Marshall vs232 making strange noise. Please help

[jon.zilla]

[phatt]

Hi jon.zilla,
               Have you tried changing the cap for another?
Do that before you start off re engineering the rev circuit. :tu:

It does not even have to be the same Value, I think it was 100uF so even 10uF would be fine as long as it's the same voltage rating.
In fact there can be benifit from less bass in the reverb path.
Phil.

[J M Fahey]

Hi jonzilla.
Nice picture, it shows C26 *might* be related or not to the reverb cirduit, it certainly lives close to it, but I still donot know what it does.
Can you please post or refer to some schematic and point to it?
It might still be called C26 or not; the important thing is what it does.
Thanks.

[phatt]

Hi jonzilla.
Nice picture, it shows C26 *might* be related or not to the reverb cirduit, it certainly lives close to it, but I still donot know what it does.
Can you please post or refer to some schematic and point to it?
It might still be called C26 or not; the important thing is what it does.
Thanks.

Hello Mr Fahey,
                    Link to schem is on page one 13 down,, posted by *guitarpicker7* but here it is again.
http://schematicheaven.com/marshallamps/vs230r_2x30w_stereochorusrev.pdf
I assume it's close enough.
C26 decouples the Revpickup amp to the rev depth control pot.

Having built/tested just about every possible way to wire up a rev pickup circuit I've found that some of these reverb pickup designs are not to flash and can do some strange things under certain circumstances.

With some you only need a broken/open circuit rca cable and they squell like crazy.
To my understanding C26 is back to front (polarity) but you may know more about that?
Cheers Phil.

[jon.zilla]

Well I replaced c26 with a new capacitor of the same value, and replaced the other three on the board that were also the same value (only because I got a 100ct box from mouser).  Still noise persists unless I pull c26.

[phatt]

Have you got a 10 uF?
or even 1uF.

In fact anything bipolar or big greencap say .22uF.

The idea here is to see if you can get a signal through without the bad hum.
Something is leaking/bleeding through at low freq and wiping excess bandwidth may fix or at least norrow down the problem.

You may have to rebuild the rev pickup circuit.
Mentime check the tank and make sure it's CASE is grounded at the Pickup RCA.
and NO ground at the DRIVER RCA.

Phil.

[J M Fahey]

Well, *now* we can all talk about the same, *if* that schematic applies and C26 on the schematic is C26 on jonzilla's amplifier.
*If* that's so, the poor cap is all right, it's doing its job of coupling the beautiful sinewave generated in the audio oscillator so kindly provided by Marshall to test ..... our patience.
On a very wild hunch: lift one end of R40, that 220K resistor, *if* the numbers match between your amp and the earlier 230 .
If they don't, find it on the board, that is the 220K resistor that goes from the junction of C22/C23 to the junction (called node) of R42/R43.
Your oscillator should stop.
You *may* still have hum/noise problems, but those are conventional.
Phil: C26 is a polarized (cheap) electrolytic, and in theory you should have a bipolar (expensive) there; but ages of SS amp building have proved that polarized ones work. Conventionally we use + towards the active device, but in this case it's the same.

[phatt]

Well, *now* we can all talk about the same, *if* that schematic applies and C26 on the schematic is C26 on jonzilla's amplifier.
*If* that's so, the poor cap is all right, it's doing its job of coupling the beautiful sinewave generated in the audio oscillator so kindly provided by Marshall to test ..... our patience.
On a very wild hunch: lift one end of R40, that 220K resistor, *if* the numbers match between your amp and the earlier 230 .
If they don't, find it on the board, that is the 220K resistor that goes from the junction of C22/C23 to the junction (called node) of R42/R43.
Your oscillator should stop.
You *may* still have hum/noise problems, but those are conventional.
Phil: C26 is a polarized (cheap) electrolytic, and in theory you should have a bipolar (expensive) there; but ages of SS amp building have proved that polarized ones work. Conventionally we use + towards the active device, but in this case it's the same.

Mr Fahey,,, you are a Gem!

You have the knowledge,, I only have hunches
I did see that 220k R and head scratched because I had not seen that input used before.
It will be interesting to hear the results from Jonzilla.
Thanks for the crarification on using electro's for decoupling.
Phil

[jon.zilla]

ok, pulled one side of r40 and reinstalled new cap in c26 now the high frequency oscillation is completely gone, but turning up the reverb level creates distortion/static sound.

[phatt]

 Some success after all this time, you're doing well :tu:

Try changing *R41 220k* down to 22k.
R43 may also need tweaking try 47k or just in stall a 100k trimmer pot and be done with it.
Other options C24 can be reduced,, try 100nF,  I'd also look at C25 try 1nF.

This will alter the bandwidth of the rev,, so it may take time to perfect it.
Cheers Phil.

[J M Fahey]

Hi jonzilla.
My first hunch was right, now we go on; I'll ask you do a couple of tests .
Not all at the same time, but either 1, 2, 3 or 4.
Post results
with a short piece of wire
1) short across R41 (Phatt was going into that direction, I'm going a little further) *or*
2) lift one pin of C23 *or*
3) lift one pin of either R39 or C24 (it's the same) *or*
4) short across C25.
Post results.
All those actions will kill reverb, one or more will also kill the noise.
See you tomorrow night.

[DARKMAN23]

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.

JUST WANT TO SAY A BIG THANK YOU!!! TO PHATT...I'm new here but with his advice I got my amp repired.  GOOD MAN YOURSELF!!!  KEEP THE GOOD JOB