Marshall VS100 - strange noise when using reverb

[Davidnhs]

Hello guys,

i'm new here and i hope you can help me out with my problem:

My Marshall VS100 Combo suddenly has pretty loud noise when i engage the Reverb.
The noise reminds me of the noise you get when you hold your phone near your amp, its not a hum or something, its more like the amp is picking up some sort of radio signal or something via the Reverb circuit.

The Amp has two channels, clean and overdrive as well as two reverb knobs for either of the two channels.
The problem occurs on both of them.
Both channels have their own volume knob. They have no effect on the volume of the noise.
Even with the volume on zero i can still control the volume of the noise by turning up the reverb knob.
The problem also occurs when nothing is plugged in.
I also disconnected the reverb tank but the problem still remains. This lets the think that the cause of the problem needs to be somewhere in the reverb signal path after the return from the reverb tank.
I already checked for loose components but didn't find any.
I also checked for bad soldering spots and found a few. I resoldered them but the problem still remains.
With the reverb all the down the amp works just fine.


here are the schematics for the amp:
https://www.thetubestore.com/lib/the...-Schematic.pdf

Do you guys have any idea about what could be causing the problem?
If you need any more information, pictures or anything, just let me know!

Greetings from Germany,
David

[nickb]

Does the reverb work normally aside from the noise?

Unplug the tank and short out the red phono (RCA) . Does the problem go away? If yes, try a new set of leads. If no check the connections to and/ot try replacing it.

[Mick Bailey]

I think either a bad connection or the reverb output coil is open-circuit - very common, or one of the internal connecting leads inside the tank is broken (usually one of the green ones - also common). Nick, if the problem goes away with the red plug shorted, then this suggests the lead is good.

When I get a reverb problem I always pull the red lead and stick a finger on it - a buzz means the recovery circuit is amplifying. I always twist/pull the connectors a few times to get a better contact to see if this is the problem. Next step is to remove the tank and check the resistance of the coils - about 30 Ohms input and 220 ohms output. 99% of reverb troubles are down to a faulty tray. Sometimes an output coil will go high resistance but still give a small amount of reverb, but very noisy. I rewind lots of coils (2 already this week). Driver coils rarely fail, other than the green/black lead coming off.

[Davidnhs]

First of all thanks for you answers!

I tried a new cable but it didnt change anything.
I also changed every M5201 and LM348N IC. Still nothing changed though.
Since the problem is the same when the tank is unplugged, doesnt that mean its cause has to be after the tank?
What could it be? A filter capacitor maybe?

[Mick Bailey]

If you unplug the red cable and short it out, what happens? Also, did you check the output resistance of the tray? An open-circuit output coil will give the same problem with noise whether or not the tray is plugged in.

[Davidnhs]

Im not sure what exactly you mean. Do i unplug it from the tank and short the outer contakt with the inner contact? And the black one stays the way it was?

[Mick Bailey]

Yes.

[Davidnhs]

I tried that, but it doesnt have any effect on the noise. Its not a normal buzz or hum eiter, it sounds like my amp is picking up some digital message or something.

[Mick Bailey]

If you touch a finger on the centre pin of the connector does it buzz? Just trying to establish if you're getting any signal at all through the reverb recovery stages.

[Davidnhs]

Yes then it hums. Also the Reverb does work just fine with the exception that there is this unusual noise added to the signal...

[Davidnhs]

Whats also interesting is that when i take out IC1 (LM348N), which is the first ic after the return of the reverb tank cable, the noise remains the same! Could it be one of the caps in that area? Maybe C13, C16, C17ß

[Davidnhs]

Marshall-VS100R-100W-Schematic.pdfMarshall-VS100R-100W-Schematic.pdf I tried to draw out the signaal path. The problem must be somewhere in the green or yellow area i think.

[Mick Bailey]

Well, pulling the IC1 pretty much eliminates everything before it. Would you just confirm that the reverb pot is the dual 10K VR2/3?

I'm thinking the problem may lie with C17 or C18.

Because the noise goes away with the reverb on zero, then it has to be before the reverb pots. Is IC1 socketed?

EDIT: Forget my question regarding the pot, that's just added to the confusion -I can see how it works. Also, because your problem lies with both channels I now think the problem is before C17/C18. Make sure the connector CON1 is making good contact.

[Davidnhs]

Yes the Reverb pots are VR2/3 10K.

But can it really be C17 C18? I am wondering because of the following:

The IC2 (M5201) which is the first IC after both reverb pots, it has two pairs of inputs. One for the clean channel and one for the overdrive channel. The IC switches between them according to whats on pin one the switching input if i understand correctly.
If either C17 or C18 was bad, wouldn't the problem be only on one of the two channels?

And yes i pulled every IC and socketed them.

[Mick Bailey]

See my previous edit.

I would pull IC1 and ground pin 14 to see if that kills the noise.