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Amp instantly shuts off

jbweld42

Member
Messages
5
So i just got done playing my marshall 1974x, absolutely worth every penny, but it just shut off and cant get any power. I know these have transformer problems, but this has the newer transformer already in it and from what I hear weird noises, possibly smoke come with a blown transformer. So anyone have an idea to what could be wrong?

Thanks

CURRENT UPDATE:
I have a 1974x and recently while playing it shut off and wouldn't power up. I checked the fuses to find the main fuse blown, so after checking all the innards for anything suspicious I replaced it and powered up. It blew in a fraction of a second. I continued doing this procedure of checking then powering up with a new fuse first removing the power tubes and then the rectifier tube with the same result. Nothing looks blown or damaged on the inside, and I believe all the tubes to be fine.
 
Last edited:

Guinness Lad

Senior Member
Messages
15,860
Are you blowing fuses?

Check fuses, if ok look at the power tubes for a dead short, the only way to know is to replace them. Usually when a preamp tube is bad it just wont produce sound, but from my experience they wont shut off.

If it's not tubes you need to check out for failing components. If it has a tube rectifier this could be the problem as well, but I'm pretty sure your amp doesn't have one.
 

Structo

Member
Messages
9,556
Radio Shack.
Make sure you get the same kind as is in there.
Probably a Slo Blo fuse, and make sure to get the same current rating as the blown one.
It could be a shorted power tube or rectifier if it has a tube rectifier.
I would throw in a new set of power tubes before replacing the fuse and power up.

It could also have taken a screen resistor when it blew the fuse.

I would pull the chassis out, look at the guts around the power tube sockets and look for burnt looking resistors that are connected to the sockets.

Don't touch anything in there as the caps can hold a lethal charge and kill you.

If you don't feel comfortable doing this take it to a tech.
But I would inspect those resistors before powering it back up because it could end up taking out the output transformer.

If you saw smoke I would guess a screen resistor and or transformer got hot.
 

Guinness Lad

Senior Member
Messages
15,860
Sometimes fuses can still not be blown, but can cause trouble. Why this is I don't know, perhaps they wear out after a while. I've had starnge issues where something would act up, I'd try everything then replace the working fuse and never have another issue. If the fuse blows again, I'd check your power tubes because you probably have a shorted power tube which needs to be replaced.
 

glman

Member
Messages
629
Are you blowing fuses?

Check fuses, if ok look at the power tubes for a dead short, the only way to know is to replace them. Usually when a preamp tube is bad it just wont produce sound, but from my experience they wont shut off.

If it's not tubes you need to check out for failing components. If it has a tube rectifier this could be the problem as well, but I'm pretty sure your amp doesn't have one.
the Marshall 1974x actually DOES have a tube rectifier


At the very heart of its simple, but effective, two channel design can be found six valves:
three ECC83s in the preamp, an EZ81 rectifier and a pair of cathode biased EL84s in the power stage.
the tube rectifier could be the problem - but you would have to check it against a know good working EZ81 rectifier tube.
 

Guinness Lad

Senior Member
Messages
15,860
the Marshall 1974x actually DOES have a tube rectifier


At the very heart of its simple, but effective, two channel design can be found six valves:
three ECC83s in the preamp, an EZ81 rectifier and a pair of cathode biased EL84s in the power stage.
the tube rectifier could be the problem - but you would have to check it against a know good working EZ81 rectifier tube.
Your correct, I read 1974 Marshall, so I was thinking big amp, not the reissue amp, my bad.
 

zoooombiex

Member
Messages
2,548
Sometimes fuses can still not be blown, but can cause trouble. Why this is I don't know, perhaps they wear out after a while. I've had starnge issues where something would act up, I'd try everything then replace the working fuse and never have another issue.
I also had a fuse go bad, even though it wasn't clearly "blown." Usually it's obviously like someone blew something up in there, but not always. And if it's getting absolutely no power it's a prime suspect. Good luck!
 

nnick

Member
Messages
1,489
I had the same issue with my AC30CC. The slow blow fuse would blow up after 10 minutes of playing. It was a bad rectifier tube that caused the problem. I replace the tube and fuse and the amp has played wonderfully since.
 

jbweld42

Member
Messages
5
Checked everything prior to power up and found nothing , so I put the replacement fuse in but when I powered up it still blows the fuse. Anything?

Thanks
 

abro163

Member
Messages
35
The fuse blowing shows there is obviously something very wrong, you are taking a big risk powering it up. As iff the fuse doesnt blow in time your transformer is toast.

Get it looked at by a tech if you dont know what to look for.

You know what they say a transformer will blow to protect a 10c fuse.
 

DGDGBD

Member
Messages
7,025
Yeah, I'd say that repeatedly powering up and blowing the fuse while troubleshooting is asking for trouble. If you've ruled out tubes, just take it to a tech and have them troubleshoot it.
 




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