Marshall MG15 CDR. Please Help

[Heinonen]

I bought this amp about a year ago and it worked fine for a month or so. Now when I turn it on, it either makes a loud helicopter-like sound, or has a very faint signal from my guitar. All the lights (internal and external) are working and all the visible connections are okay. I would probably have to spend more than its worth to send it to marshall and back, so I'd like to try and fix it myself. Any advice?

[Garydean]

"Motorboating" is what you're hearing. Low frequency oscillation is usually a symptom of a bad decoupling capacitor between stages. There could be a bad solder point or colder solder point or a cap got disconnected or a broken cap lead or trace. I would open it up and do a visual inspection of the circuit board. Keep the amp unplugged while you do this.

[Enzo]

For reference, in my shop, the minimum charge for small amps is $45, so chances are it would be a $50 repair more or less.


Turn the reverb up midway. Now give the amp a bump to crash the reverb springs. Does that reverb crash come out the speaker loud? If so, the power amp works. The CD player input is at the same point in the circuit as the reverb, so you could plug in there and see if the powr amp works.

Power supply could easily be an issue. The amp runs on some voltage - +/-25 or 30 voolts maybe? See if that is present. I don;t care so much what the voltage is, I care that both positive and negative are about the same thing. Then those are dropped through resistors to a couple of zener diodes to make +/-15v for the op amps. Has either of those 15v rails collapsed?

And in case the power amop is not OK, we can still test teh preamp section. The CD input is also the line out jack. SO plug the guitar into the regular input, then connect a cable from the line out to the input of another amp and see what it sounds like over there.

[godonnell]

Marshall MG15CDR

Thank you for the schematic. I had a problem with no sound except for a high frequency that only my kids can hear. LED's on the circuit board were both on when I started & then both off, I measured +- 20V on the rails. I think it was a loose or dirty connection on the guitar input, now both LED's dance with the music.

[bobbybird]

While on the subject. I have the same amp and the speaker has gone dead for no real reason. It works fine through the headphones though! Any ideas please?

Thanks.

[52 Bill]

While on the subject. I have the same amp and the speaker has gone dead for no real reason. It works fine through the headphones though! Any ideas please?

Thanks.

The speaker could be dead. Remove the wires from the terminals and touch a 9 volt battery to the terminals and see if it makes any noise. If it does, replace the wires, then the speaker is ok.

The headphone jack has a switch that shuts off the speaker when the phones are used. Use a little spray cleaner like DeoxIt to clean the jacks contacts.

[walteroa]

For reference, in my shop, the minimum charge for small amps is ............

Hi Enzo, do you have the MG15CFX service manual too?

Txs!

[Enzo]

CFX or DFX?

[walteroa]

confirmed: is Marshall MG4 Series MG15FX

CFX or DFX?

Sorry, maybe I wasn't clear enougth: Marshall MG4 Series MG15FX

link to guitarcenter.com : model MG15FX

[Enzo]

OK

I had never heard of an MG15CFX, but I do have MG15DFX, MG15CD, MG15CDR, and MG15RDC. Now you want plain old MG15FX, and I do not have that one.

[walteroa]

txs anyway.
please keep in the loop just in case someone else can get it... Txs a lot!!!