Rumbling Marshall 5210 combo, HELP !!!

[sebgreen]

Hi,
I own an old Marshall solid state 5210 amp. For a few months it has started popping and crackling after about 15 mins playing time.It sounds a bit like wind blowing. The loudness of the rumbling is only influenced by the master volume and is present on both the clean and boost channels. I have tried changing some oxidised capacitors but to no avail.
Here's the URL to a schematic if it helps: http://www.drtube.com/schematics/marshall/5210.gif

Any advice ( exept buy a new amp cause I've got no money ) would be most welcome.

Cheers
Seb

[Enzo]

Probably a noisy semiconductor. If the master volume turns it down, then the problem is before that point. Do ANY of the other controls affect the sound in ANY way? Any that do are after the noise source. I'd wager on one of the couple op amp ICs or the couple of JFETs being noisy.

[sebgreen]

Hi, only the master volume affects it so it's before the pot. The wierd thing is it stopped doing it for about four months and started again on friday.
I'm not experienced enough to be able to understand the schematic but someone told me there isn't much before the master volume. The main problem is finding out which component is crapping out, and I live in France so amp repair is hideously expensive.
Cheers
Seb

[J M Fahey]

Hi SebGreen.
Try to find a friend who at least can solder and read a schematic, even if he's not specifically into guitar electronics, and we'll suggest him a couple tests. Offer him the usual Pizza, Beer, or introducing some of your hot girl friends to him. Your problem *can* be repaired, the parts themselves are cheap, the problem is finding *what* is causing it. Good luck.

[sebgreen]

I know how to test an op amp but I aint got a clue how to test a JFET. Thing is I don't really want to be taking out various components to test them. I've already changed a couple of oxdidised caps cause when I first started to have this problem I just opened it up and looked for anything dodgy looking.

[J M Fahey]

You don't have to pull anything, because you can't test noise on a component on its own.
I'll suggest some tests with the amp on, but you must supply somebody who can easily navigate the schematic and the actual board which is quite different. The schematic is not the layout, far from it.

[Enzo]

ANd the VAST majority of bad parts do not LOOK any different.

Frankly I'd replace U1 and see what happens. There is one JFET in each channel, unlikely both have the same problem, but even so, if you suspect one, just replace it, they are not expensive.

The input signal in this amp splits right after the jack and goes to the two channel, then the output of the channels are mixed together into one half of U1, just prior to the master volume. There is very little common to both preamp channels before the master volume control.

[sebgreen]

Hi, I just bought a tremolo pedal and put it in the effects loop. I realised when the noise started that the trem affects it.This must mean that the noisy component is on the preamp side of the effects loop but after all preamp controls. Is that possible ?
Cheers,
Seb

[sebgreen]

Hi could you plz indicate where U1 is cause I can't figure out where it is on the schematic.(what component it is) The writing is incomprehensible...
I'm probably going to get a new amp soon and i'll try replacing that one i've got it.
Thx
Seb

[J M Fahey]

Hi sebgreen.
I'm quite sure that U1 means IC1a ; besides it's the only active component after most controls but before the master volume and Loop Jacks.
What Enzo suggests is that there is a Fet (Qi) injecting its output into IC1a , so he finds it somewhat unlikely, but not impossible.
Of course there might be *mechanical* problems there, such as cracked solder, rusty/worn contacts, etc.
I personally find these random occurring problems nastier than burnt/smoking parts and similar stuff.
When somebody enters my shop and says " weeeeelll , it *does* work, but ..... " I'm sure that but will mean a pain in the butt

[olddawg]

You might try tickling the suspected components with a little freeze mist. Could be a jfet, an op amp, a bad small cap, even a failing resistor or a bad solder joint. withouut a scope you may have to isolate an area and shotgun the parts.

[Enzo]

Very sorry. If the schematic is calling the intergrated circuits IC1, IC2, etc, I regret using the alternative style U1, U2 etc. SChematics are common in both styles, I can see how that would be confusing. I meant of course IC1.

[Chris C]

-
hey guys I'm having the same issue with my Marshall 5212 Twin Split Reverb. Anybody find a solution to the problem? Seriously. It sounds like my amp is on the top of a mountain in a tree, blowing through the arctic.
I'm guessing it's a leaky cap, I have the schematic and I study Engineering in school so I can read circuit diagrams (yeeeah!) I just don't know which cap to replace. I'm guessing its in the power section.

[Chris C]

There actually is no capacitor at "C14" on the board if you look at the left of the circuit board itself. There is however a wire on the underside of the board connecting one lead of C14 to one lead of R20, which is a 1K resistor. I was thinking maybe I can start by changing these components but I cannot read what C14 is on the schematic...tough to make out. It looks like "478p" on the schematic which could mean a .47 pico farad cap.... The fact that there is nothing physically there doesn't help... It is right next to C13 which is a 330 micro farad cap but I'd rather not guess and blow out my amp .__.

[Enzo]

His problem was noise that came up after warm up. Channel selection made no difference, but master volume could turn the noise off. And in fact the noise was present at the FX send. You have the IDENTICAL case?




As I and others mentioned in earlier posts, it is more likely a noisy semiconductor than a leaky cap. And if the volume controls turns it up and down, it won;t be a power amp problem. Or by power section, do you mean the power supply? Even then I doubt it is the supply, since that would ignore the volume setting.

I don;t know why you would change C14, but 470p means 470 picofarads, not 0.47pf.

What you need to do is isolate the problem. Any control that affects the noise in any way is after the noise source. COntrols that do not affect the noise at all are either before the source or not in the signal path. For noise the same in both channels and affected only by the master volume, IC1 is the prime suspect.


There are more than one versionj of this amp, and some part numbers are different. Here is an alternative, make sure you are using the right drawing.

http://www.webphix.com/schematic%20h...s/50w_5210.pdf