers you need to remove first).

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.

Bearings have gone in it... The fans are standard part. Take number off the label and search for the same unit. Note if it has two or three wires. If it has only two wires, replace with one of same voltage and current or wattage rating and of course that fits size-wise. Three wire ones have a sensor and you are best to replace with exact unit.

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.

Make sure that when the audio from the speaker cuts, the headphones are still working (try it several times). If this is correct the problem isn't at the preamp or driver tubes. Try to locate the wire that is connecting the speaker with the amplifier. There must be a bad connection at this cable or at the speaker's coil itself. Try hiting with a solid tool (the back of a screwdriver for example) the parts and the connections (not the tubes, just the mountings) while the amplifier is working in order to locate a loosing contact. Once you find it use a soldering gun to fix the connection.

In case of a problem or clarification, don't hesitate to post.

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Stelios
direct FixYa link: http://www.fixya.com/users/technical114

Often the bypass contacts on the FX insert jack fail to close. These jacks are really poor and after plugging in a plug several times the bypass switch contact becomes sprung so as to not make good contact causing symptoms like yours.

Open the unit and inspect the jack. You can bend the contacts to tighten them BUT they will fail again.

Externally sometimes you can make a short jumper cable to connect the preamp out to the PA in to bypass the failing contacts.

On an insert jack, you can just wire a TRS plug tip to ring and plug it in.

Overdrive is distortion. Essentially flattops the waveform.

Assuming that the amplifier has no other problems (till this one come up) and in case you don't use the "send" connector frequently, I believe that it's some kind of "dirt" in the connector. Switch of the amplifier and use a contact cleaner spray (available at any electrinics' shop) in order to spray in the connector. Plug - in and out many times a free male connector into the female one for the contacts to clean up. Allow some minutes the amplifier to dry and check the results.
Also make sure that the cable that you use for "send" audio is working. In case of a short-circuit cable you may come to the same problem.

In case of a problem or clarification or further details needed, don't hesitate to post me a reply before rejecting my answer.
If you are satisfied, accept my solution.

Thanks and regards
Stelios
direct FixYa link: http://www.fixya.com/users/technical114

I have a 250 gfx, and I have had the same problem intermittently, I have concluded that the digital fx section is to blame, as it is also responsible for the speaker modelling for the line/headphone jack, and I have also had problems with my fx section.
If you have anything in the effects loop that boosts signal volume remove it, a small boost in the loop gives the power amp in these a nice glassy sound, but seems to be (in my case at least) responsible for the failure of the fx/ emulated line out.
Also check all your leads and headphones cord, try some different headphones if you have them.

Has it always done this? If it's just a mild hissing sound, chances are it's normal. If it's a loud obnoxious buzz, is your gain cranked all the way up? If it is, turn it down about half way and see if it goes away. If not, take it in to a good guitar store for service/diagnosis.

Using a multimeter VERIFY the amp is not putting out a DC bias into your speakers. If it is it may cause damage to the speakers. I would not TOLERATE a bias in excess of 100 millivolts.

If you leave the input float... ESPECIALLY with no speaker loads attached, the amp may be internally feedbacking... DO NOT permit this to continue!... ALWAYS have your speaker loads SECURE and inputs attached, especially in tehse high pwer amps.

Check C131 which should have 5 volts across it. This powers the analog switch chip that has to select the audio channel and routing. If 5 volts isn't there, check cap for a short. There are too many other possibilities to enumerate here. You need electronic test equipment to troubleshoot this problem and some skill at troubleshooting.

Here is your schematic:

http://elektrotanya.com/marshall_mg50dfx_sch.pdf/download.html

Surge protector would NOT have protected it so don't kick yourself. If you had your footswitch plugged into the amp, the surge may have ruptured components in the circuit. Most likely are components on page 2 of the schematic in the upper left corner associated with the channel selection, C77 and TR1 would be first suspects. Deeper into the circuit is the channel switch chip IC9.

A voltmeter test of the FS_B input from the footswitch to verify that with the footswitch disconnected that this approaches 5 volts is first step. If it doesn't then the three components mentioned should be tested in the order I mentioned them. If it does go to nearly 5 volts, then IC9 is likely the culprit... it is a DG212 analog switch chip. It is readily available from DigiKey.com and others... in several different type packages. If yours is surface mount, you need to find someone with the proper tools to change the chip.

Here is link to schematic: http://elektrotanya.com/marshall_mg50dfx_sch.pdf/download.html

this is a solid state/tube amp. One of the tube's went bad.

Likely one half of the full-wave rectifier has failed. Take in for repair. Sometimes output amp shorts to one power rail causing this. DO NOT continue to operate this until repaired as it will destroy the speaker!!!

TRY GIOVANIA FOR HELP.

Bo, the fans cannot be lubricated. If the noise is from bearings, replacement is the only option.

This is likely a bad switch contact in the headphone jack... they are really poor quality in most amps... plastic case and EASILY damaged by side forces on the jack. REplacement is the solution.