Marshall JCM900 4100

[titan7]

Man I am having no luck.

I pick up a used 900 head, works fine played it a couple times. Sunday I am playing, take a break, put it on standby, then switch it off, I get a loud pop.

Tonight I turn it on, power light comes on, no sound, thinking its on standby I flip the Standby switch, POP, blew the main fuse. I replace the fuse, turned standy off, powered back up, no sound, but the power light is on. I looked in from the top and the power tubes are lit but I can see light on the pre-amp tubes?? Not sure if I should?

So it powers up but no sound, if I put in on standby it blows the main fuse?

any suggestions?

thanks!

[Enzo]

Yeah, same suggestion I have for anyone getting a used amp - new power tubes.

Aside from that general advice, the things you describe are classic symptoms of bad power tube. Pops and crackles are often signs of failing power tubes. Blowing fuses is usually bad power tubes. Few other things in there can blow the fuses.

DOn't you have a mains fuse AND a high voltage fuse on that? Are they BOTH good? Pull them and check with a meter, don't rely on eyeballs.

Remove the rear grille if it is still there. Now watch the power tubes closely when yuo flip the power on and then teh standby on. ANy tube that sparks inside is bad for sure.


If it is not tubes, then something more dire is going on.

[titan7]

Enzo,
I looks like there is onlt the 4A 125v main fuse, and two valve fuses. Those are fine. The guy I got it from said he had the tubes replaced a year or so ago and had not played the amp?


Man I though I got a good deal on this thing for $800 with the 4x12 cab.

[Enzo]

New does not guarantee OK. If all the fuses are OK, then we need to start looking for missing voltages and go from there.

[ALEXZANDEREPAIR]

I Have A Jcm 900 4100
Ive Been Trying To Get Help With Mine To
Try Connecting Your Guitar To The Effects Return In The Back And See If You Get Sound If You Do Then Is The Pre Amp
Then Youll Be On The Same Boat Im On
The Shit Out Of Luck S.s.