Off Topic Scored a 50W JCM900 Marklll needs repairs!

[craigny]

Well I basically was given a JCM 900 Marklll 50W head by the guitarist in my old band. Hes been out of playing and it has been sitting at a friends house for like two years. He said if i can have it fixed i can bacically have it. If he feels the need to have it down the road he have to pay what i put into it, or perhaps i can just buy him out for whatever it was worth before the repairs. I dont think hell be asking for it anytime soon and i doubt if hed even take any $ seeing its been sitting at a buddys house that long collecting dust! Anywho....(sorry for the ramble) it seems that this amp was working fine and from what i can gather there was a little smoke and it went out. My buddy replaced the fuse and put a new set of tubes (Mesa EL34's) in there (not sure if he knew what he was doing as far as biasing goes or if the tubes were bad but he figured to just put new ones in) and turned it on and blew the fuse again, he tried one other time with a fuse and again it went on and blew the fuse. It went on the shelf and there it has stayed, untill now!. So what do ya think? Any ideas of the problem, or how much it would cost? I know it could be any number of things. Any one know a reputable shop on Long Island Ny to take it to. If I can get this fixed reasonablly I could have a nice head for a great price..It looks pretty intact...I dunno whadaya think?!?

 

[koogie2k]

No offense but you may want to ask on the manufacturer's site or take it to a tech. We knicks can't solve a problem based on what you said. Really, this is more for ebmm products and not amp fixing. Again no offense.

 

[patpark]

i would look at the power transformer if it's blowing fuses. or could be bad component on the pcb. either way it has to go to an amp tech and put on the bench.

 

[megaroel]

your friend is probably a clumsy guy and droped some kind of liquid in the head and it damaged the transformer i think a new transformer will run you well go here dude check it out.Transformers for Tube Amplifers
example.. it might even bee the same as the 800 belive me marshall didnt change much
over the 80s to the 90s good luck dude..

Output Transformer for 50w Marshall JCM 800 and most JMP
Output Transformer for 50w Marshall JCM 800 and most JMP 40-18025$44.04

 

[craigny]

Thanks guys. I know that this is not an amp fourm, but i figured id ask cause i know one or two of ya use amps to play these EBMM'S LOL!! Mega if you say a transformer for like 45 bucks plus labor i may be lookin good!!!! (actually there looks to be two transformers in the amp one on each end)

 

[guitfiddle]

Hi craigny,

When they blow fuses, they are usually doing it to save our lives. There is plenty of electricity in that amp to kill you many times over, so unless you know what you are doing, don't poke around it there (even with it unplugged - the capacitors store a charge).

I'd be very suprised if it was the input (power) transformer. When those go (which is VERY rare), there is a lot of smoke and an unforgettable smell (your friend would be reporting how bad it was).

One thing you can check easily, it make sure someone hasn't changed the input voltage selector switch on the back. At lot of Marshall amps are dual voltage 110V or 240V. In the US, you want it set to 110V. The switch being in the wrong place often won't blow the fuse, but the power tubes will not see the correct voltage and this can cause problems elsewhere that could lead back to a blown fuse. Stranger things have happened.

My picks:
- Power tubes (may not have been changed correctly, or may still have a bad new one - they don't make them like they used to when it comes to tubes).
- Capacitors (especially since it hasn't been energized much, the caps dry out and fail).
- Output transformer (the smaller one), especially if it hasn't always had speakers connected to it when powered on (or possibly if it was used a lot with speakers of the incorrect impedance - most transformers handle the wrong impedance for a while, but not forever).

I'm not heading to New York soon, or I'd offer to fix it for you for free (I'm an amp tech in my spare time). I'm pretty sure it's not going to be an expensive fix though, no matter where you take it. It's easy to find and solve problems that result in blowing fuses. It's faults that cause tiny transient noises and such that are hard to find/fix.

Good luck and enjoy, they are nice amps.

 

[craigny]

ghynes thank you so much for your info. My buddy told me about the capicators holding a charge so i a'int messin with anything!! Like i said my buddy said that it stated to somke a little and then it went out. he replaced the fuse...went out again. He just slapped in a set of Mesa el34's and changed the fuse.....went out again and on the shelf.. In your opinon you think it shouldnt be too expensive that would be really great, as im eager to get this baby up and running again. Too bad your not comming to new york the beer would be on me!!!!!

 

[guitfiddle]

Mesa doesn't actually make any tubes. They buy them from wherever to meet their fixed bias specs and slap a Mesa label on them. Mesa has a good reputation to protect so I suspect that they would use decent ones (but these days the amp market is so competitive that everyone is trying to keep their costs down), but any tube can be a dud. Even a brand new good quality tube can take a knock and fail. They are a lot like light bulbs. Knock 'em and the filament will break.

Do you know what speaker cabinet your buddy is using when he is testing it? Do you have lots of spares fuses? I have a couple more things that can be tried, but each time you try one that isn't the cause of the fuse blowing, you will burn out the fuse.

Wish I was coming through NY soon too. Going to Toronto next week to see Eric Clapton and Jeff Beck, but that's as close as I'll get.

 

[craigny]

Hey thanks ghynes. I cant tell you what speaker cabinet he was using this was a few years ago, and he probably wouldnt remember if i asked LOL!!My other firend who has been holding it was the one who attempted the fix. He replaced the fuse and it went again. He figured it may be the tubes so he just swapped the ones that were in there for the "mesa" EL34's and put another fuse in. Powered it up and it blew again. Thats all the info he gave me and he hasent touched it since. I do not have any fuses and im a little leary of poking around back there as I am a total tube noob. guess ill just have to take it to a tech, just dont wanna get rolled with the price ya know?

 

[jamminjim]

go to these forums, post your question/problem and find out if there is anyone near you that will fix it. I'd fix it for you but I'm not nearby.

Tube Amp Building - Tweaks - Repairs

Marshall Amp Forum - MarshallForum.com

New York Tech Registry

Amps n More - Index

 

[craigny]

thanks jammin!!!!

 

[beej]

Ok man, good luck with the fix, hope it's not expensive. (Make sure you check out the used values of those amps before you sink too much money into it).

I'm gonna closer 'er down now since, well, this isn't really an amp forum.