Beloved Little Marshall MG15CDR Severely Ill, Please Please Help!

[X-MM]

So my amp decided to fall sick on me today. I'm a complete electronics n00b, but I have a friend with a bit of electronics experience and he says the circuit's pretty simple, but it'd help him a lot if I could diagnose the problem first.

I was just playing on it when the sound suddenly cut, and it just suddenly started humming quite loudly. It was just a continuous low hum that wouldn't change no matter how much I played around with the pots and switches. I gave the thing a whack and the hum got quieter, but didn't stop. Plugging anything into the inputs and outputs, playing with any of the pots and switches or doing anything in general changes absolutely nothing. All I'm getting is the same low hum.

I found the schematic (Marshall Mg15 CDR, I've included the schematic below), and read somewhere that three common areas for it to be messing up are in the power amp, power supply or preamp.

Apparently, to test the power amp, I should just crank up the reverb a little and give the amp a hit and see if the reverb springs crash, which they didn't. That would apparently mean that the problem's to do with the power amp? My friend says that it's probably not the power supply and running my headphones and my bass amp through the line out yields the same low hum, which I'm guessing would mean it's not the preamp. But I *am* a total newbie to this, so I could be completely wrong.

So if anyone could give me any pointers or extra information that I could find useful, that'd be absolutely fantastic.

Thanks in advance for your help!

Marshall MG15CDR.pdf

[Steve Conner]

It's not that obvious from your post what is wrong with the amp. What are the symptoms?

If you're getting no sound whatsoever, try shoving a plug in and out of the guitar and CD/Line sockets several times, see if that helps. Also try giving it a whack to check for loose connections.

[X-MM]

Thanks for the reply.

Oops, I did forget to include any specific symptoms. My bad. -.-

There's sound coming from the amp, but it's just a continuous low hum. It was much louder when it first happened, but I gave it a bit of a whack and it got quieter then just stayed that way.

Any attempts at plugging things in or out, messing with the pots and hitting all the switches causes absolutely no change, to the hum itself or otherwise. Even plugging in a guitar and cranking everything up to full volume puts no signal through whatsoever. It's same if I try listening to it through my headphones or through another amp.

I opened it up and had a quick peek and there doesn't seem to be anything loose. And just now I tried to turn it on and plug something into both inputs repeatedly and still no change, however, an ominous smell started to come out the back (just a smell, no smoke...yet...>.<), so I'm a little reluctant to try turning it back on now...

[Steve Conner]

OK, hum and bad smell suggests that the power amp chip is blown. IC3, the TDA2050. Don't turn it on any more in case the shorted chip takes some other stuff out with it.

Replacements are cheap and easy to install for anyone with some soldering skills.

[X-MM]

Great, thanks a lot for the directions. I'll see what my friend and I can do about it and see if it works.

[Jazz P Bass]

Steve's advise is sound.
Remove the output IC, TDA2050.
Get the datasheet.
Check pin #3 (-VS) to pin #4 (output)
Check pin #5 (+VS) to pin #4 (output)
If a low resistance reading is observed then the IC is toast.
Then check that you have a good power supply Vs+ & VS- with power on.(Vdc)
Check for any undue Vac ripple with respect to chassis ground.
A large reading would indicate a supply capacitor has failed.

[52 Bill]

I don't think that I've ever seen a bad output chip respond to an Enzo whack before. My first guess would be a loose filter cap.

[X-MM]

Thanks for the info, guys. I'll try it out or, failing that, make sure to bring it with me to...whoever ends up fixing this thing, haha. ^^