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.
your sound file isn't working please re upload it.
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?
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.
Quote from: gabefauver on November 10, 2009, 02:00:07 AM
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.
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.
Wow, I have never heard one make that noise before. .....interesting.
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.
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.
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.
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.
Also disconnecting the reverb completely did not change anything.
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
Quote from: jon.zilla on November 11, 2009, 11:45:10 PM
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.
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.
Thanks for all the help I will try it right away. :)
Ok, shorting the reverb return did not work. Here are some pics of the board.
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.
Ok how would I go about testing the opamps voltage pins? I have the schematic.
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.
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.
Be sure to throw Phatt some chip points if he was helpful to you :tu:
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.
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.
Quote"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.
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.
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.
Remember C26 itself might be dead ?
Try replacing it with a new one.
Phil.
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
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!
(http://i743.photobucket.com/albums/xx75/gabefauver/vs232c26Small-1.jpg)
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.
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.
Quote from: J M Fahey on January 09, 2010, 02:10:38 PM
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 (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.
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.
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.
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.
Quote from: J M Fahey on January 10, 2010, 03:19:20 AM
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
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.
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.
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.
Quote from: phatt on November 11, 2009, 07:40:19 AM
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