Marshall MG100DFX Help Please!

[btnmq7]

My Marshall MG100DFX was victim of a beverage being spilled on the powerstrip it was plugged into during practice, and fried, after taking the amp apart and pulling the head out I found the component that was attached to the fan immediatly behind the heat sync (sp?) was torched, I looked up the schematics and they weren't much help for my limited knowledge of electronics. Its just one very small circuit board (1" x 1-5/8" in size) That has a 15 wire jack (they are all wired to the same jack) comming from it that plugs into the main circuit board in the head. This Small part that was fried was attached by 1 screw to the heat sync block. I am hoping this might be able to be fixed by simply replacing this part, however I realize that is somewhat of a long shot. After some very thorough searching I still am unable to find out what this part is. Does ANYONE out there know what part this is? I would greatly appreciate ANY advice/input about my amp. Thank you very very much.

I have attached a picture of the part next to the heat block. Thanks again.

[Enzo]

To nitpick, it is a heat SINK.

The chip is a TDA7293V. Mouser sells them for 6 bucks or so.

www.mouser.com

Chances are that is all that is wrong. The small parts on the little board rarely fail. Make sure the solder happens on both sides of hte board. Sometimes the copper traces on that board get damaged. A Q tip and some isopropyl will clean off the soot from the IC burning.

Your 15 pin cionnector to the rest of the system is upper right on page 1 of the schematic. The little board itself is upper right of page 3.

[btnmq7]

Enzo your a lifesaver

Ok first off thank you very much for taking the time to help me out, I am very greatful. Second I looked up the part online, and I guess im a little confused by what you mean, are you saying the small circuit board I showed a picture of that fried is still good and just needs to be cleaned? Because the picture of the part you said didnt actually have the small circuit board attached to it or is it just implied its there and comes with it? On mine, some of the small electronics have actually melted on it and im pretty sure will need to be replaced (I apologize once again for my limited knowledge of electronics and if I am just completely misunderstanding you). But I see the circuit that connects to the heat sink and is soldered to the board, I guess I just dont understand about the actual board itself. Thank you again so much.

[Enzo]

No, circuit boards do not come with ICs. Remove the dead IC from the small board, then Qtips and alcohol rub down to remove the soot. It usually LOOKs a lot worse than it really is. If one or two of the copper traces on the small board are damaged they can be repaired with bare wire. Usually.

On the board are three small electrloytic caps which appear intact in your photo. COuld be bad. There are also four smaller caps. If any of then were damaged, then replace them.

I have focused on repairing the existing board. If you are not comfortable doing that, or if the damage is just too great, you can order the small board with IC complete from any MArshall dealer or a MArshall parts supplier like www.marshallparts.com. It should cost maybe $20. Marshall part number is: PCBCD0053

[RR1]

I have Same Problem

Both myself and my other guitarist both own the MG100HDFX Full Stacks. Mine is 3 years old, his is only 5 months old. My IC went out about a month ago. His went out 3 days ago. After reading thru this forum site, I ordered IC's and replaced mine at the time it went bad. I have had no other problems. I thought that there was a possibility that mine had been shorted to ground due to the fact that I measured voltage on the back of the IC and it's mounted to the Heat sink which in turn is mounted to the Chassis with only some plastic grommets to isolate the mounting screws from the Metal Chassis. My thought was that the fan screws could have possibly come loose and touched the Chassis.

I just replaced his IC and now when the amp is plugged in, speakers and guitar, etc.. There is only a loud sort of "ground" crackling noise coming out of the speakers. We cannot hear the guitar at all.

Any ideas? and thanks for posting info to this problem previously.

[Enzo]

Are not both your amps under warranty?

[RR1]

It takes too long and it's too much hassle to send the thing out and wait to get it back. We have a gig the 13th and the Store that we bought them from doesn't have an exact type to loan. My understanding is that Marshall is updating all of their MG series amps. We like our sound and don't want to just use any loaner head. I'd rather just fix it myself. If it ends up not being a quick fix, he said he'd just buy a Randall. I feel bad because he bought the thing because I had one and it sounds as good when the volume is on 1 as it does when the volume is on 10. We actually never even have to play any louder than 4!

[Mars Amp Repair]

I guess as an addendum,
the TDA7293's are particularly sensitive to blowing if the heat dissipation is not up to snuff. Really any solid state device like this will blow if not adequately heat sinked. On many of the Marshall models the heat sinking is really minimal.

That said, be certain if you fan ever starts to make any squeaking or unusual noises that you get it replaced mondo-pronto...the IC will blow for certain if it is not cooled properly. Also take a look at the blades on the cooling fan from time to time & be certain that there is not any dust build up as that can diminish its ability to move cooling air over the heat sink. Clean it carefully if that occurs.

At one point Marshall updated the size of the fan from this reallllly cheesy thin thing to a deeper more efficient one. Hopefully your amp has one of that variety. glen

As you can find with a search of this forum, there has been much talk about these Marshall's & the TDA7293 IC. If you're going to try to replace the IC instead of the board, you need to be careful of the double clad pcb traces.

Without special vacuum desoldering devices or at the very least this product called 'Chipquick', it can be a rather delicate job.

the only way to get this IC out safely without any special desoldering devices is to clip the IC out at the body of the IC & then carefully remove each leg one by one. the you really would at the very least need to use solder wick to wick out the remaining solder in the ic holes.

happy IC changing...glen

[RR1]

Thanks for the helfpful tips Glen. My MG has the old style "cheesy" fan and his has the updated bigger fan and heat sink. I have already replaced the IC on his board and then the noise began. My first guess is that I lost a ground.

[52 Bill]

Double check your work on the little output board. As Glen mentioned you can damage the traces while removing and replacing the chip. Check all traces for continuity from pad to pad, etc.

If you send a signal into the loop return or the cd input will it pass a signal?

[RR1]

You are right Bill and glen. I got the thing under a microscope. I actually had lifted a pad during wicking. I removed some material off the trace near the pin and ran a little bit of solder from pin directly to trace very gingerly. It is now operational.

Now, I am just worried as to how long I can expect the Power Amp IC to last. Sure would hate for it to happen at a Gig.

[nyge23]

mg100dfx blowing 1.6A internal fuse

Hello!

i am fairly knowlegable on electronics.
my marshall 100w dfx amp is blowing its 1.6A internal fuse, i was wondering if anybody has had the same problem and been able to fix it?
this would save me alot of time if anybody can tell me what is the most likely part to have gone faulty.

thanks for your time and any information you can offer me

nyge

[Enzo]

Hello nyge, welcome to the forum.

If you are training to be an engineer, then you know the inportance of being analytical. There are many reasons a fuse can blow, and it may or may not be what someone else experienced.

Isolate your problem.

Verify that your rectifier diodes are not shorted in the power supply circuit. And check for shorted filter caps.

The low voltage supplies could also be bad, check their rectifiers as well.

Disconnect the wiring cable from the power output IC - the TDA7293 on its own small circuit board mounted to the heat sink. If it no longer blows fuses with that disconnected, then your TDA7293V is bad. That is a common failure mode.

DO NOT connect or disconnect that cable when power is ON.

[nyge23]

thanks enzo.

i am aware that different faults can have the same symptoms, it was just me trying to save time. i was surprised to see how simple the circuit was. i have recently had to repair my trace elliot amp (valve pre amp, 12 band eq, dual band compressor etc) which has 4 large multi layered PCBs where as the marshal (being all solid state) only has one single sided PCB.

i will put up any info i find that may be useful after i have sorted this amp.

there is another good website i use: freeinfosociety.com it has schematics of loads of different things, not just music based. i have used it to make my own bass synth and then experimented by changing the values of caps in high/low/bandwidth filters etc. its a bit bulky so i wouldn't use it on stage but i learnt a lot that way. its worth a look.

thanks again

[Enzo]

Go visit Duncan's Amp Pages and find his tone stack calculator. It is a free little program you can download, and it shows freq response graphically for several common EQ circuits. You can adjust the values of the components in the circuit and see the changes in reponse in real time. There is other great stuff on that site as well.

You should find it interesting.

Not to belabor it, but if I told you the last one I fixed for blowing fuses had a shorted rectifier, and yours has a blown output chip, how much time would I have saved you?