Dead JCM900

[mr seeds]

This is my first post on here so a big hello to all

I have a jcm900 dual reverb 100W combo which was fine until the volume started fading out and then finally it gave up the ghost.
I've taken the chassis out and inspected and continuity tested the fuses and all is well there but Ive noticed that R30 which connects to the full/half power selector switch and one leg of a smoothing cap has burnt out (22k/1W?)

Do any of you learned people know what may have caused this?

I will be fitting new tubes soon when watford valves re opens after Christmas so could it have been a dodgy valve which caused the burnt resistor?

The amp is usually run at half power.

thanks in advance for any light you can shed on this problem.

Gaz

[Twist]

Does the power light come on and do you see the filaments glowing inside the tubes? Besides JCM900 does it have a model or chassis number on the back?

[mr seeds]

Yes, power light is on and all heaters are lit.

The model is 4210, hi gain dual reverb 100W 2x12 combo.

There is a slight hum from the speakers when the amp is powered up but this does not increase when the volume is turned up.

cheers

Gaz

[Alex R]

R30 feeds the PI and the preamp with high voltage. Looks like something is drawing/has drawn too much current in those areas - ie, not the power tubes. Possibly a short to earth.

It is possible for preamp valves to short, but unlikely. Have you tried Hot Rox? Good quick cheap UK supplier.

Here are some prime suspects to check:

The filter caps in the preamp, particularly the one that R30 connects to. Any leaks or bulges to be seen?

Check values of R22 and R23. These are the PI plate resistors, 82K and 100K, which R30 feeds. Also C14. I have known these to fail.

That's enough checking for now...

[mr seeds]

Cheers Alex

I've left the amp at work so I'll have to give it a look over after xmas.

I did give the boards a good look over for damaged components and only saw R30 damaged but I will test the components you mentioned and post my findings.

Gaz

[Alex R]

...the other possibility is the rectifier that feeds the preamp heaters with DC. That fails quite often. One step at a time though...

[mr seeds]

All the heater elements are lit Alex so I would say the bridge is ok.

cheers

Gaz

[mr seeds]

Ok folks,

A little update.

I went into work today to have a look at the amp and measured r22 and r23, both measured ok with no signs of burning. C14 is also ok.
There are also no leaking caps
I also checked the onboard 250mA electronic fuses which measured ok for continuity.

I did notice one thing with a couple of the output tubes. There were 2 loose pins in the bases of 2 of the EL34's, one very loose, could this have blown R22?

Cheers for the help so far guys but we aint done yet!


Gaz

[Alex R]

I did notice one thing with a couple of the output tubes. There were 2 loose pins in the bases of 2 of the EL34's, one very loose, could this have blown R22?
Gaz

- shouldn't be loose with the tubes in, bound to be loose with them out. Not a likely cause.

Are you getting any HT voltage? (pins 3 and 4 power tubes, counting from below c/w).

[mr seeds]

I havent tested HT voltages yet Alex.

I wasnt refering to the socket pins being loose on the output sockets, Its the tubes themselves.
The pins at the bottom of the glass tube where they go through some sort of bakelite are loose on a couple of tubes, maybe three in total.
Do the pins go through into the vacuum of the tube or is there a connection within the bakelite?
Could a blown output tube cause the R30 burnout I wonder?

Cheers

Gaz

[Alex R]

- if the top if the tube inside is still silvery or black, not white, then the vacuum is ok.

R30 burned suggests a preamp fault, not a power tube failure. Was its value still ok?

[mr seeds]

Its not giving a value as its burned out

[Alex R]

Something in the preamp is passing too much current to ground. If you can't identify it visually or with your meter you're going to need some equipment that will register the short without blowing a fuse each time - the simplest way is a light bulb limiter - ask if you want to know more about this.

[mr seeds]

hi all

Its up and running now after changing R30 and re-valving.
I gave it a good 2 hour test at rehearsals last night at full half power (50W) and it never missed a beat!

Maybe a dodgy preamp valve?

Thanks for the help folks, I,ll be on again soon I think as I have a dodgy Laney coming in

cheers

Gaz