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Old 20th September 2010, 04:40 AM   #1
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Default Marshall MG100HDFX - No sound

I bought this half stack used for an insanely cheap price (Marshall MG100HDFX) I know, bad reviews, but I couldn't refuse. $125.00 used. It worked great for a few months, and then the head just stopped producing any sound. I am not sure whether this amp is considered a chip amp or solid state, although it has a ton of JRC 4558DD 2004C chips in it (5 to be exact). It has one JRC 5532D 210BB, and one DG212BDJ. It also has an effects board that is surface mounted on a separate PCB. None of the electrolytic caps are leaking, and the ceramic discs appear to be fine. The input jack is clean and all solder spots look great.
The amp cuts on, and the cooling fan immediately runs. I replaced the fuse (just for kicks), and the power module (located on the heat sink of the fan) which are the most common problems. If I run my headphones in the headphone jack, I get a tiny bit of sound, but all other output jacks do absolutely nothing. I am lost at this point. Should I start at the transformer, capacitors, IC's??
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Old 20th September 2010, 06:47 AM   #2
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Measure the power supply output. I would get a diagram first, though.
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Old 20th September 2010, 12:12 PM   #3
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Thanks Bob. Should I measure that in volts? And would I do that on the output side of the transformer?
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Old 20th September 2010, 02:58 PM   #4
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BTW, if anyone can help at all, here are the schematics if it makes it any easier. : )

http://www.ampix.org/albums/userpics/10003/MArshall_MG100DFX.pdf
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Old 20th September 2010, 03:53 PM   #5
llwhtt is online now llwhtt  United States
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99% of the time the only thing wrong with these amps is the output IC. The remaining 1% was the last one I repaired the output IC fried like no other I'd seen it also wiped out part of the stby circuit. Check the 6.8V zener, D14, and the other parts around it. The output IC maybe stuck in stby.

Craig
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Old 20th September 2010, 04:12 PM   #6
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Well thank you Craig, that sounds like a great idea. How exactly can you tell if an IC has gone bad, or should I just change it out and cross my fingers? I was thinking the DG212CJ was the one I should start with.
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Old 20th September 2010, 05:15 PM   #7
llwhtt is online now llwhtt  United States
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Check for signal at the input of the TDA7293, if the signal is good there replace the TDA7293. You might even check to see if there is DC on the output.

Craig
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Old 20th September 2010, 06:11 PM   #8
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I didn't see a 7293 in there, just a bunch of JRC's.
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Old 20th September 2010, 06:15 PM   #9
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I just realized, that schematic must be for an older version that the one I have. Hmmm.
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Old 20th September 2010, 06:19 PM   #10
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Nevermind, thanks man, I found it. Ill give it a whirl and let you know how it goes. One more thing, when I am checking for signal, I need an analog tester don't I. All I have is a multimeter. Is there a way to check it with a digital multimeter?
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