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Update: 2009 JCM 2000 blows HT fuse

jcs

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8,093
Update: its a 2009 JCM2000, the very last version apparently.

I bought this amp cheap.....I will take it to a tech soon but am gathering info from you techs familiar with this amp.

The last EL34 (by the phase inverter) has carbon on the base of the tube between pins 1, 2 and 3.

All of the power tubes tested fine in my testers and all are what appears to be original Marshall (Svetlana) EL34's, tested very strong as strong as a few of my NOS Svetlana.

2 of the Chinese Marshall branded had 1 triode that tested dead fwiw.

I have looked the amp over with a magnifying glass, nothing appears liked burned resistors or leaking bulging caps fwiw.

The amp made a hum as soon as I turned it on WITHOUT the standby switch in the ON position.

I bought it cheap so I am not concerned about repair costs!

It does have reverb as well.....any insight is appreciated and Happy New Years!
 
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Blue Strat

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30,243
That socket may need to be replaced. Carbon traces will often continue to conduct, arc, and take out fuses. Rather than have people who can't see the amp guess, I recommend letting your live, in person, tech let you know.
 
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jcs

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That socket may need to be replaced. Carbon traces will often continue to conduct, arc, and take out fuses. Rather than have people who can't see the amp guess, I recommend letting your live, in person, tech let you know.
Thanks!.......well, ive read enough about these amps to know that after we find the "repair issues" I will likely go ahead and shotgun replace some other parts that could fail down the road.

Yes, I will insist on replacing that socket.

Those parts that seem crucial (screen resistors, bias supply etc.) and aren't too big of pain to replace, I plan on replacing.
 

Blue Strat

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Thanks!.......well, ive read enough about these amps to know that after we find the "repair issues" I will likely go ahead and shotgun replace some other parts that could fail down the road.

Yes, I will insist on replacing that socket.

Those parts that seem crucial (screen resistors, bias supply etc.) and aren't too big of pain to replace, I plan on replacing.
Good idea. Might as well hit anything associated with that socket and possible loss of bias, which may have been the catalyst.
 
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pdf64

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It would be a good idea to investigate for likely root causes of the arcing, eg intermittent circuit in the OT secondary such as a dry joint.
Don't expose the amp to further fault current unnecessarily, eg power up via a light bulb limiter after repairs are made.
If you've got sufficient time and inclination, the bad EL34 may be able to be repaired, eg replace the tube base, dremel out the carbonised material.
 
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jcs

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In a 4 x EL34 amp, is the fact that only 1 socket arced showing anything as far as potential cause of the problem?

Anything else that should be addressed?
 

jcs

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8,093
How do the filter caps look? (Don't touch them).
They don't appear to be bloated.....I need to drain off the caps before I probe any further but I am going to my tech in the next week or so.

Just trying to gather as much info as i can from the fine folks at TGP!
 

TimmyP

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I would not trust the tubes without a good test - which may or may not reveal a short or arc owing to the voltages in the tester being lower than those in the amp. And sometimes, tubes like to mess with you and not misbehave when you need them to.
 
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jcs

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I would not trust the tubes without a good test - which may or may not reveal a short or arc owing to the voltages in the tester being lower than those in the amp. And sometimes, tubes like to mess with you and not misbehave when you need them to.
I agree 100%!
 

Tone Meister

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3,266
No one has mentioned it yet so I will.

Many (I didn't say all) of the early DSL/TSL amps suffered from HV arcing at the power tube sockets due to a design issue with the board itself. What happens is the HV at pins 3 & 4 can arc over to pin 5, which wreaks havoc with the bias and in turn blows the fuse. EDIT TO ADD: The HV can also arc to ground.

IF that is what's happening with your amp, the "fix" is to mod the board by drilling a hole in the PCB between the HV pin and pin 5 to prevent the HV from arcing. In most cases that tube socket must be replaced also as has been mentioned already.

Of course it could be something less sinister, but I have seen more than one of these amps with the same exact issue and drilling that hole does stop the arcing. In fact, I still have a DSL100 head here that was arcing across the PT socket and was fixed that way.
 
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jcs

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Thanks so much folks! I can always count on TGP for info!

Now, I need to determine the year of this JCM2000.
 

Tone Meister

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If it is one of they early ones, the 175$ board replacment may be the way to go.
Yes, if the board is damaged beyond repair. But if there is no damage and it's arcing from the HV, then drilling those holes does work. The DSL I have here was done several years ago and is still alive and kicking
 

jcs

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8,093
I want to update this amp: Any caps or resistors that are known to have caused issues with other amps, I want to shotgun replace after finding the repair issue.

So the tiny holes need to be drilled on the PCB between which pins?

I am not seeing any burned spots on the board but I have only lifted the top assembly enough to barely see anything....the burned socket has to be on the last EL34 next to the PI tube and frankly, I cant see much at all on that socket...no carbon on the top od the socket fwiw.
 

TimmyP

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Even if it is OK, you should replace C46, which is a 22pF/500Vcap from one of the screen grids to ground. This cap likes to blow short, which can take out the screen grid resistor and the HT fuse. Use a 1kV cap.
 

jcs

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8,093
Even if it is OK, you should replace C46, which is a 22pF/500Vcap from one of the screen grids to ground. This cap likes to blow short, which can take out the screen grid resistor and the HT fuse. Use a 1kV cap.
1000V cap? What about the bias supply in general, anything that should be replaced?
 

jcs

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8,093
So the amp is a 2009...apologies for not identifying the build date originally.

I assume some things have been changed by Marshall....and it sounds like I have an amp with the latest revisions...

Which means, afaik, less to deal with in terms of repairs overall?

I will take note of everything suggested here and hand it to my tech.

Anything observations you tech folks would like to add since I have now identified the build date?
 






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