Marshall 5210 thunderstorm problems

[booj]

The amp works, but has an annoying "thunderstorm noise" type static that is apparently coming from somewhaere close to the master volume and reverb control circuitry. If I turn the master down the noise goes away completely. If I turn the reverb control up the noise gets brighter. I've had problems like this that I solved with replacing coupling caps. That hasn't worked so far. I've re-flowed the solder on all the controls, power supply caps and emitter resistors as well as changing the caps that are circled in the scat. By the way, for some reason I can't make colored circles using paint brush for this drawing (???) I've included a cleaner scat of a slightly different model that is cleaner to see the parts values better. I'm getting frustrated and could use some "Did you try this?" type comments. I'm out of ideas.

[g1]

Did you try... freeze spray on the IC's and small signal transistors (preamp)?

[booj]

Hey G-one.. Ran out of the stuff. I did try a heat gun to warm things up. No effect. Then tried dabbing alcohol on the hot parts. Made one of the chips stop working. Replaced. No effect. Re-flowed entire circuit board. Nothing. I've changed about 20 signal and small electrolytics so far. No change. I've got new filter caps on order. Will also order some more freeze spray.
Oh.. I was thinking if all of the controls are turned down except for the master control would that indicate the problem is AFTER the tone circuit?

[nickb]

booj,

Any difference between boost and normal channels? How does changing the tone controls on the two channels affect things? Is this a steady rushing sound or more of a popcorn kind of thing?
Oh and get some more freezer spray, saves lots of time!

Nic

[booj]

I haven't tried switching channels. All the controls are at zero except the master and the reverb. It's a "windy day with lightning strikes all around" kind of noise. the sound gets brighter with the reverb control up. None of the other controls make a difference.

[nickb]

If the channel volumes are at zero, then you are left with gain stage of IC1a and the FET switches. Try lifting R10 4.7K. If that makes no difference try lifting C11 too. That will narrow it down to one of the FET switch stages or the IC1a buffer stage.\

Nic

[booj]

I dig what you're saying nickb. I'll post the results.

[Enzo]

Have you scoped to see the noise signal? Look on the amp output to see what it looks like, then look for it at lower levels through the amp. In my experience caps are rarely the source of such noise, it is almost always a semiconductor.

You can follow the signal path watching for the noise, you can also ground one end of a largish cap (a 0.1uf or something) and use its other end to "ground" points in the circuit, moving back until you pass the point of having effect. The cap prevents any DC upsets, but any signal will be either shunted off entirely or seriously muffled, which is just as good when looking for the source of a noise.

[booj]

fixed it. Lifted one end of R10 and the noise stopped. Replaced C12, a tantalum cap. Put R10 back down and tried it. No noise! Thanks Nic!
And Enzo, I tried scoping it out and could see the noise, but when I got to the input stage of say, IC1a, I was at the threshold of what the scope could see, and I couldn't nail it down.
G-one, I got some freezer spray. Replaced few parts that made an "I'm being freezer sprayed" noise, but no results from that. I have lots of freezer spray now though!