New Marshall MG series schematics

[leadfingers]

Ok....HELP!!!

I've looked all over the place and can find NO info on the new(er) Marshall MG series. Specifically the MG 50 DFX.

I am doing some repair work for a local Music store and the owner is giving me all of the solid state stuff that comes in.
Seems the "tube" guy doesn't want to touch the new stuff or anything solid state.

Anyhow, I have looked at dozens of schematic sites for this particular amp and the only thing I ever come across are tons of Marshall tube amps.

I don't care if it's NOT free. I went to a site that seemed to show it had a copy in the search but it was a different company with the same model no. !!! ARGH!

Symptom is, Lights are on, no one home....restated, The lights come on, the fan comes on, but no sound.

There is no obvious sign of physical damage, there is signal at the volume pot, but from there, the signal appears to go through some components then into a piggyback PCB with FIVE major ICs.

No signal at the output and the speaker is fine.(connected it to a different signal source and it's working.

From what I can figure is part of the Power Supply, it seems that is OK, too.

I just don't want to go poking around near that Piggybacked IC board and smoke it accidentally.

Does ANYONE know where I can get these schematics? I went to the Marshall site and all they have are tons of downloads of Operator manuals. No schematics.

Supposedly, the customer is not in a hurry, but I'd like to get this done and move on to the next piece.

So, if anyone can point me to the right source, It would be GREATLY appreciated.

Thanks!

Andy

[sumgai]

Andy,

Mucho commiserations! I'm a long time member of the Have To Fix It Without A Schematic Club!

First, have you contacted Marshall directly on this? I mean, either by email, or more preferably, via telephone? If you are a legitimate support operation for an Authorized Marshall dealer, then you should be able to get the whole book of support materials from Marshall. If not........

Next, I see that you've done some preliminary checking for power and signal. So, can you put the arm on a like unit that is known to work correctly? If so, then can you lay it out on the bench for comparison testing? That little trick has saved my bacon more than once, and taught me a thing or two into the bargain.

And finally, is this thing under warranty? If yes, then box it up and ship it! If not, then you're correct in your caution against smoking the unknown components. Can you bypass any part of the circutry, thereby forcing signal to go from the preamp straight to the power amp, without going through the DSP circuitry? If so, then ask the customer what he thinks/wants. If not, then tell the Marshall support team, and see what they think. Perhaps you can get away with installing a new DSP board, if they'll sell/send you one.

At that point, you are a "module plucker". Nothing wrong with that.... you know what they say, "the check's the same at the end of the week!" But to go beyond that, you'll need a pretty good background in detailed component analysis and troubleshooting techniques. That's a bit advanced for this board. Hmmmmmmmm...........

Have you given thought to asking some of the other guru's around the net? Steve Ahola, Steve Bench, R.G. Keen, people of that ilk. If Marshall shines you on, you might try one of those guys. Bear in mind, troubleshooting blind like this is difficult. It will probably take someone who's very familiar with the exact symptoms for that exact unit to set you on the right path.

Good Luck!

HTH


sumgai


p.s. Sorry, but I deleted your duplicate post from the Schematics section - that's for Member-originated circuitry only.

[leadfingers]

Hi Sumgai,

Sorry about the dbl post. I'll be a lot more focused on what goes where in the future.


Ok, No, I'm not an authorized Service center. I just recently started getting back into repairs. The store I am doing repairs for WAS, a long time ago, authorized. But in the rash of MARS Music, Sam Ash and Guitar Center super sales, On line sales places and some not so frugal investment decisions, the store slipped down in the ranks and lost most of the big name franchises.

So, anyhow, I'm doing this on a part-time basis. I was going to e-mail Marshall and see about getting some schematics since the store still gets quite a few requests for this stuff.

What amazes me is the number of folks that bring stuff in that is probably under warranty but they don't care and want him (his shop) to take care of it. My understanding is that there is a several week wait at Sam Ash and Guitar Center. Both use outside repair centers and just take the repair work in and then call the other guys.

Anyway, the Marshall site states there should be a card with each amp explaining what the warranty time is. This didn't have one so I'm guessing either the owner of it didn't know or didn't care.

The good news. I started at the back (speaker connection) and worked my way back up toward the front. I found a high wattage resistor in the output circuit that had broken one of the leads off at the PCB connection. When I touched it down against the solder joint, SOUND!!!

A quick resolder job and a visual once over for any other solder joints and the amp is cranking again.

But I still am going to follow up on getting schematics.

Thanks for the info!

(Man!! I love this site! A ton of good info and good people!)

Andy

[JohnH]

Ive also looked for MG schematics, just out of curiosity relátive to my little MG10. Cant find them, but www.drtube.com has some older ss marshalls, as well as heaps of tube schematics.

John

[sumgai]

Andy

The good news. I started at the back (speaker connection) and worked my way back up toward the front. I found a high wattage resistor in the output circuit that had broken one of the leads off at the PCB connection. When I touched it down against the solder joint, SOUND!!!

As usual, it's the simple things in life that do the best job.

And IIRC, my first electronics shop instructor was wont to say: "Visual inspection is your best troubleshooting tool".

I usually do the same thing, look it over first for obvious signs of discombobulation. Sometimes an oft-repeated mistake, such as a bad set of solder joints that escaped the factory, will be known to most techies, and we just look for it, without further discourse. In your case, I have no knowledge of such. And it didn't help that by doing a remote diagnosis, I was forced to give up my most favorite troubleshooting tool, my eyes.

If there was no glob of hot-glue holding that resistor down, you should give serious consideration to making it so. You don't want the thing back next week because Joe Six-String tossed the amp in the trunk without a care in the world.

What amazes me is the number of folks that bring stuff in that is probably under warranty but they don't care and want him (his shop) to take care of it. My understanding is that there is a several week wait at Sam Ash and Guitar Center. Both use outside repair centers and just take the repair work in and then call the other guys.

I'm in the same boat, although without the loss of having originally had Dealer status. "My" shop is a mom-and-pop store, catering mostly to the beginners and intermediate players. But due to the quality of the teachers in there (nearly a dozen studio pros, a few of them having played with some very big names, and more than 3 dozen albums to their credit, collectively), we have a lot of 'referral' work - their buddies who also don't wanna wait for the box stores to truck their stuff up the street, once a week.

It's a good gig, hang on to it.


sumgai