Think My JMP Might be Dying

[greyskull]

So at practice last night my Amp Started making horrible popping and crackling noises... You couldn't hear them whilst playing; but you sure could when you stopped!
To make matters worse; the iffy non Marshall standby switch I Have on it; the light has died on.
Definatly gonna get them replaced ewith proper ones...But any ideas what the problem may be?
Too broke to pay someone else to fix it at the mo!

Wolfe?

 

[guitarguru777]

Dude honestly i wouldnt even bother. I would throw it on a shelf till you get the cash to have it looked over properly. They are too rare and too expensive to fuck with.

 

[Straight Shooter]

Crackling-static w/pops problem is usually a part failing, and can be difficult to nail down. You could start with tubes first (and sockets), filter caps second, plate resistors third, coupling caps fourth. If the caps are original (old) that might be the issue. I had a '75 JMP with the same crackling/popping...recapped the head and that fixed the problem. Could also be a bad contact somewhere. As guitarguru777 said I would take that head out of action until you find the problem and repair it. Would be a shame to burn it up-kill a transformer.

 

[Notuern]

Ive had those sounds every time a preamp-tube has been close to failing.. but considering its an old amp, i wouldnt count on it being something that cheap.
I agree with guitarguru though.

 

[J.DavisNJ]

Have you tried switching out the tubes? Maybe take a fresh pre/power tube and swap em out one by one to see if it changes anything?

If you feel uncomfortable doing that, bring it to a tech.

Honestly, the JMPs aren't as rare or $$$$$ like previous poster said. Old, yes. JMPs are relatively plentiful - I bought mine for $1k. More tempermental, maybe, but it could indeed be a tube.

Marshalls of that breed are built quite well, and while any amp can have major problems, I wouldn't jump to any conclusions just because it is "old." Hell, my '68 Super Reverb sounded amazing and the caps were original. It had the occasional noise too.

Could need a recap, or any multitude of tubes.

Joe

 

[greyskull]

okay.. great!
Thanks guy's!
Hope it's not the caps or the tubes as they're original... But if it means it keeps going then it needs to be done.
(yes...Mullards)
Probably drive to milton keynes to the factory to get it looked at properly.

 

[Notuern]

The tubes are original?
Fuck me backwards that amp must need a tube change.

 

[Straight Shooter]

Marshalls of that breed are built quite well, and while any amp can have major problems, I wouldn't jump to any conclusions just because it is "old." Hell, my '68 Super Reverb sounded amazing and the caps were original. It had the occasional noise too.

Could need a recap, or any multitude of tubes.

Joe

They may be built quite well but a short (if the mains fuse does not blow quick enough) can fry the PT. The filter caps wear out, especially after 20-30+ years. Over time filter caps age, the dielectric paste dries up and essentially breaks apart and becomes nothing but powder. A bad filter cap may not be the problem with Greyskull's Marshall but the trick with these amps is to keep them from breaking in the first place. If you play'em loud replace those old worn out/dried up 25+ year old filter caps and maintain power valves /rebias to preserve your PT and OT in vintage Marshalls.

 

[greyskull]

Yeah... Bugger, regarding the valves; it's a closer classic so to speak, so been unused then serviced. Hey ho.
Any good valve recomendations, or shall I get jj's ?

 

[Wolfeman]

Tubes first, then I would look into a failing component.

Tung sol for preamp, power tubes are dependent on how much you are looking to spend.

 

[greyskull]

Fuck it.. gonna ring Marshall this afternoon and get them sort it good and proper.
New pots, new knobs, new handle probably new power amp valves.... (anyone know what the put in em?)

Lasse.. what valves have you got/tried in your JMP?

 

[Sloan]

Surely this is a joke thread.

 

[greyskull]

ehh???
No joke Mate.

 

[Lasse Lammert]

as always I'm using =C= in the poweramp...the only alternative for me is the JJ el34L (that "L" is important!)
atm I have super old and super expensive NOS in my JMP (actually the original ones, I think Mullard and Siemens, not sure though)
Usually I like Tung Sols in 2203 with a jj for the PI

 

[greyskull]

RIGHT!
well.
According to Marshall it needs.... (deep Breath)

new resistors, mains sockets, screening cans, pots, bias trim, output valves and capacitors.

and all they want in return in 130 quid, included couriering it to/from me.
AWESOME

 

[Sloan]

oh wow, that sounds cheap.

I thought this was a joke thread because it was about a JMP and you didn't know about changing tubes yourself. I'm a firm believer in that anyone that owns a tube amp should educate themselves about changing tubes and biasing (at the very least). A bias probe makes biasing very easy. I've got a Weber BiasRite, but they have been discontinued. There is more than enough information on the internet about biasing.

I live in an area with absolutely no 'amp-techs' so I have had to teach myself how to do my own repair work and have been able to make a little money on the side repairing gear. The more you educate yourself about your tools, the better off you will be. There's no way I would even be able to play music if I had to pay someone to repair everything. Cables, amps, speakers; if they go bad at a gig, i gotta fix them before the next one!

 

[greyskull]

yeah.. if it was just the valves I'd have been more than happy to do it myself!

 

[AHChris]

Wow man that is really cheap for Marshall support

Last time I contacted them, they asked for almost 50€ for the metal cover plate on the back that protects the tubes (for my JCM900)... AND THEY REFUSED TO INCLUDE THE SCREWS!

 

[greyskull]

It's coming back tommorrow!
Awesome!

 

[greyskull]

the EAGLE has Landed!

Marshall Have....
Fitted 5w 1k resistors to output valve bases
Replaced the mains socket
Replaced the Bias caps
Replaced ALL the pots
REplaced inout and output Sockets
Replaced dead smoothing block and removed incorrectly fitted caps on main pcb
REplaced output valves and re biased
replaced
first pre amp valve base
Replaced missing preamp screeening cans
Rewired transformer for 240v (quite worrying!)
Replaced mains and standby switches

and tidied up the mess inside, including previous bodged repairs and mods.. not to mention replacing all the chassis mounting screws, the backplate screws, knobs and other thngs i haven't noticed yet.
Can't wait to plug it in!!!
It was always a little wierd before and a touch toppy, so I'm hoping it's gonna be a new beast all together!!
Pics to follow!!