Marshall MG100DFX Unusually Low Volume

[bose]

Hey guys,
I've been searching for a solution to this problem for about a week (which probably means I'm just a poor searcher) and haven't been able to find anything exactly like the problem I'm having with my amp.

I bought the amp a couple weeks ago, everything worked fine until about a week ago when I unplugged my guitar to tune it. When I plugged my guitar back into the amp, and began playing, there was no sound coming out of the amp. I tried using a different guitar, different chord, pressing all of the different buttons on the amp, nothing worked. When I turn up the master volume to full, I get a lot of static and some sound, but that's it.

Nothing appears to be fried on the circuit board and I don't recall smelling anything burning when the amp broke.

Below is a link to a video of me playing the amp at full and half master volume (gain turned all the way up on the clean channel).

[ame="http://www.youtube.com/watch?v=C2HZ6I1z6a0"]YouTube - Broken MG100DFX Amp[/ame]

Any ideas on what the problem might be? I'm currently stationed in middle of nowhere Mississippi and there aren't any amp repair shops anywhere near me.

[bose]

Update to the problem: I tried plugging my guitar into the FX Return and the volume on the amp works just like it should.

[Enzo]

So plug a spare cord from FX send to FX return and see if the amp now works.

[bose]

Sorry, forgot to mention that I tried that earlier and it didn't work.

[Enzo]

OK, so it looks like your preamp is a problem. APply a test signal to the input, and trace it through the stages.

Since your master makes the background noise come and go, I'd be that it and everything after is OK. How about the EQ and gain controls? Turn up your master and try those controls. DO they affect the background sound in terms of tone and level? if they do, then sound is travelling to your speaker through them. And the break must be before them.

For some odd reason, I often find the cap at the very input broken free of solder more often than I would think. C60 on your model. I have no idea why.

[bose]

All of the controls (pots and switches) seem to change the volume/tone as they should and there isn't any static between them.

C60 doesn't appear to be bad (though I'm not sure what to look for on this type of capacitor) and nothing appears to be missing solder, short circuiting, or have fallen off.


http://img121.imageshack.us/img121/9241/img0918c.jpg

[ame="http://www.youtube.com/watch?v=QIT1jZBJtic"]YouTube - MG100DFX Broken Part 2[/ame]

[Enzo]

So your background sound tone and volume are adjustable by the panel controls. SO the noise is coming through them, so presumably so would the signal.

C60 was just an example, and I wasn;t expecting it to be bad, I was expecting it - for whatever reason - to be cracked free of its solder on an end.

Either apply a strong signal to the input and trace it forward, or start at a point from which the amp works and inject a signal, then work backwards until you find a break in the circuit path. That is look for the signal on the inoput end of C60, then again on the output end, then pins 1 and 7 of IC4, and so on. or inject a signal using either a signal generator, or just hum. small screwdriver touched to the input pin of an op amp should send hum through it. We don;t care about fidelity, at this point all we want is to get signal passing through.

[bose]

Ok, thanks Enzo. I'll let you know how it goes.

BTW, I'm originally from Nashville and might be going back up there some time in the near future. Do you know of any good amp techs in that area if I'm not able to fix it myself?

[Enzo]

I don't know anyone in Nashville myself, but i am sure there are competent techs there. Look on Ted Weber's site for his referral listings.