Marshall Valvestate vs100 smoking cigars

Alex Carrington

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Hi im Alex,

Im new to this forum so thanks for having me! Ive had this Marshall Valvestate vs100 for like 10 years now and it was working perfectly fine until one time I powered up the amp and after 10 seconds saw smoke.

So i pulled the chassis and powered it up again and started hearing this popping and crackling sound. I also found that the r20 and r21 resistors on the preamp board were smoking. So i pulled the circuit to take a closer look at solder joints. Didn't find anything other then the circuit board being burnt around the r20 and r21 resistors also around the two voltage regulators REG1 and REG2.

So what i did was check the power coming in at CON3 pin 5 and it was 40v (Seems normal). Probing the end of r21 resistor it showed 5v. Now i check the output of the REG1 and it showed about 0.3 volts. So there was an issue. I replaced REG1 and now the output was 4 volts and r20 and r21 were still smoking.

Not sure where i should look next. Im suspecting the r21 is getting too much voltage. But i could be wrong. Could you guys help me troubleshoot this thing? I just purchased my first Gibson Les Paul and i cant even run it through the Marshall... How devastating lol I have some basic circuitry knowledge but nothing crazy. I also have an oscilloscope, multimeter, and variac so i can check things if needed. Page 2 bottom right section has the r20, r21, REG1, REG2, and CON3 pins.


I cant paste the schematic link because im too new but a quick google search of Marshall vs100 schematic will give you the right one.
 

SkyMonkey

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I'm probably no help either, but the only time my VS65R smoked was when I fired it up without the speaker connected.
I was changing the valve and had the chassis pulled and disconnected to access the main board.
In a brain fade moment I fired it up to see if the valve glowed and immediately saw a wisp of smoke.
All I can say is that the smoke came from the right hand side of the main board, with the knobs facing you.
I shut it off straight away and manage to get away with any damage.
I know most will say a Solid State amp can survive having no load, but I found the opposite to be true of my VS65R.

So I wonder if your 'out-of-the-blue' problem stemmed from a speaker connection failure in the first place?
Check the internal speaker connections and clean the jacks before firing it up again, after you get the main problem fixed.

Welcome to the forum too @Alex Carrington :welcome:

Hope you find a fix.
The Mk II Valvestates are great little amps.
 

Alex Carrington

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I have no ideas on how to fix your problem but those smoking resistors will probably have to be replaced as well I would think.
I have no ideas on how to fix your problem but those smoking resistors will probably have to be replaced as well I would think.
I tried to replace them but they burnt In 10 seconds, Thanks anyways!
 

Alex Carrington

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I'm probably no help either, but the only time my VS65R smoked was when I fired it up without the speaker connected.
I was changing the valve and had the chassis pulled and disconnected to access the main board.
In a brain fade moment I fired it up to see if the valve glowed and immediately saw a wisp of smoke.
All I can say is that the smoke came from the right hand side of the main board, with the knobs facing you.
I shut it off straight away and manage to get away with any damage.
I know most will say a Solid State amp can survive having no load, but I found the opposite to be true of my VS65R.

So I wonder if your 'out-of-the-blue' problem stemmed from a speaker connection failure in the first place?
Check the internal speaker connections and clean the jacks before firing it up again, after you get the main problem fixed.

Welcome to the forum too @Alex Carrington :welcome:

Hope you find a fix.
The Mk II Valvestates are great little amps.
I didn't have the speaker connected but I had a 4ohm Dummy load hooked up. I'll try to hook speakers up again and see if it helps. Thanks for the help
 

Spezia

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Hi im Alex,

Im new to this forum so thanks for having me! Ive had this Marshall Valvestate vs100 for like 10 years now and it was working perfectly fine until one time I powered up the amp and after 10 seconds saw smoke.

So i pulled the chassis and powered it up again and started hearing this popping and crackling sound. I also found that the r20 and r21 resistors on the preamp board were smoking. So i pulled the circuit to take a closer look at solder joints. Didn't find anything other then the circuit board being burnt around the r20 and r21 resistors also around the two voltage regulators REG1 and REG2.

So what i did was check the power coming in at CON3 pin 5 and it was 40v (Seems normal). Probing the end of r21 resistor it showed 5v. Now i check the output of the REG1 and it showed about 0.3 volts. So there was an issue. I replaced REG1 and now the output was 4 volts and r20 and r21 were still smoking.

Not sure where i should look next. Im suspecting the r21 is getting too much voltage. But i could be wrong. Could you guys help me troubleshoot this thing? I just purchased my first Gibson Les Paul and i cant even run it through the Marshall... How devastating lol I have some basic circuitry knowledge but nothing crazy. I also have an oscilloscope, multimeter, and variac so i can check things if needed. Page 2 bottom right section has the r20, r21, REG1, REG2, and CON3 pins.


I cant paste the schematic link because im too new but a quick google search of Marshall vs100 schematic will give you the right one.
Hi Alex,

I've had this amp for a year,great amp by the way.

i think,there is a design failure on power supply rail.REG1 and REG2 feeding (+)(-) 12V for opamps.R20 and R21 reduces incoming (+)(-) 40V to 27V.REG's input voltages too high.(27-12=15V x 0.2 A=3W).Both resistors and regulators are frying.

So, you need to reduce REG's input voltages.Here is what i did:

2x 180 ohm 5W or 7W resistors in series connection, both sides.i rewired resistors to a small circuit board.They are getting hot, but not much than before.And REG's input voltages now 18V.REGs are dissipating
(18-12=6V x 0.2A=1.2W).No more heating issues.

Also, i pulled out 12AX7 from pcb and shorted pin3 to pin7.Now we've got fully transistor amp :) There is no tonal difference, tested👍This is useful for volume control.You know this amp is loud when volume at 1-2.

Hope this helps.
 

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