Problem With My bro's JCM900 - BAD SMELL!<---REPAIRED!

[Superfuzz]

He was playing here..suddendly, the sound became lifeless, then it was intermittent, then bad smell..so we put it off...we've opened it but nothing seems burned..it has never been revalved I think, but does dead valves do bad smells?

[northern_dirt]

My transformer went in my 900 once..
Don't remember a smell..

new tubes + new transformer = $500 dollars..

[mjet]

He was playing here..suddendly, the sound became lifeless, then it was intermittent, then bad smell..so we put it off...we've opened it but nothing seems burned..it has never been revalved I think, but does dead valves do bad smells?

A bad smell typically isn't a good thing with electrical equipment. I'd get it checked by a tech who can measure everything to see where the problem is. The only thing worse than having a blown transformer is having a pair of blown transformers and tubes...

[Superfuzz]

just reassembled and tested..now it seems to work fine and without any smell..that's strange, the only different thing he was doing when the amp started to work bad was putting the tremolo in the fx loop..but I can't see any relation..
All the valves glows except one that glows less, I think that's the phase inverter, because also on my HR Deluxe it glows less..

[fend14]

i suggest some deodorant

[Superfuzz]

I resurrect this old thread beacasue here we are again..the problem is happened again..the other day while he was playing the amp started to smell..nothing looks burned btw.
Then volume started to drop back and forth...so I gave a look on the back and one of the power tubes was glowing, it was purple!!!
So we put on standby..wait a moment, and then nothing..it worked perfectly as ever..
Today..we have tried it to look better and check here and there..nothing..then suddendly here again, bad smell, then volume drop, and then the valve glows...
HELP!

[northern_dirt]

I resurrect this old thread beacasue here we are again..the problem is happened again..the other day while he was playing the amp started to smell..nothing looks burned btw.
Then volume started to drop back and forth...so I gave a look on the back and one of the power tubes was glowing, it was purple!!!
So we put on standby..wait a moment, and then nothing..it worked perfectly as ever..
Today..we have tried it to look better and check here and there..nothing..then suddendly here again, bad smell, then volume drop, and then the valve glows...
HELP!

Take it in man

[Superfuzz]

I resurrect this old thread beacasue here we are again..the problem is happened again..the other day while he was playing the amp started to smell..nothing looks burned btw.
Then volume started to drop back and forth...so I gave a look on the back and one of the power tubes was glowing, it was purple!!!
So we put on standby..wait a moment, and then nothing..it worked perfectly as ever..
Today..we have tried it to look better and check here and there..nothing..then suddendly here again, bad smell, then volume drop, and then the valve glows...
HELP!

Take it in man

Sorry, what does it mean?

[northern_dirt]

I resurrect this old thread beacasue here we are again..the problem is happened again..the other day while he was playing the amp started to smell..nothing looks burned btw.
Then volume started to drop back and forth...so I gave a look on the back and one of the power tubes was glowing, it was purple!!!
So we put on standby..wait a moment, and then nothing..it worked perfectly as ever..
Today..we have tried it to look better and check here and there..nothing..then suddendly here again, bad smell, then volume drop, and then the valve glows...
HELP!

Take it in man

Sorry, what does it mean?

Bring it to a shop

[Superfuzz]

ok now I see 

[Just A Dude 2o8]

those amps suck..im not suprised..

[Superfuzz]

those amps suck..im not suprised..

Talking about sounds it doesn't suck so much..it does only the marshal thing, but it does that pretty well..speaking of the damage, yesterday the amp has been brought to a tech and he says that it should be just one of the output valves..pretty normal when you buya 15+ years old amp that has never been revalved since now..

[Superfuzz]

Something's burning

You in all likelihood have an intermittent power tube and it's shorting from time to time and something is heating up.

It may be the output transformer or a screen resistor or something.  Let's hope it's the latter.

I would replace the power tubes.  Certainly DO NOT USE THE AMP until you at least do this much.

Then open it up, but DO NOT TOUCH ANYTHING unless you know how to discharge caps.  THE GUTS OF AN AMP CAN KILL YOU, even when off and unplugged.

Sorry for the caps-lock, but when it's a matter of life and death I think it's fair.   :D

Anyway, look around the tube sockets and see if you see any burnt-looking spots on the circuit board or any burnt-looking components (especially resistors).

If not, then you're probably fine with new tubes.  I'd guess the OT got a little hot and the shellac in it started to heat up.  If it still works, no harm no foul.

Marshalls are not exactly known for robust output transformers, for the record.

Also, make sure the impedance is matched between the head and the cab, and that the impedance selector on the cab or head is not coming loose or making a bad connection.  You're not using an attenuator or anything are you?

Don't worry, before I can do something bad the amp has just been brought to a tech! yesterday for the records..
What he has said for now, is that one of the power tube is gone for sure, it has a black dust inside the bottle and going overheat has made burning some white substance that I really have no idea of what it is..(probabily this is what was smelling)
BTW he'll give a full check, and he will add a resistor after each power tube in the way to prevent transformer burning..plus the amp will be completely rebiased and some matched svetlanas will be put in it! He said it will sound completely new!!

[øøøøøøø]

Good!

That's what I thought it sounded like. 

[andtrea]

Harrr!
E mo chi ti ferma più col Marshall revisionato?