Marshall MG10CD Amp woes...

[BinaryFu]

Hi,

I'm completely new to the board, and pretty new to working with amps as well.  I've got a little experience with electronics (note LITTLE) and I've got a problem that I haven't figured out yet (big surprise).

I'm hoping some of the highly knowledgeable minds in here might be able to peg the problem just from the explanation (which follows this long diatribe of an introduction)...

My little Marshall MG10CD Amp has died, but only partly.  The clean channel is practically dead.  Cranking it up to 10 will get you a small amount of sound and that's about it.  However, push the OverDrive button and adjust the volume to even 3 on Gain and 3 on Volume will get you some fairly clear sound with a bit of the cheap distortion that comes with this amp.  Turn them up higher and your ears will start to bleed.  Sound comes out loud and strong.

So basically, my clean channel (that I love on this tiny amp) is shot and my overdrive (that I hardly ever use) is working just fine.

So is there any place on the amp I can hit it with a hammer to swap the two around?   Failing that, can anyone give me some suggestions on what I should check for, what could be the main culprit, what my next step should be, etc?

Have multimeter, will zap myself.

[J M Fahey]

As far as I know, this amp has one channel, with a switch that adds more gain and a couple clipping diodes ... but the active channel is still the same.
So your problem must be other thing.
Please search and post the schematic.
Good luck.

[Enzo]

Actually on this one there is an extra stage that switches in.

Here is the schematic.

[J M Fahey]

Hi Enzo, thanks a lot. I had thought it was similar to the older one, with a total count of one chip, but now they use two.
Hi Binary.
Start by checking that each 4558 has +15V on pin 8; -15V on pin 4 and practically 0V on pins 1 and 7.
If all normal, the suspects are mechanical:
1) the Clean volume pot might be bad, or its soldering or pcb tracks that touch it (after receiving some hit on its knob), or
2) SW1A might be dirty, or have splder pads or tracks cracked.
Squirt some cleaning fluid into it and push-pull it a few times to remove grime, look carefully at the solder pads, resolder (adding very little solder, don't turn it into a blob), do the same with the pot.
3) If still not solved, bypass the mechanical part by soldering a short piece of wire from the left end of R5 (as shown on the schematic, on the actual PCB it might point anywhere) to the right end of C5 (same considerations)
You should regain your amp with its full volume, although it would be on 10 always.
This last test is not a repair, but simply to confirm  that the active part works.
Good luck .

[BinaryFu]

Wow, thanks for the quick replies all!

JM: I've checked the chips (With much trepidation hoping I was grounding properly) and found +/-15.07v solid on both sets of chip pins.  That quick n' easy tip helps a lot...now I know it's not something major, but something even I might be able to fix...an ohm test on the volume pot shows 840ohm - 3,500ohm (if I read it right, still learning the multimeter.  20k ohm reading of .84 to 3.5...just a thought, but should the 840ohm be @ 0 for volume 10?)

Does all that sound about right so far?  If so, the next step would be what exactly?  I'm nervous about stripping this thing down to the board to do more tests, but if it's necessary, I'll jump in with both feet - after all, to paraphrase Boba Fett, "It's no good to me dead..."

Thanks again so much for the fast help!

[BinaryFu]

So...ummm...yeah.

I figured while I had down time because of the amp issue...I'd shield my guitar.  Heard a lot of great things about it, and considering that I have a cheap little Jay Turser (21 fret - not the horrid 20 fret POS) that hums and buzzes all the time...I figured it'd make a fair amount of difference.

Well, it made A LOT of difference.  In fact, it magically fixed my amp for me. 

Yeah, so...very sorry to have wasted everyone's time trying to track down an issue in my guitar via the amp.  :lmao:

The good news is, the guitar is now so silent, I plug it in, turn on the amp, crank it up a little...and then scratch my head and wonder if the amp's dead...because there's zero sound...until I strum the strings and wake the neighbors.  0:)

Thanks for all the help!

[J M Fahey]

.very sorry to have wastedeveryone's time

Wasted? Not at all.
It was a pleasure doing Business with you !!!
What did you say your Billing Address was?
In a couple days one of our Associates  :trouble in your Town will ring to arrange payment details.
Please have your House Ownership papers at hand, as well as your Credit Rating in quintuplicate.
$2500 in cash for the Down Payment will do, for now.
Enjoy your Amplifier !!!!!!!!!!!!!!!!
8|  <-- this would be our Lawyer.
he he.

[BinaryFu]

How about 1 free guitar shielding instead?  Apparently I'm REALLY good at them...

[J M Fahey]

Ok, that will do ... for now.