Marshall MG100DFX Combo problems!

[nbriles2000]

Ok so let me preface this by saying that I am basically a completely noob when it comes to working on amps, but I'd like to learn more about it (hence taking on this task in the first place!)
A friend of mine has a Marshall MG100DFX Combo amp and whenever you turn it on the amp sounds like a blowhorn even if the volume is turned all the way down. Its extremely loud and happens even when a cable isn't inserted.
I've seen people say that the fans crap out on these amps quite frequently and it makes the TDA7293 (whatever that is) overheat and blow fuses. That doesn't seem to be the case on this amp because the fan runs properly and I dont see that chip smoking or anything like others have reported. Any advice/ comments/ questions would be greatly appreciated! Sadly the amp hasn't been in working condition in awhile and I would really like to try and return it to my friend in working order!
Thanks in advance!
-Nate

[Amp Kat]

You are going to need some Electronic experience to repair this amp as it's going to require some voltage,ohmage and possibly some amperage testing to repair. The amp is going into Oscillation and coming out of the speaker so it could be several things causing this. The TDA 7293 aka as spider chip is the amplifier usually to the outputs and if it's blown can cause that symtom but it's not written in stone that it is the problem. You'll also need some soldering skills and you have to be real careful working on this board or you can mess something else up very easily and double the problem troubleshooting process. I would recommend letting a tech handle it until you get some experience under your belt because this isn't a good amp to learn on nor is this problem.

[acorkos]

if it is the tda7293, you can buy one of these and skip the soldering:

Power Module - Marshall, MG100DFX | Antique Electronic Supply LLC

[Jazz P Bass]

Or take the amp to a reputable tech.
Sounds like the OP is in way over his head on this one.
These are not simple amps.

[nbriles2000]

I think I made myself sound far less competent than I intended to lol I'm very solid at soldering and I've made several custom speaker cabinets/ crossovers, I just don't know that much about amps. I know how to use a multi-meter and I know how not to shock myself and get killed. I would greatly appreciate if you'd give me some insight on what components on the amp I should test and how to properly test them. If I decide I'm in over my head I will definitely take it to a reputable repair shop! Thanks for the concern guys!

[Jazz P Bass]

The very first item to check is the power supply.
Are the high voltage rails both the same?
Are they relatively free of Vac ripple?
Are the low voltage opamp power supplies good.
These should have zero Vac ripple.
Prove out the supply.
If it is good, we move on.
If anything is bad, we fix it & then move on.
The amp cannot operate properly with a faulty supply.
That is the reason for checking it first.

[Jecarroll7]

I've had the 100, and now have a couple of the 250s, I agree these are not easy amps to work on.

Being solid state, what ever you hear will happen pretty quick. I wouldn't leave it turned on for long particularly if it is a power supply problem.

I assume the master volume is off as well, What happens when you plug in headphones?

[J M Fahey]

I'd start by unplugging that speaker NOW and measuring for DC across speaker cables.
I bet that oscillation is of the 120Hz variety, a.k.a "Ripple".
Just sayin'
And agree it's a tricky amp to repair..

[J M Fahey]

>>Deleted<<

[Steve Conner]

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[Enzo]

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[Steve Conner]

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[oboi]

A friend of mine has a Marshall MG100DFX Combo amp and whenever you turn it on the amp sounds like a blowhorn even if the volume is turned all the way down. Its extremely loud and happens even when a cable isn't inserted.

Hi OP,

I have the exact same problem with my MG, were you able to fix it yourself?

Thanks!!

[J M Fahey]

Please upload a short YT video or MP3 showing the defect.