Marshall vs232 making strange noise. Please help

[jon.zilla]

I am working on a marshall vs 232 stereo chorus 2x12 all solid state amp.  Can't figure out the source of this strange noise.  I am attatching a zip file with a recording of the sound.  Never heard anything like this before.

[gabefauver]

your sound file isn't working please re upload it.

[Enzo]

I can't play sounds at all presently anyway.  perhaps you can describe what it is doing?


And isolate the problem.   Does the sound come out with the controls all at zero?  In fact do ANY of the controls have ANY effect on this sound?

[jon.zilla]

Well with all controls set to zero it produces a sound that is a bit like if you were to hold down a button on a touch tone phone.  It increases with volume, does it regardless of which channel, with or without a guitar plugged in.  If you even barely turn up the reverb control knob the volume of the "tone" gets so loud it is almost unbearable.  Yet you can still plug in an instrument and it plays fine just with an equally loud "tone".  When amp is powered off the sound kind of dies out with a warbled? sound? dont know how to better describe it.

[joecool85]

your sound file isn't working please re upload it.

Agreed.  It seems you've uploaded a zipped version of an empty .wav file.

Not entirely certain on your sound issue, but have a good guess.  I'm thinking it is bad power supply caps.

[jon.zilla]

ok here is the sound file, this one works.  the recording is with mic next to cabinet, power switch cycled on for ten seconds and back off all controls set to zero.

[gabefauver]

Wow, I have never heard one make that noise before. .....interesting.

[phatt]

Ok I had a listern to your prob,,, If it has a spring rev tank pull it out, completly disconnect it. The tank will likely be fine but the circuitry running it may have gone futt. See if that helps,, Phil.

[J M Fahey]

Hi.
I agree with Phatt, and think your reverb circuit is oscillating.
*Maybe* one ground wire leading to your tank opened, and the circuit is howling.
After all, a reverb circuit is made of a "power amp" (the driver) and a "mic preamp" on the other end.

[phatt]

JMF, just gave me another thought, You may need to short circuit the *Input/Return* Socket to stop the howling. I had that issue when designing my maxiverb unit.
And since then I've noticed that some Amps squeal if the return pickup rca falls out or goes open circuit. These are usually just RCA type plugs.
don't short the *Output socket/Driver side* on the Amp as that may cause more problems.
Phil.

[jon.zilla]

So you are talking about the rca jacks that go to the reverb?  If so the return one I need to connect a shorted plug to ie: inner conductor to outer shield?  Just wanted to clarify before attempting it.

[jon.zilla]

Also disconnecting the reverb completely did not change anything.

[guitarpicker7]

Sounds like a headphone connector gone bad- when you plug in the HPs, a lot happens! IF one of the leaf switchs inside the 1/4" connector got bent, you sometimes have a feed-back loop just waiting to go crazy.

Try the amp with some headphones plugged in- it might help lead you to an answer.

I didn't find the exact schematic but Schematic Heaven Dot Com has the VS230R, which might help.

http://schematicheaven.com/marshallamps/vs230r_2x30w_stereochorusrev.pdf

[phatt]

So you are talking about the rca jacks that go to the reverb?  If so the return one I need to connect a shorted plug to ie: inner conductor to outer shield?  Just wanted to clarify before attempting it.

YES, one RCA *Sends* the signal the other *Picks up* the reverb sound.
It can get confusing I know but as with most audio stuff you get a mental picture of something going *Out* has to connect with something going *In*.
So looking at the back panel of the Amp chassis there should be one RCA sending the signal to the reverb tank and the other one is picking up the returning signal. Try shorting out *The Return RCA* socket.

I doubt shorting the *Send* will do harm but I have no circuit to work with.

Phil.

[J M Fahey]

Hi.  Agreeing and clarifying (I hope).
The reverb circuit is made up of a small power amp driving a "speaker" , placed at the end of a long, tiled corridor "the spring" which provides the reverb (delayed and back-and-forward bouncing sound), which is picked out on the other end by a "microphone".
This sound is amplified by a mic preamp and mixed again with the original signal.
The mic volume is the reverb control. Bright and simple idea, huh?
The send signal goes through the "out" RCA jack, and the mic signal comes back through the other one.
As anybody knows and has experienced, the mic inputs are troublesome, prone to picking noise, radio, hum, feedback, howling, the works.
As Phatt suspects, you lost your ground there which causes all of the above symptoms in varying degrees.
You may have lost ground or have an open live wire *anywhere* along the path, so to check it, start by shorting the return RCA that plugs into your reverb tank.
The howling/oscillation should stop.
If not, follow the wiring until it physically connects to the board.
I don't remember it by heart now, having repaired so many amps , but I distinctly remember that Laneys have a couple red and white  RCA connectors at the board itself and *think* that Marshalls don't (Imight be mistaking them with Fenders), rather they solder the two grey cables straight into two pins at the PCB, please post a picture of that.
2 possibilities:
a) you have 2 RCA jacks straight in the board: plug a shorted (hot to ground) RCA plug into the return one. The howling should stop.
b) you have 2 "pins" as return connection (you should have 2 others as "send"). Carefully, trying not to melt everything, tack-solder a piece of wire shorting them . The howling should stop.
c) also check with good light and good glasses or a loupe the PCB around those connections, you may have a broken track which amounts to an open circuit, or some cold solder joint.
Maybe somebody pulled those reverb wires and cracked something inside.
Take a couple pictures of that area.
d) Nothing works.
Consider shaving your head, buying a saffron orange tunic, and becoming a Tibetan Monk. There are many important things in our life besides Music.
Good luck.
Juan Manuel Fahey.