Marshall Mg15dfx 15-Watt Combo Guitar Amplifier - Answered Questions & Fixed issues

A wild guess is that the soldering on the cold side of the volume control pot MAY be bad. When it vibrates and loses contact where it is soldered, the unit will go to full volume. Check soldering of ALL the pot leads into the circuit board. Look for a thin grey line around the lead part way up the menicus of the soldering.

The 30 Watt version uses MJ3001 (NPN) and a MJ2501 (PNP) in a standard emitter follwer arrangement. TR4 looks like a BC184 but drawing is not too clear and this transistor is CRITICAL as it sets the bias for the two output transistors, and if it fails, the outputs will fry.

Here is typical schematic:

http://elektrotanya.com/marshall_30w_5203.pdf/download.html

Since you are not a registered user of the site, scroll down hte page and nearer the bottom is a small link that says "Get Manual" to click on to start download. The is a statement next to it that as unregistered you are only allowed two downloads a day.

There are many things it could be... I often quote the probabilities based on many ears experience and the symptoms the person posts.

Based on what you have posted, there is an intermittent... the unit vibrates while operating... there could be a loose connection, however my guess is you will find bad soldering at one of the pots that control the distortion. The solder often breaks loose as the sometimes support the circuit board from the leads of these pots. Also switches that solder to the board and sometimes jacks for foot switches break loose.

Open the unit and inspect the solder joints. It often takes an experienced eye to spot the tiny grey ring around the solder meniscus that signals a failure.

Either the speaker is blown OR more likely, the switch on the headphone jack fails to connect the speaker when headphones are unplugged. The contact gets bent causing the problem.

I suspect either the power supply rectifier developed a short which would likely blow a fuse till repaired OR the power amplifier chio went bad. Most of the smaller amps use some version of the TDA2050 audio amp chip. It looks like a power tab transistor (TO220 stylle case) with 7 leads.

If it went, it did so for a reason... time for unit to be serviced.

Behind the amp, right around where the power-supply is. Usually the right hand side at back of amp. Serial numbers are labeled as: S/N########

could be effects module or sound output i.c. also check send and return jacks

probably a bad solder joint on the circuit board where that pot is soldered in. check it out and reheat and apply a little more solder. or its just a dead pot.
yostamplifier.com

oh yes it can be fixed quite easily,...but you are best to bring it down to your local music store and have them do it, they'll be fair in price and your amp will be done right,....(no time for that breakin down stuff when your playin your guitar), thanks, cracklin joe

Try checking the preamp out which would be located at your line out plug. Plug that into another amp or get a 1/4 inch to RCA splitter and plug it into your home stereo. By doing this you can eliminate the preamp circuitry. That would point to the power amp. Let me know what you find. Good luck.

Most of these use an amplifier chip in the TDA family of 5 or 7 pin audio amps.

If you play real loud you may overheat the chip and it will go into shutdown to protect the chip.

Sometimes htey don't put adequate thermal conductive grease on the chip causing it to overheat.

Remember this is a small practice amp, not a powerhouse.

It sounds like it is going into "motorboating" as it is called.

It sounds like your volume control is dirty. You will have to disassemble the amp enough to get access to the volume potentiometer which is soldered to the circuit board. There will be a square hole in it that you can spray some contact cleaner into the control. Use only contact cleaner as anything else could damage the control. Use a small squirt and rotate the shaft. If this doesn't solve the problem you may have to replace the control. While you have it apart, examine the solder side to make sure all the solder joints are perfect especially those on the controls. A bad solder joint can also cause the same problem. Before you take the amp apart unplug the power with the amp on and the volumes turned down. This will discharge the capacitors so it will be safe to work on. I hope this helps.

If memory serves me correctly, these units use a 4558 OpAmp Ic. Both inputs pass thru this IC. One side provides a clean amp stage and the other is set up to run in a higher gain mode. This IC needs to be changed, It should be a small IC with 8 pins.

Dan

If you still want the schematics for the Marshall MG30DFX Steve I have them and can email them to you if you want

The fuse in a Marshall MG100DFX amplifier is located on the rear panel of the amplifier, near the power cord. To access the fuse, you will need to unplug the amplifier from the power source, and then locate the fuse holder, which is a small, cylindrical component with a cap that screws off. The fuse holder should be labeled with the appropriate fuse rating for your amplifier. Once you have located the fuse holder, you can unscrew the cap and remove the old fuse. You can then replace it with a new fuse of the same rating, and screw the cap back on securely. It's important to note that if your amplifier blows a fuse, it may be an indication of a more serious issue, such as a short circuit or a problem with the power supply. If you replace the fuse and the amplifier continues to blow fuses, it may be necessary to have it serviced by a professional technician.

More information required.

Why is there no sound when my wired headset is connected? To check that the headphones are connected correctly and there are no loose connections, try unplugging and replugging the audio device. Make sure that the headphones are not plugged into an audio-in or microphone jack if your audio equipment has more than one headphone jack accessible.

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.