JCM900 (used) problem

[the_stryder]

I recently received a used JCM900 dual reverb 50watt head
and it happened to short out on me. There is a legititmate
reason I suppose. I'm not that great with guitars; especially
the techinical stuff. But would the problem be solely caused
by my lack of connecting the head to a speaker while the amp
was on? and yes i'm really dumb with guitars. Please, any help
would be awesome.

 

[msplines]

Re: JCM900 (used) problem

If you put any serious signal levels through the amp while the speakers were disconnected, then you've probably blown the output transformer and perhaps some associated circuitry. What are the symptoms?
if you are new to this, take it to an amp tech and explain what happened.

 

[the_stryder]

Re: JCM900 (used) problem

The only symptoms I can come up with is that it won't turn on.
Anybody know how much this might cost me to fix this? and how long it would take?

and thanks...

 

[msplines]

Re: JCM900 (used) problem

The only symptoms I can come up with is that it won't turn on.
Anybody know how much this might cost me to fix this? and how long it would take?

and thanks...

Might be a silly question, but if the power light won't come on, have you checked the fuses?
I'm afraid I can't help you with the cost, you should ring a tech and ask for an idea of the cost of repairing a blown output transformer, they should be able to give you some idea (min and max).

 

[flank]

Re: JCM900 (used) problem

if you blew the output transformer......you lookin to pay around 200 to get it all fixed up......and not having the head connecting to the speakers for more than 30 secs -1 min is a serious no no and theres a very good chance that you could have killed your output transformer

however, the light not turning on tells me that you may have blown a fuse..... pull out the fuses and see if there burnt up

 

[dystrust]

Re: JCM900 (used) problem

It's possible that the output transformer may be blown, but if the indicator won't turn on it's more likely a fuse (as said before) or the power transformer. Starting with the 900s Marshall began using substandard transformers that aren't nearly as dependable or sturdy as the ones the 800s and older Marshalls used.

BTW, one of the easiest ways to vastly improve the tone of a JCM 900 is to convert the power tubes to 6L6s or EL-34s (get rid of the 5881s) and change the transformers to something decent. If it turns out that you've blown one or more of the transformers, I would change both to mercury magnetics JCM 800 transformers.

 

[yes-fan]

Re: JCM900 (used) problem

i have one of those but i am stupid when it comes to amps and that has never happened to be before.

 

[italic zero]

Re: JCM900 (used) problem

My JCM 900 keeps blowing fuses for some reason... even when it's not cranked. When I first blew one I took it out, and it said something completely different than what it said on the amp. So I replaced it with a T3 250 fuse (it said T3 120/240), but that one blew pretty quickly as well. So now I have another one of those in there, and it's held up for a couple hours... but what should I do if it blows again?

 

[mrid]

Re: JCM900 (used) problem

My JCM 900 keeps blowing fuses for some reason... even when it's not cranked. When I first blew one I took it out, and it said something completely different than what it said on the amp. So I replaced it with a T3 250 fuse (it said T3 120/240), but that one blew pretty quickly as well. So now I have another one of those in there, and it's held up for a couple hours... but what should I do if it blows again?

Change the tubes?

 

[italic zero]

Re: JCM900 (used) problem

Old tubes make the fuse blow? I never knew that...

 

[lastwinj]

Re: JCM900 (used) problem

My JCM 900 keeps blowing fuses for some reason... even when it's not cranked. When I first blew one I took it out, and it said something completely different than what it said on the amp. So I replaced it with a T3 250 fuse (it said T3 120/240), but that one blew pretty quickly as well. So now I have another one of those in there, and it's held up for a couple hours... but what should I do if it blows again?

fuses can blow for no reason. i have had it happen. be sure you are using the correct rating of fuse. it should tell you on the amp (3 to 4amp is common in 50 watt marshall's) it needs to be a slow blow type, not a fast blow. if it still blows fuses, it will need to be serviced. good luck!!

germ

 

[McLaughysSN]

Re: JCM900 (used) problem

an amp that is blowing fuses constantly sounds like something a tech should look at. i had the unfortunate oppurtunity to have an amp start blowing fuses like no ones business and its something that could be cause by many things including shorts somewhere in the amp

your best bet, if you have the resources (money) is to take it to a tech and get it totally serviced.. tell them everything that is going on.

best of luck to you, i hate to see an good amp such as a 900 have problems like this

 

[italic zero]

Re: JCM900 (used) problem

fuses can blow for no reason. i have had it happen. be sure you are using the correct rating of fuse. it should tell you on the amp (3 to 4amp is common in 50 watt marshall's) it needs to be a slow blow type, not a fast blow. if it still blows fuses, it will need to be serviced. good luck!!

germ

Yeah, the last two have been slow blow 3 amp fuses, it says both T3 and T4 on the amp, although I used a 250 fuse and it says 120/240 for T3... (it's 100 watts btw). I'll wait to see if it blows again, then take it in.