Marshall Park Series amp repair questions... possibly the preamp?

[andrewklope]

BACKGROUND:  I have a Marshall Park series 30w amp that has recently stopped putting out sound, and its got me banging my head on the table.  When I turn it on, and turn the volume up to 10, it still puts out very loud white noise like it always did before any signal comes in from the guitar.  It sounds like it's ready to split your eardrums if you pluck a string, but when you do no sound comes out at all on the clean channel.  When I switch to overdrive and turn the gain all the way up, almost the same thing happens, but the guitar sound does come through very, very faintly and clean, as if every knob on the amp was at 0.1.  You can barely hear it when it should be full blast.  I thought it might be the input jack, because once I jiggled it and it rocked to life for a split second, but it only did that once, I could never make it happen again.  I replaced the jack anyways, and it didn't fix anything. 
HELP NEEDED: I think that the preamp must be bad, but I cannot find anything on how to repair it myself.  I am a decently competent amateur with electronics.  I feel like I can fix it if I can just find a how-to or some guidance... can anyone out there help?  Its become sort of a quest for me know, gotta prove that I can fix this thing haha.

Thanks for any help.

[DrGonz78]

I think the schematic might be online... Although it would be good to know what exact model amp we are trying to fix. If you have the schematic then please post it or show us a link. Of course cold/cracked solder joints come to mind instantly as a possible cause of this symptom. Also, may want to just clean the headphones jack to make sure to cover that base.

[andrewklope]

It is a Marshall Park series G30R CD.  I've been looking all over google for a schematic or how to, so yeah any help finding one would be much appreciated.  I had thought cracked solder as well, but shouldn't that more than likely only affect one channel, since there is no master volume control?  I've been searching for a crack and can't find one.  I've tried the headphone jack and speaker both, no difference between them.  Also I replaced the input jack with the CD input jack b/c I never used it, but I don't think that should make any difference, just figured I'd better mention everything I could.

[DrGonz78]

I had a problem with a solid state amp where the clean channel was almost mute sounding. When I switched the amp to overdrive it let a little more signal through but was nearly mute too. The culprit was one solder joint on a leg of a tone cap through the signal path. So, I think it can effect both channels if it shares the preamp down the signal path. Meaning that amp shares bass, treble, or mid EQ or possibly each channel has it's own separate EQ paths.

Also, have you tested anything voltage wise on the amp yet? If you see good numbers everywhere on the amp it might point to a connection on the signal path. Check if you have +/-15 vdc on any opamps on the preamp side.

[Enzo]

"Two channels" in an amp does not mean there are two entirely separate circuits in the amp.  Think of a two-seat bicycle compared to a single seater, that is more like a two channel amp.  There are many parts of the amp that are common to both channels.

The first place to look is the input jack itself.  It may have cracked solder that doesn;t allow the guitar signal to even enter the amp.  The hiss you hear is just the self noise of the amp.

ANY control that has ANY effect on the sound is AFTER the source of that sound.  SO if the volume controls can turn the noise up and down, it is coming from before them.

From the input, the signal flows through probably a resistor and a cap, and if that cap breaks its solder, no signal.

[J M Fahey]

Search for Marshall MG G30R CD , it should be *very* similar.

[Roly]

Clue #1

andrewklope> it still puts out very loud white noise

Clue #2

andrewklope> input jack, because once I jiggled it and it rocked to life for a split second

I think @DrGonz78 and @Enzo are right on the money; you most likely have a cracked PCB track or dud solder joint, and the first clue tells us that it is going to be somewhere close to the start of the signal chain because you can hear noise generated in transistors right down near the input.

The fact that it came to life is really good, and only once (together with replacing the input socket) tells us that it isn't right at the socket, but it can't be far away from there because it responded to your wiggle, and you can hear preamp noise.  Ever since manufacturers started mounting controls and sockets directly onto PCB this kind of broken track fault has become horribly common.

If it isn't the input socket itself then it is likely to be one of the connections between the PCB and front panel controls close by.  You'll need to go over the input area very carefully with a bright light and a lens 'cause sometimes these breaks are almost invisible.

If you can safely run the amp out of its case you can try the "wet finger" or "blurt" test.  Using your multimeter probe start at the mains volume control and when you probe you should get some sort of crackle.  Work you way back along the signal chain towards the input and the crackles should get louder.  Eventually (we hope) you will find a point that doesn't respond, and it is between there and the last response you need to look carefully.  *Gently* poke/wiggle components while looking at the solder side to see if you can spot any movement *within* a solder joint, or a PCB land floating.

HTH