Marshall VS100 clean problem

[yanyan39]

Hi I have a Marshall vs100, It's been noisy and unreliable for a while. I've taken it apart and found a capacitor and input jack with multiple cold joints on the preamp board so have fixed those. The amp is now much improved, but there remains a problem on the clean channel, As the volume is increased there is nasty breakup which responds to louder playing. Both distortion chanels are fine so i don't think it's the power amp. I have the schematic and was wondering if this problem could be a faulty TL072 chip in the preamp, any ideas how best to proceed with sorting this. I'm pretty sure there are no more bad joints on the preamp board and I've used contact cleaner and all plugs, sockets, pots and jacks.

thanks for any help.

[drewl]

Does sound like a bad opamp.
Is one of them getting warmer than the others?
Measure the voltage on the pins of the TL072's. Pins 1 and 7 are outputs for the two amps in that chip, is there a high dc voltage on any of the outputs pin 1 & 7?

[yanyan39]

Thanks for that, I'll check it out

[Amp Kat]

You may want to also take the send from the preamp to the return of another amp just to verify the preamp is indeed the culprit. The power supply in those amps can cause some issues as that circuit isn't the best design and can get unstable easily. Make sure the tube retainer isn't digging into the circuit board also.

[yanyan39]

Sorted it out, the culprit was a mn5102 opamp wjch controls the chanell switching, swapped it out all working well now. replaced it with a NJM2121D which is quite an improvement.

[brstma]

I am having a similar issue with a VS100 I just got used. It only happens on the clean channel. I first assumed it was the volume pot so I replaced it only to find the problem still exists. It works fine until it gets to 3.5, then there is a pop sound, scratching noises as the knob is turned, and the volume gets really low and doesn't change as the knob is turned. This happens from 3.5 up to 9, then at 9 it pops again and works fine from 9-10. I also notice if the power dimension switch is out there is a loud hum present only while in the 3.5 - 9 range, but if the switch is in the hum is not there.

I have very basic knowledge of electronics but I'm not willing to invest a lot fixing it so I'm hoping someone can give me enough advice I can repair it myself.

Thanks in advance

[yanyan39]

I am having a similar issue with a VS100 I just got used. It only happens on the clean channel. I first assumed it was the volume pot so I replaced it only to find the problem still exists. It works fine until it gets to 3.5, then there is a pop sound, scratching noises as the knob is turned, and the volume gets really low and doesn't change as the knob is turned. This happens from 3.5 up to 9, then at 9 it pops again and works fine from 9-10. I also notice if the power dimension switch is out there is a loud hum present only while in the 3.5 - 9 range, but if the switch is in the hum is not there.

I have very basic knowledge of electronics but I'm not willing to invest a lot fixing it so I'm hoping someone can give me enough advice I can repair it myself.

Thanks in advance

I found that there were loads of bad solder joints on my amp. Some were obvious, some invisible. I think that 90% of my succesful repair was from resoldering joints. It could be that the opamp for the first stage of the preamp is bad, (or has a bad joint) I'd try reflowing any joint that seems at all dodgy, I identified bad joints by tapping on the components with a piece of plastic while the amp was running, the worst offenders caused a clear bump or click noise through the speaker.

[brstma]

I found that there were loads of bad solder joints on my amp. Some were obvious, some invisible. I think that 90% of my succesful repair was from resoldering joints. It could be that the opamp for the first stage of the preamp is bad, (or has a bad joint) I'd try reflowing any joint that seems at all dodgy, I identified bad joints by tapping on the components with a piece of plastic while the amp was running, the worst offenders caused a clear bump or click noise through the speaker.

I don't see any obvious cold solder joints. I did spend some time tapping around and I think it may be the same opamp (IC2 5201A ?) When I tap the pins while the volume is in the 3.5 - 9 range the noise from the speaker is noticeably louder. I tried the same thing with all the opamps with each channel selected and I only get the noise from the IC2 opamp while clean channel is on and only when the volume is in the 3.5 - 9 range.

Another thing I noticed is moving the effects level knob will also make noise only when the clean channel is selected and only when the volume knob is in the 3.5 - 9 range, and it does it with the effects loop switch on or off. The noise is much quieter than the noise from moving the volume knob but can be heard

[g1]

If you have trouble finding a 5201 IC, you can use NJM2120D or JRC2120D.

[brstma]

The op amp wasn't the problem either, not sure where to go from here.

[DrGonz78]

This may sound silly, but still a logical question. Have you tested this volume pot to see if it might just be going bad?

[brstma]

This may sound silly, but still a logical question. Have you tested this volume pot to see if it might just be going bad?

That's the first thing I replaced, second was the op amp on the clean channel. I don't see any bad solder points or obviously bad components. I tested a few diodes and got mixed results but I didn't remove them from the board so that may be why.

To sum up:
-it only happens on the clean channel
-it only happens when the volume knob is between 3.5 - 9. Setting it at 0-3.5 and 9-10 works fine.
-I'm getting two different symptoms depending on whether the power dimension button is in or out:
1) with power dimension out I hear a pop at 3.5 and 9, there are scratching noises when the knob is turned between 3.5-9, there is a loud hum present when knob is at 3.5-9, and the volume works as it should.
2) with the power dimension in I hear a pop at 3.5 and 9, there are scratching noises when the knob is turned between 3.5-9, there is no loud hum present, and the volume drops significantly when in the 3.5-9 range. Adjusting the volume in the 3.5-9 range does change the level but it's very quiet.

Any ideas on what could cause such an issue? I've only been suspecting the preamp circuit board so far since it doesn't happen on the two distortion channels http://elektrotanya.com/PREVIEWS/634...100w.pdf_1.png

[brstma]

Anyone have an idea of what could cause this issue?

[Jazz P Bass]

Assuming that you do not have an oscope, you could use your volt meter to try & find where the noise is originating.
Marshall schematics are lacking in the fact that the controls are not labeled.
So looking at the schematic is a lesson in futility without the amp.
I would get on each opamp output pin (1 & 7) & try to 'see' the pop with a meter.
Once you identify the output you can work backwards to the source.