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Marshall MG100DFX & Mucho Problemos

Started by hellomydog, February 19, 2011, 11:08:17 AM

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hellomydog

So I'm a drummer but I have an amp related question...

My band's guitarist has been having a terrible time lately trying to get his equipment to function. First, he was using a 200w Randall RG200 Head and a 4x12 Randall cabinet - which was awesome - but the Randall was experiencing some volume loss and a bad signal so we put it aside and decided to look for a new used amp. I assumed he would try to get a tube amp, but he ended up buying a Marshall MG100DFX in so-so condition instead. After he got it, he plugged the Marshall head into the Randall cab and it immediately fried itself (with a small smoke puff). Now there is no power whatsoever.

I really know very little about this kind of stuff, but my FIRST impression was that he got screwed on ebay buying a bad amp. Now, I'm thinking it was the Randall cabinet that fried the Marshall head?

Does anyone have any ideas what happened?

DJPhil

It's hard to say what happened without further investigation, but your hunch may be right.

First I'd check the cab and see what resistance you get across it on an ohmmeter. It'd have to be a decent meter, or one that allows a relative measurement so you can cancel the resistance in the leads, because you're dealing with small amounts of resistance. What you're looking for is a short, or less than an ohm of difference between shorting the leads together and running them across the cab inputs. It can get touchy with a four ohm cab, as the difference is so small. If you get a really low reading (or a really high one, like over fifty ohms) you may have a dead speaker or a bad connection. You'll have to test the wiring and the speakers and look for loose connections or fried chunks of speaker brains.

Then there's the Marshall cab. The schematic is available at Enzo's (awesome dude that one), so that'll help a great deal. If you're confident you can pop the case on it and we can help you find which part let the smoke out (though it might be obvious) and what to replace. Odds are decent that you can get the Marshall up and running again with a little tinkering.

The more experienced folks here might be able to bullseye your problem, but that might get you started.

Hope that helps. :)

joecool85

I'd follow Phil's advice and check the resistance on the speakers in the cab and the cab itself.  After you verify it is ok (or fix what needs fixing on the cab), I would repair the Randall.  Sounds like it would be a better fit for your guitarist than the Marshall in this case.
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J M Fahey

Agree and add: the Randall may even be fine; you could have been hearing cabinet problems, or faulty cables.