Cause Or Effect Jcm 900

[unclej]

i had a customer bring me a 2-3 year old marshall jcm 900 combo that kept blowing fuses on him. while he was at the store i replaced it and had him play for a while and sure enough after fifteen minutes or so the amp started "crackeling" like there was a bad input jack or output jack on the guitar and the fuse blew.

that afternoon i replaced the fuse with a new one, removed the gutar cord and just let it idle. after 15-20 minutes it made the same noise and blew again. so i pulled the chasis, tested the tubes, replaced one 12ax7 that tested extremely weak, re- tensioned all of the input and output jacks, re-tensioned the tube sockets, cleaned all the pots and replaced the fuse. i turned it on with all of the gain and volume controls in the off position and after 15 minutes or so it started making the same noise but didn't blow the fuse. so i started tapping components with my trustry chop stick and when i started taping on the pin end of the two can capacitors the noise reduced and then quit.

i turned the amp off, let it cool down and then turned it on again. the noise began again after warming up but there was much less of it. i tapped on one of them fairly smartly and the noise quit altogether. i repeated the turn off/warm up cycle several times leaving the amp on for over an hour the last time and the noise is gone, all controls work properly and the amp sounds great and it didn't blow the fuse.

i'm going to re-flow the solder on all the pins on top of the cans in the morning just to make sure that it wasn't a cold solder but my question is: what is there in a can cap that could be loose and would whatever that is cause a blown fuse?

[JoeAArthur]

What you have on your hands is a couple of mechanically unstable caps. The only thing inside are the two plate layers, the electrolytic and the mechanical connections to the outside tabs. It's those mechanical connections inside the cap, between the outside tabs and the internal plates that are mechanically unstable.

The fuse blows because the cap is shorted - internally across the plates.

Do your customer and yourself a favor and replace the caps. It'll only take a trip or so to a gig in some vehicle to bounce those connections back to where they are close to shorting the plates. The heat will do the rest of that gap.

Each fuse blow is a little shorter life on the power transformer.

That crackling sound is usually the dead giveaway on a bad electrolytic - it's caused by the electrolytic inside the cap reaching the boiling point. If you've never had a cap blow due to internal pressure, let me assure you it ain't a pretty sight. You do wear goggles... right?

[unclej]

What you have on your hands is a couple of mechanically unstable caps. The only thing inside are the two plate layers, the electrolytic and the mechanical connections to the outside tabs. It's those mechanical connections inside the cap, between the outside tabs and the internal plates that are mechanically unstable.

The fuse blows because the cap is shorted - internally across the plates.

Do your customer and yourself a favor and replace the caps. It'll only take a trip or so to a gig in some vehicle to bounce those connections back to where they are close to shorting the plates. The heat will do the rest of that gap.

Each fuse blow is a little shorter life on the power transformer.

That crackling sound is usually the dead giveaway on a bad electrolytic - it's caused by the electrolytic inside the cap reaching the boiling point. If you've never had a cap blow due to internal pressure, let me assure you it ain't a pretty sight. You do wear goggles... right?

thanks..you confirmed what i suspected and yes, i did have one blow on me. 'course in this case it was my fault. i was in a hurry and wired one in backwards. shot flaming schrapnel all over my work bench and burnt holes in one of my favorite hawaiian shirts. not to mention scaring the hell out of me. and after that i do indeed wear safety glasses.

thanks again

johnny