Marshall MG15CD not working properly.

[KatierRax]

Hi All,

I got given the above Amp by a friend and being the practical sort am enjoying the challenge of getting it going. So far I fixed a dry joint on the input jack socket, cleaned the pots and (as a cheap/quick trial) swapped out the power amp IC.

But she's not working properly, there is life so I do want to try to get her going.

From the Guitar there doesn't appear to be anything, however if I take the headphone output from my hi-fi (used as it's the same 1/4" jack so I can use the guitar lead) I get the following :-

Via the MP3 input I can hear the sound nice and clearly - this tells me the Power Amp section is definitely working as looking at the schematic this input goes straight into the PowerAmp.

Via the Guitar input (I started nice and low as I didn't want to blow anything up but needed an alternative input to test) I can hear 'something' but doesn't seem to respond to volume inputs particularly and it's much quieter than via MP3.

I also get some fairly loud 'pops' from the speaker.

I don't know what could be causing the pops but the fact the volume doesn't appear to do anything noticeable would seem to indicate the volume circuit has a fault, which is where you guys come in!!

I need help trying to diagnose the popping and apparent lack of amplification/volume control via the guitar input.

Ta!!

Kat

[DrGonz78]

Welcome to forum Kat. Do you have a multimeter to check voltages? I would first check that your opamps are getting -/+15 volts on pins 4 and 8. You need to know that your power supplies are intact. Also, signal tracing will help pinpoint where the signal drops out. You did good so far by troubleshooting that the power amp was good and the fault lies in the preamp section. So you have cut the circuit in half and know better where to look. Many of these practice amps get abused and can have broken pots that will usually cut the signal out completely. Try banging on top of the amp to see if those pops are the signal trying to come back full blast. Could be a loose intermittent connection or even just a bad solder joint. Look up signal tracer and make one. Good luck.

[Enzo]

And also check pins 1 and 7 of the op amps for unwanted DC voltage.