Marshall MG not working, i need help!!

[roast]

So, i borrowed a marshall MG 100dfx from a friend the other day. Yeah, i know its a shit amp. I took it to band practice, and we were jamming away, then suddenly the MG stopped.

The breakdown:
Power comes on, the fan whirrs, theres a *very* faint humming from the speaker.....but yet no sound whatsoever. I tried out the headphones port, nothing. Tried the CD input. Nothing.

Im an electronics newbie, but i really wanna fix this amp!
If you can help, it would be GREATLY appreciated.
Thanks.

BTW....my friend always played bass through the amp....would that harm it?

[tim]

So, i borrowed a marshall MG 100dfx from a friend the other day. Yeah, i know its a shit amp. I took it to band practice, and we were jamming away, then suddenly the MG stopped.

The breakdown:
Power comes on, the fan whirrs, theres a *very* faint humming from the speaker.....but yet no sound whatsoever. I tried out the headphones port, nothing. Tried the CD input. Nothing.

Im an electronics newbie, but i really wanna fix this amp!
If you can help, it would be GREATLY appreciated.
Thanks.

BTW....my friend always played bass through the amp....would that harm it?

Chances are you blew the power amp module...I think they use a TDA7293 (or two) but usually it blows the main fuse when that happens...it's a crappy design, I'm really surprised that Jim Marshall allowed his name to be put on those amps that use that power amp module....after you replace the module (assuming that's the problem) make sure you get all the dust off the cooling fan blades... it could also be a bad switched contact in the fx loop return jack. Try jamming your guitar cord into the fx return jack and see if you hear anything...if so amp is OK and it's something forward of the jack.

[roast]

ok, i plugged the guitar into the fx return, and i got some sound, at very low volume. What do i do now??

[roast]

hmm.....now, when i plug my guitar thru a distortion pedal, and into the input jack of the amp(thru the clean channel) i get a little sound.......what do i do????

(thanks for the help so far tim)

[Amp Kat]

Try patching a cable from the send to return and see if it comes back. A guitar into the return is not hot enough for the power amp to amplify. Try sending a cd player or preamp out or send from another source to it but DO NOT send a speaker out to it. These amps have very cheap pcb's and the components inside can break solder joints very easy which is also possible. There are some resistors inside it that looks like a 3rd grader installed and sometimes they aren't soldered in very well.

[roast]

um, what comes back when i patch a cable from the send to the return? (sorry, im a noob)

[Amp Kat]

What's a noob ? Ok heres what happens. The signal from your guitar gets amplified and the preamp amplified signal gets sent to the send jack so you can insert it to your effects box. From the effects box output to the return in to the power section of the amp. When nothing is plugged into the effects loop the signal gets routed by the jacks touching each other. They can loose contact quite easily so if you take a cable and insert it to the send and return you verify that the loop is ok and making good contact.

[roast]

ok. So, i did:

guitar-->effectsbox-->FX Return and i got a little bit of sound. Very hard to hear.

and then, i did:
guitar--->amp input, while having a cable from fxsend into the input of my effects box, and a cable from the output of the effects box into the fxreturn jack. The results from that=absolutley nothing, no sound.

What happens now?

Btw, thanks so much for the help so far, youve been great!

[Amp Kat]

Hmm, I hate to tell you this but I'm thinking you have a solder joint problem on the PCB board. On one of those PCB's I literally pulled a power resistor right out of the board. You can try this. Take the send out and route it to the input of another amp and if it works you know the problem is in the power section.

[roast]

Ok. Well, i tried that, and there was still no sound. Does that mean that the problem might not be in the power amp?

I doubt that it could be a loose solder joint. The amp was working fine, and it never moved for the whole practice session....

[Amp Kat]

Doesn't matter if the amp moved. The amp warms up, cools down, heats up, cools down. If you plug your guitar in the front and take the send out to another amp and use the preamp or gain controls and send the send to another amps input or return and it doesn't work your problem is in the preamp. Could be a bad jack,grounding problem ect... Vice versa if you take the preamp out or send from another amp and run it into the return of the MG and it works your problem is definitely in the preamp section. In that case the first thing to check is the op-amps making sure you have +/- voltage on the rails which are most likely pins 4 and 8 depending on the op-amps used. Once again I'm still betting on a bad solder joint on that pcb.

[roast]

nope, none of that will work....
Now, if i turn all the pots up, it crackles a little if i pluck a string really hard. Seems to show theres a little bit of hope for this amp....
how can i find a find a faulty solder? I took the PCB out, dosent look likes theres anything wrong on the bottom too.....But your right, it is a piece of poop, the pcb is quite fragile!!!

[Enzo]

Oh. Crackles on the peaks of a loud pluck? Look for DC on the output of one of the op amps. SOunds like the output pin of one of the ICs is slammed over hard to one rail or the other. When you play real loud, some of the peaks manage to cram themselves through it.

I have no problem working on those pc boards. They may not be sturdy as a 2x4, but I have not had any board failures while swapping parts.

Try this. Plug the guitar into the effects return, BUT ALSO plug something into the input jack on the front. If nothing is plugged into the input jack, the amp is muted.

If I had to suspect a chip to look at first, it would be the one that the input jack feeds a signal to.

Does the headphones jack work?

Are both + and - power rails present? Both HV for the power chip and LV for the preamp?

[roast]

ok, whats an op amp??

I plugged a guitar into the FXreturn, and then plugged another guitar thru an FX pedal into the front input. Got nothing from either guitar.

The headphones jack dosent work, unfortunatly.

I dont know what to do to see the + and - rails......??


Im really a beginner at this, but seeing as no-one around is an electronics technician, im gonna try get it workin myself (with your help of course :P).

Cheers for the help so far

[Enzo]

Op amps are operational amplifier ICs. They are the little black plastic components on the circuit board. They have like 8 legs, sometimes 9.

Rails refer to the various power supply voltages in a circuit.Each different power supply voltage has some wort of wire or copper trace to distribute itself around the circuitry. We refer to that wiring as a rail, and by extension the supply itself. SO when I want you to check your rails, I mean I want you to verify that each power supply voltage is actually present. I generally measure the voltage at the corners of one op amp here and there.

The little ICs run mostly on +15v and -15v, so I refer to those voltage sources as the +/-15v rails. There are a ton of tiny little transistors inside the IC, and if one fails, it can short circuit one of those power supply directly to an output pin on the IC. That is what I casually referred to when I mentioned the thing slamming the output over to a rail. Instead of resting at zero volts so it can move both positive and negative with your music, the output pin of the IC winds up at 15v. And the music signal trying to run through the amplifier will have a hard time overcoming that obstacle. Only the very peaks are strong ebough to do it.

This work will require voltage readings on a live circuit board, removing bad parts and soldering new ones in place on the circuit board. I don't mind helping, but without trying to be unkind, this may be over your head. Where are you located? There are more electronic techs around than you might think, and some willing to help. Aside from amplifier techs like me and numerous others here, there are also amateur radio operators (hams, we call them), instructors in local schools and colleges, and other resources.