Marshall VS100 combo noise

[jbmdisco]

Have a Marshall in at moment with 2 separate issues. It is a valvestate model.

1st problem - noise 1. On both clean and valve channels there is white noise after being switched on for about 5 minutes which I believe to be the resistors on the amp board so no real concerns with that one.

2nd problem - noise 2. On valve channels od1+2 only when volumes are pushed up to 7 and beyond there is a frequency tone (eg 1khz). When you plug a guitar in it goes away. Could this be a fault with the tube?

The white noise (1) is present whether or not an instrument is plugged in.

I had this amp in a month or so ago with the outputs fried which where replaced along with a few diodes. The amp, since then, has been used for 2-3 days a week since repair so not immediately suspecting original repair.

[g1]

In general, I don't consider problems that require nothing plugged into input & high volume settings to be valid faults. Is there an issue during normal use? It could be related to a microphonic tube as you suspect. With a guitar plugged in and guitar volume off, the 1Khz tone is not there? How about with amp volume maxed?
The other white noise issue seems more like a fault, but depends on the level of the noise. It could be a problem, or could be normal amount of noise for this model. You don't seem too concerned about it so it must be fairly low level?

[jbmdisco]

Hi g-one, thanks for reply. With instrument plugged in and vols at max i get feedback, to be expected as i work in an 8x8 room but no khz frequency noise. I will swap out the tube tomorrow and hopefully sort that out. The white noise happens in all situations hence the suspect being with the power amp side. Being a fault i've had on a few occasions usually solved by changing the power resistors. It's another job i'll 'get around to'.
Regards as always

[g1]

With instrument plugged in and vols at max i get feedback, to be expected as i work in an 8x8 room but no khz frequency noise.

Here I meant amp volumes up, guitars volumes at zero.
There are 2 switches in the input jack. When nothing is plugged in, one grounds the jacks' tip connection, one ground the grid of V1A via a cap. It is important that both those switches are closing and not dirty/oxidized when nothing is plugged into the input.

[J M Fahey]

Noise is a Reality of Life and is always present, annoying in high gain amps but that's the price you pay.

The tube is quite late in the preamp chain and actually used more as a clipper than anything else, so noise basically comes from the front end IC .
And it's probably normal.

Marshall often uses a harmless trick: an extra contact in the input jack grounding a later gain stage, so with everything unplugged amp *appears* less noisy than it actually is.

When that contact fails for whatever reason, "noise suddenly or randomly appears" leading to endless troubleshooting.

No big deal; when actually playing that jack switch is irrelevant, it's only important at the Music shop to help you choose a Marshall instead of other brand.

What G-one said by the way: follow the lower switch in Js1 input jack (S)leeve contact until 100n cap/Lk12 link (they might have fitted any) which grounds V1 grid AG .
This mutes at least 2 gain stages, one of them very high gain, and probably the effects loop.

[g1]

follow the lower switch in Js1 input jack (S)leeve contact until 100n cap/Lk12 link (they might have fitted any) which grounds V1 grid AG .
This mutes at least 2 gain stages, one of them very high gain, and probably the effects loop.

Yes it does mute the effects loop. This is one of the silly Marshalls that you can not use the FX return unless you have something plugged into the input. Extremely frustrating for trouble shooting, and hard to see on that type of fragmented schematic.

[jbmdisco]

I have sorted out the khz/frequency noise. I had a poke about the input jack and cleaned with no difference but when pressing the tip connection to 'force' a good contact the noise disappeared so a new socket was fitted. Thanks guys for this guidance.
The white noise I described is coming from the pre-amp stage, after eliminating the power amp. The noise is likened to a radio between channels - white noise with crackles + pops. This comes through both clean and both o/d channels, hence my original suspicion being a fault in the power amp. So tomorrows job is to scope backwards from the pre-amp output.
Despite the noise the amp actually functions very well which leads me to believe something reasonable simple is to blame.

[J M Fahey]

White noise is normal and unavoidable.

Now pops and cracks are not, so if you eliminate these consider yourself happy.

There is a kind of noise called popcorn (scientific name, not kidding) which is typical of SS stuff and often appears on damaged junctions, think somebody connected a speaker output to the input jack.

Burst noise - Wikipedia, the free encyclopedia

The Transistor (or IC) may survive and still work, but present this noise.

So perhaps you can carefully replace the first Op Amp IC4ab, which to boot is common to all channels (hint hint).

That is one of the few places where one of the new, fancy, expensive (U$2.89) premium Op Amps may prove worth using.

[jbmdisco]

Have now swapped out ic4 with some good results. But the noise has remained within the tube drive circuits.
I connected scope to input with khz signal and to the 'emulated' line out on the rear panel. the only noticable difference was that the output signal was approx 1/4 of a wave out but no sign of any interference.
Next I ran a line from the line out to slave amp. the noise returned but again only on the tube channel. I then ran the 2nd scope channel to the effects send. As soon as this was plugged in the noise completely disappeared. So cleaned the contacts of this socket and tried again. All at this point seemed great with a 'as quiet as it could be' amp.
Left switched on for about 1/2 an hour and then gradually the noise resurfaced. By now I know that the amp is laughing at me
But as a sort of result the noise is no longer on the clean channel but remains within the tube driven channels. Being a Sunday with plenty of time to myself I persevered and went back to the basics checking resistors and caps. I identified 1 little cap 470uf 35v which seems to be faulty. This cap reads +11vdc at both ends. This will be replaced tomorrow and hopefully resolve the noisy issue.

[g1]

That cap would have to be in the tube channel to cause the problem. What is it's designation on the schematic?

When you connected to the FX send and the noise was killed, I think any signal would have also been killed. So that is probably a false indicator. Just to be sure, try playing through the amp, then plug something into the FX send. Do you still hear your guitar?