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Author Topic: Marshall ValveState VS232 repair  (Read 511 times)

EddieH

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Marshall ValveState VS232 repair
« on: January 21, 2021, 10:34:52 AM »
Hello and thanks for letting me on the board.   I am starting to re-learn electronics after 20+ years.   Built a couple of guitar pedal kits recently. 

I acquired a Marshall Valvestate VS232 to learn how to repair with.   It was producing a loud hum when I first plugged it in.

It appears someone else had been inside the amp.   
C1 on the poweramp / power supply board was missing.  I went ahead and replaced both c1 / c2 with new caps.
R10 and R18 (same function left vs right channel) were both burnt on the power amp board they were both replaced.    R38 on the main board was burnt also.  It was replaced.
I did re-solder a few of the electrolytic capacitors that appeared to have cold solder joints.

Initial fixes seem to fix right channel and R10 is holding.  There is a very little hum on the right channel now.

I have also tried unplugging the conn cable between the 2 circuit boards.

R38 on the main board will not burn up on the main board with the conn cable unplugged.   
Leads me to believe something is off on the VE- side or the VE+ side that the conn cable provides to the main board.  Looks like from the schematic that R38 is the first resistor on the main board that VE- hits.

R10 continues to burn up regardless and has a strong hum on the left channel. 


Schematic for the power supply / poweramp circuit board

https://drtube.com/schematics/marshall/v230-61-02.pdf

Schematic for the main board.

https://drtube.com/schematics/marshall/v230-60-02.pdf


With the conn cable unplugged I am thinking the problem is still on the power supply / power amp circuit board.   Possible the IC2?

Looking for any other ideas on where to check?

Thanks

Ed

Enzo

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Re: Marshall ValveState VS232 repair
« Reply #1 on: January 22, 2021, 01:03:36 AM »
Loud hum...on a solid state amp, that so often means DC on the speaker.  And that will damage speakers.  SO check for DC voltage across the speaker that hums.  Or look at the speaker cone.  When you turn on the power, does the cone move one direction and stay there?  That means DC voltage on the speaker.

REsistors do not burnt up on their own, there is almost always a dead semiconductor involved.

R38 is in the -15v supply, if it burnt, then either the zener or cap is shorted or one of the ICs using -15 has a short to ground.

EddieH

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Re: Marshall ValveState VS232 repair
« Reply #2 on: January 27, 2021, 10:45:49 AM »
Thanks Enzo!!!    Sorry for the delayed response I was waiting on parts.

So the solution was the IC2 TDA2050 was leaking DC out Pin4.   Replaced that with one from amazon.  They are only 7.99 for 10 of them.  Thought I would have to replace both IC1 and IC2 to get a more matched sound since this is a stereo amp.   So far it sounds equal and balanced.  That fixed the problems with the Power Amp board. 

The zenor diode zd1 was shorted causing R38 to burn.    Replacing that fixed the amp.

Once I had good sound, I found that VR3 for the clean channel is broke.   I have one of those on order.   The reverb seems weak as in not having much verb.   I will dive in to that next and post if I find anything.

Hope this helps anyone else experiencing issues.