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Marshall 5203

Started by Stu, September 01, 2014, 01:18:27 PM

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Stu

Hi I have a Marshall Master Reverb 30.  It's a great little amp.  It has recently developed a strange fault - when turning the amp up using the master volume the amp gets louder but as the pot goes past half way the volume decreases?!?

I have removed the mater vol pot ad checked it with my multi meter and it looks fine.

any help, advice etc would be much appreciated

Roly

The main chance is that it's a pot problem, and that cleaning with a contact cleaner, or perhaps replacement, might be the cure.  But this sort of symptom has come up when one of the by-pass electros for one of the preamp sections has died and you are getting unwanted inter-stage negative feedback.  Maybe.  It could be one of those rare cases where one of the caps really is to blame.


Needs circuit and more diagnosis.
If you say theory and practice don't agree you haven't applied enough theory.

primofacprima

Hi Stu,

5302 brings up a 12/20 watt keyboard amp. I think you mean 5203?..

http://www.drtube.com/schematics/marshall/5203.gif


Enzo

That's better.  here it is again but in a pdf, if anyone prefers.

http://bmamps.com/Schematics/marshall/Marshall_30w_5203.pdf

Roly

 :(    A bit stumped here I hafta admit.


Okay, up the river; somewhere on the board there are two zener diodes ZD1 and ZD2.  These should both have 15 volts across them.

Please find, check, and post (then we at least we know are starting our fault finding with good supplies).

Internal pic of the board would be helpful too.

Q. Does the position of the Treble control change how the Volume control acts at all?

{I can't see anything obvious that would cause this odd volume control behaviour.  I suspect it's impedance related, the level rises and falls as the impedance of the Volume wiper, which suggests that it might be something to do with the following input of IC2a, but that is only a wild surmise at this point.}

BRAINS TRUST! - this is a curly one we got 'ere maties.  Ideas?


If you say theory and practice don't agree you haven't applied enough theory.

Stu

Thanks for the replies folks.

Yes, primofacprima it is a 5203, my bad...

Roly, zeners checked and they both have 15 volts across them!  I am also sure the pot is ok, the resistance increases smoothly when the pot is turned, no jumps or sticky points.... 

The Treble control has no effect on volume.

However, I have spotted that as the Master Vol reaches half way and the volume starts decreasing that LED2 begins to illuminate and gets brighter as the MV is increased (and the volume reduces) - I'm thinking this could be a clue?

the circuit board looks exactly the same as the one in this thread: http://www.ssguitar.com/index.php?topic=2851.msg21069#msg21069

Roly

Well the LED is certainly a clue, I just don't know what it means.


Have you given all the pots and signal sockets a good cleaning with metho or Deoxit yet?
If you say theory and practice don't agree you haven't applied enough theory.

g1

  Sounds like it may be breaking into oscillation, causing volume loss and turning on the LED?

Roly

Quote from: g1 on September 03, 2014, 11:35:32 PM
  Sounds like it may be breaking into oscillation, causing volume loss and turning on the LED?

ooh .. ah ..ooh .. ah ...

We've been here before.

Something about a long trace back to an earthing contact on the input socket, having to mod the trace because the instability is inherent when the contact gets dirty, ...

... something, something?

If you say theory and practice don't agree you haven't applied enough theory.

Stu

It looks to me as tho the output of the second op-amp must have some sort of +ve bias which increases with the master volume (hence LED2 illuminating and not LED1).  But I can't understand how this is happening as the MV is on the output of op-amp3, it seems strange that it is effecting the previous stage!!  Oh, I wish I had a scope!

Enzo

A couple thoughts:
Clip a couple extra filter caps across the 15v rails to ground, there might be insufficient decoupling from the power supply.

The schematic specifies 1458 dual op amps.  ANy chance someone has put more recent op amps in the circuit, like even 4558 or newer?  There are some Marshall solid state amps of that era that were sensitive to it.  The newer ICs had more bandwidth or something, I forget just what.

Once I replaced a bad 1458 with a 4558, which usually is OK, but I found the amp unstable.  After chasing it a while, I talked to someone at marshall who pointed out to me that these amps really did need the 1458.

Just a possibility.

DrGonz78

Quote from: Roly on September 04, 2014, 05:40:52 AM
ooh .. ah ..ooh .. ah ...

We've been here before.

Something about a long trace back to an earthing contact on the input socket, having to mod the trace because the instability is inherent when the contact gets dirty, ...

... something, something?


Hey Roly was this the one you were thinking about?
http://www.ssguitar.com/index.php?topic=3070.30
"A person who never made a mistake never tried anything new." -Albert Einstein

Stu

I may have discovered the answer.... http://translate.google.co.uk/translate?hl=en&sl=nl&u=http://www.circuitsonline.net/forum/view/31529&prev=/search%3Fq%3Dmarshall%2B5203%2BLED2%26biw%3D1118%26bih%3D584

sounds like a very similar problem, he doesn't say which op-amp he changed but at 60p a pop I'll do um both.. will post my results in a few days.....

Roly

Yeah, think so Doc.

Good find Stu
If you say theory and practice don't agree you haven't applied enough theory.

g1

  Could you please do them one at a time and see which was the problem?  That way we will have a definitive solution to the issue, which may come up again.