Valvestate Chorus farts

[drewl]

Well today we have a Marshall Valvestate VS230 making a farting noise, like when you have your cell phone next to an amp.

It's coming from the chorus, as the continuous fart sound matches the oscillator speed.

Pretty sure it's coming from IC7, MN3101. Thought I had an extra to try, but can't find it.

Probing around D3, D4, R50 and C31 changes the fart to a whoosh.
Increasing C31 to like 270pf just about kills the noise but makes the chorus too warbly.

Any other ideas to verify IC7 as the problem?

Pulling it kills the noise, killing the oscillator kills the noise.
Kill the brain, you kill the ghoul.

[drewl]

Just want to add that it makes the noise with chorus on or off.

Chorus switches on/off fine.

[g1]

C30 on the bias pin is good? VCC on pin1 is clean?

[Jon Snell]

It is not going to be IC7. IC7 is the modulator from the low frequency oscillator.
It is more likely to be a bias issue from the junction of R37 & R66. Ensure it is at 7.5v and the emiter follower sets up a 7v DC bias for the MN3101.
This chip is rarely at fault.

[Enzo]

The 3101 is the clock chip driving the 3007 bucket brigade chip along. The 3007 is in the signal path.

[drewl]

Bias pin 8 of IC7 is only 1 volt.
junction of R66 and R37 is 7.5v

Vcc +/-15v

[g1]

Bias pin of those 2 IC's are shown connected together and nothing else. I don't see how those resistors or TR1 are involved with the bias. Am I missing something?

[drewl]

Hmm, turns out C34 was the culprit.

Chorus works fine/no noise on or off.

Bias still only 1volt

[g1]

Bias still only 1volt

That's right. When VCC is positive with respect to VDD (pin3), bias will be output at 1/15th of VCC.

Faulty C34 will have been throwing off T3 input of MN3101.

[drewl]

DAFUQ????

Was working fine, put it back in box and noise is back, wtf?

I'm gonna' replace every damn cap in the chorus circuit.

Another thing is this amp has a high frequency oscillation of some sort on the rails and chassis.

[Enzo]

BY the way, I believe you will find this circuit runs between +15 and ground rails, no -15 connections.

I don't see, did you check what Jon pointed out in post #4? There is the BIAS pin on the ICs, that is separate., but the whole circuit is biased to +7.5v by the two resistors. R37, R66.

[drewl]

Yes I did check the 7.5v bias circuit.

[g1]

With the intermittent nature of the fault, and the HF oscillation issues, you could have a solder issue or maybe a grounding problem. Both of could come and go at various points of disassembly/assembly.

[drewl]

Well after some research, it seems this chorus noise is a fairly common problem.

One guy came up with a solution.
Soldering a .1uf cap across the chorus chips IC 4 & 7 pins vcc to ground kills the oscillation.

Sure enough it works.

[g1]

Well after some research, it seems this chorus noise is a fairly common problem.

One guy came up with a solution.
Soldering a .1uf cap across the chorus chips IC 4 & 7 pins vcc to ground kills the oscillation.

Sure enough it works.

That's interesting. I guess the ones with the issue did it since new? If not, C17 is also from vcc to ground and is a possible suspect.