Question about Marshall Mg50dfx 50-Watt Combo Guitar Amplifier

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When I turn the amp on it makes a loud squealing noise. If I turn it on and off again (several times) the amp will work normal. It doesn't do this all the time; but it is beginning to happen often. When it works, it sounds great. Also; sometimes when I turn the amp on I can hear the fan whine, but that clears up fast. Thanks, David

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Have a look at this link:http://www.geofex.com/ampdbug/squeal.htm then Back to the Home Page. You may have a bad solder joint some place on the circuit board also. Unless you have experience with electronic repairs you are best served by consulting an accredited Marshall dealer and repair person as there are voltages present that can kill or injure you.

Posted on Aug 27, 2010

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My fender fm 65 dsp amplifier squeals and feeds back every time i turn it on


Is it plugged into a guitar at the time? If so, it sounds like you're using a speaker cable instead of an instrument cable. There is a big difference. Look on the cable to make sure. If guitar is not plugged in would be a completely different diagnosis.

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Marshall MG30FX loud tapping sound in time with delay even when no guitar plugged in and effects turned off


Hi J Wade. I'm Dave P. I'm not a guitar amp expert or technician but I've been using amps and effects for 35 years and there are a couple things I can suggest.

That kind of periodic noise/switch noise can be a symptom of having the polarity of the amp reversed. If your amp has a 3-position OFF/ground switch, try reversing the ground. If it doesn't, take the plug and turn it around. Just don't get near a microphone unless you're sure your amp is grounded the same as the P.A. It could fry your lips off.

Dirty switches to that too. You said the noise is in time with the delay's tap time which is set by a foot switch. If tapping that switch the first time generates a pop, the delay may have catch it and is repeating it. Turn off the amp, remove the delay foot switch and turn it back on. Still noisy? If not, give the foot switch a good cleaning.

When you turn the EFFECTS knob up, is it scratchy? Might need the potentiometer cleaned. What happens when you specifically turn the DELAY level down all the way while using the other effects? Noise still there?

That's all I've got. Hopefully, it was helpful. Don't wrack your brain over this too much. There could very well be an internal problem with your amp that will require a trip to the shop. You don't want to open it up yourself. Rock on but be safe.

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Hello, I own a hotrod deluxe. I take great care for it and never move it. Normally on the clean channel, the volume is regulated by the volume and the master. Today the amp sounded only half as loud as...


Do you turn on standby every time and wait for the tubes to heat up first? If you don't, it can severely damage the tubes.
But a common cause of this could be just old tubes. Like any other piece of technology, tubes get old. If you like to play your amp LOUD then they wear out a lot quicker. You have to replace tubes every now and then. Because I'm not there to check out your amp, I can't give you an exact answer but it sounds like that's the problem. This isn't always a bad thing. You can make the sound of your amp exponentially better with nicer, new tubes. Your amp has 12AX7 tubes. Very common tubes to use actually. I'm not sure what kind of music you play, I'm assuming mostly rock and blues. But here is a list of tubes and reviews on them.
http://thetubestore.com/12ax7review.html
This is how to replace a tube:
http://www.ehow.com/video_4438285_change-pre_amp-tubes-guitar-amps.html
Upkeep on a tube amp:
http://www.ehow.com/video_4438295_replace-tubes-guitar-amp.html
A note he doesn't make in the video is to dust out your amp. I use the compressed dust spray for the inside of computers.
So overall, stop turning up your amp to 11, make sure it's well ventilated and dust it out from time to time. Hope this helped.

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1 Answer

When I plug in my guitar to the Fender Frontman 64R Amp and I just barely move the cord, it make this really loud screeching type noise. It's not an old amp; I got it about five months ago. I can't...


Yes, there are several possibilities. First thing is to disconnect your guitar and then plug in the cord to the amp. Expect a bit of hum. If it screeches, then problem is not feedback via the guitar. If it doesn't MAYBE your guitar is "hearing the amp" which the guitar is then a microphone and can feedback. Moving the cable generates electrical noise which can trigger the feedback to start.
If you get the screech without the guitar connected, then TRY a new guitar cable. Make sure it is an INSTRUMENT cable... I have seen some try to use speaker cables or junky 1/4 inch cables intended for home stereo... these will NOT work and can cause the feedback. The next possibility is a broken jack on the amp. This would require repair. The common cause of this is to fail to run the cord through the handle of the amp... one trip on the cord or step on it pulling the plug sideways in the amp and the jack is broken and it is a trip to the shop as well. While on the subject ALWAYS run the cord at the guitar end either through the strap or your belt loop. Cords left dangling keep rotating and soon wear out the plug and also the jack in the guitar and then that has to go to the shop.

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1 Answer

My hot rods clean channel sounds great, but as soon as I go to overdrive, there is a loud squeeling sound that is only produced when the drive is on and makes different sounds the more i play with the...


I need you to run a few tests... one at a time.

Plug a set of headphones into the preamp output and simultaneously a set or a guitar or other low impedance device into the power amp input jack (this disconnects the power amp from the preamp)

Now, does the squealing noise appear in the headphone when in OD?

I also want you to test if the normal/bright switch has any effect when in normal configuration in OD?

If the normal bright switch has little effect on the squealing that helps isolate which stage is allowing the feedback.

I suspect that the source of the feedback is either a bad filter capacitor in the power to the preamp OR one of the contacts in the two realys that are for OD are making poor contact. These relays are switched "dry" which means there is little current to clean oxides off the contacts so they can make poor or no contact. An open contact would leave a grid floating as a high impedance "antenna" to pickup and feedback.

The first thing I would do if you had an oscilloscoppe would be to see that C36, a 22mfd 500 volt cap does not have high frequency ripple across it. One could try paralleling a new 22mfd cap across it to see if problem goes away... you will GUARANTEED have trouble finding a 500 volt one of these and have to settle for a 450 volt one which SHOULD be OK judging from the circuit. You can probably get one from Antique Radio supply... use Google to find them. Remember this amp has lethal voltages in it so if you are not adept at working on electronics of this type, best to take it to a shop that is.

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1 Answer

Baldwin Model 230 DE when turned on makes a loud thunk like the volume is turned way up, then when you disturb or shake the organ or stomp on the floor it makes a loud noise like if you twanged a coil...


If the organ has a reverb spring "tank" in it, that is likely where the spring sound comes from.

The initial noise is likely due to the initial unbalance of the power audio amp.

Lesser organs don't disconnect the speakers till the amps attain balance so the inrush pulses the speakers..

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1 Answer

Marshall MG50 DFX makes a squealing noise on power up


Yep, chalk that one up to poor design...The amplifiers are going out of balance during power up and down. Two seconds is pretty long... Try turning down the gain during power on and see if that affects it.

The pop on turning off is normal... None of these have speaker disconnecting hardware that is necessary for very high power amps. The biggies have hardware that checks amplifier is balanced before connecting the speakers and a relay also cuts the speakers immediatley on power down. As power goes up and down the circuitry comes to equilibrium and until it does, strange things happen.

I would investigate the CONDITION of the filter caps for the main power because if they age to a higher internal resistance that COULD cause the high freq oscillation on power up... Two seconds is MUCH too long.

There should also be some non-electrolytic caps NEAR the power output transistors to handle high frequency supply power bypassing.

Check the power bypasses that service the preamp stages. Look for pregnant electrolytic caps that have failed.

Ultimately, you MAY need an oscilloscope to isolate the high freq problem.

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1 Answer

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Sounds like your master volume is turned up too much. The channel volume will make the squealing go away(caused by high gain and humbuckers). The best thing to do is dial in your favorite presets and change the channel volume to the correct level with the master volume where you usually keep it. I use my Spider Valve at master volume at about noon and the channel volume between nine o'clock and noon. Let me know if this does not solve the problem......

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