Marshall vs232 making strange noise. Please help

[jon.zilla]

Thanks for all the help I will try it right away.

[jon.zilla]

Ok, shorting the reverb return did not work.  Here are some pics of the board.

[phatt]

Oh dear!

Well check for broken tracks and solder joins especally on the PCB mounted RCAs.
The *RED RCA* short it right on the board.
(Rev circuit is all at the top left corner.)

My other thoughts are;
IC, 1 is blown, as these current drive reverbs using stacked opamps can leed to an early death.

Possibility that chorus or delay circuit has gone belly up,, never had that happen but something is definitly osscilating.
I don't know how good you are with electronics but if you have the schematic that *Guitar picker * kindly posted then check all the opamps voltage pins and see if something is wrong in the supply.
Phil.

[jon.zilla]

Ok how would I go about testing the opamps voltage pins? I have the schematic.

[phatt]

Before you go much further,,Un solder *C26* (1uF /100V) It hangs off pin 14 of IC1d. That will totally cut the reverb return from the audio path. if the amp then works ok,, replace IC1. ( I still have a strong feeling it's the reverb circuit.)

Next find ZD1 and ZD2 (top left corner page 2) Establish the voltage between ground and the +/- supply rails for the preamp. (they should read close to 15+ 15-)

Now follow all the rails around the board not all are the same pinouts, most of the power pins are noted on the schematic. The idea is to make sure power is getting to all the opamps.
(Note the chorus has a seperate positive supply)

If no luck then pull *Con 1*
which should isolate the power amps from *ALL* the preamp circuits.

Inserting some kind of line audio (CD line out or similar source) onto pins 3 and 5 should give the poweramps full power. (pin 4 is common /ground for both amps)
That will establish that the power stages are ok.
(Don't short out pins 1 and 2)

Bear in mind that the schematic is a different model so it may not be exactly the same,, use your head.
Phil.

[J M Fahey]

Clap clap clap clap.
Bravo !!!
A flawless performance.
In this case, incredibly accurate and detailed service explanations.
Congratulations, Phil  :tu:
Jon:zilla: you couldn't have gotten a better coach !! 
Anyway, life is life, it still *might* be something else, but I see you two very motivated.
Good luck.

[Brymus]

Be sure to throw Phatt some chip points if he was helpful to you  :tu:

[phatt]

Hi JMF, Thanks for the vote of confidence, My brain was switched on I guess, lol.
Brymus, Thanks for the thought also, maybe I don't deserve to much credit as I just made a complete stuffup drawing a simple diode circuit.  :duh
Phil.

[jon.zilla]

Ok, pulling "C26" fixed the noise!!!!! Now "IC1" is a "TLO74CN" , are there any compatable alternative replacements? Having a hard time finding this part.  Thanks for everything you guys.

[J M Fahey]

"TLO74CN" , are there any compatable alternative replacements? Having a hard time finding this part. 

You *will* have a hard time finding a "tee el oh seven four" part, but you will easily find a "tee el zero seven four" quad op amp IC. 8|
Although lazy typists often use letter O for number 0 and get away with it, here it is not the case.
The important part of the name is TL074 ; the letters that come later (C, CP, etc.) refer to temperature ratings, case, etc., which are not that determinant here.
Be sure to get the most common DIP or PDIP size, like the one you have, not the sub-miniature SMT part.
Be careful when pulling it to avoid damaging the board. You might order some solder wick, it works wonders.

[jon.zilla]

Ok I replaced IC1 (TL074) and resoldered the capacitor back in place (c26) and the noise came back.  Removing the capacitor (c26) again and the noise goes away.  I really appreciate the help so far but what can we try next to eliminate this noise. I think we are close.

[J M Fahey]

Hi jon.zilla.
Well, now we know the TL074 is not guilty.
Is this a joke? Not at all, we stop wasting time with a very suspicious part.
As you say, we are getting closer.
Now my last neurons are running out of fuel, but tomorrow I'll open the Msh schematic and take a good hard look at it.
Please post its link, if you have it.
*Maybe* I have it somewhere, but I'm not quite sure about the "where" part.
Good night.

[phatt]

Remember C26 itself might be dead ?
Try replacing it with a new one.
Phil.

[J M Fahey]

Hi jonzilla
Please post the VS232 schematic to know *what* C26 is or does.
If not available, open the VS230 schematic kindly offered above, it can't be *that* different, specially the reverb part, but certainly component designators will not match, so point there which capacitor is the same as "C26"
Thanks.
JMFahey

[skaomatic]

Gents (and Ladies, if applicable) -

This problem occurred on my beloved VS232 4 years ago (give or take). I scour the inter-webs every couple months seeking an answer, but this is the first promising discussion I've seen!

She's in storage now, so I have not taken any of the steps outlined yet. When I do, results will be posted.

Just wanted to let you know OrigPoster is not alone - and I also await your wisdom!

Cheers!