JCM900 no sound after bias

[Alkaline198]

Hey all, I was working on bias my jcm900. I had it working ok and (being stupid) i tried to adjust the bias while i was playing it. As i turned the bias control the sound cut out and only hums.

Tried connecting the SEND/Recieve together to bypass and didnt get any sound.

Any ideas how to start? maybe I killed a transformer? No fuses are blown btw, none of the 500ma, main fuse or onboard fuses.

 

[fastdave]

Hmmm, you've certainly fvcked something there - but how the hell could you play and adjust the bias at the same time - did you do a video??

 

[fastdave]

On a serious note, there are many fuses on these boards - there are even encapsulated ones, so the chances are if nothing is fried, you have blown one - it is possible to check them all with an ohmmeter - if you get a resistance other than 0.1 - 0, it means the fuse is blown - check a good one and read the meter in case it's a little out of calibration. Do not replace the fuse with anything other than the correct type and value.
ONE big no no - allow the machine to discharge for a while before you work on it, and unplug it before you look at anything - I think you're probe has slipped and taken out something.
As Jon Wilder says, you should have a set of valves spare.

 

[SWM510]

If you don't have any experience working on tube amps and know all the safety precautions to take (draining caps, etc.), then you should take it to a tech.

Tube amps contain LETHAL voltages for days, weeks, possibly even longer AFTER they have been turned off and unplugged, and if proper safety cautions aren't followed this could be the first and last time you work on an amp.

Just some friendly advice so no one will possibly get injured......

 

[Alkaline198]

OK... so i found some other little black cylinder fuses on the board, testing now. What does BR stand for on a circuit board, they look like the black cylinder fuses

BR1 and BR2 have 4 pins ea. BR1 shows some continuty between a few but BR2 shows no continuty between any 2 pins


EDIT--- no fuses are blown, all good - does BR mean bridge recitfier? anyway to test these?

ALso, i can get a 'direct guitar sound' if i plug into the return on the back... i have no clue.. channel volumes control the volume like this, but no preamp or gain changes the sound


Could the power or output tranny be fried? how would i test this

 

[saranden]

I'm new here, but I do know there isn't an output traanny on this amp, the sound is amplified by the preamp which is a sort of hybrid with some IC's and 2 ecc83 tubes, then the final stage output is received fromthe preamp and amplified expanantially by the tubes, an ecc83 and 5881's or EL34's then out to you speaker cab, so, have of put the same type of tubes back in making sure you haven't snapped any wires off of e tube sockets, and have you checked the two fuses in the back.. Clean the tube sockets with switch cleaner or electricl cleqner. Before you poke your finger around it is an absolute necessity to discharge the two big filter caps as they are Likely to store electricity for long after you've turned it off. I discharge mine using a 1k0 resistor in line with one of the wires on an electric 240v bulb, that lights up and discharges the cap, do all contacts(red and yellow dots) I also touch the tube bases with the same probes just to make sure. Clean all of the jack sockets by spray cleaner, plug in and out a jack plug after spraying 20 odd times, I also sprayed into the high low switch and ohm selector switch. Overkill. All advice picked up from the Marshall forum. Great bunch of technicians and diy'ers who have been listening and taking advice.

 

[Jonathan Wilder]

I'm new here, but I do know there isn't an output traanny on this amp,

There IS in fact an output transformer on these amps. If it has a valve based power section, it DEFINITELY has an output transformer.

 

[RussBert]

I'm new here, but...

what Jon said!

 

[Ken]

Sounds to me like you craked a solder connection when you were fiddling with it. Start with the parts you were fiddling with and see if any have cracks or are loose.

Ken

 

[RussBert]

Sounds to me like he set the bias too hot and blew the HT fuse...

 

[Ken]

Sounds to me like he set the bias too hot and blew the HT fuse...

He says all the fuses are good.

Ken

 

[RussBert]

I realize that, but I still feel the same

 

[fastdave]

Sarenden, I hope expanantially was a typo - and don't make sweeping statements like 'no output trannies' - you could cause more confusion than good - listen to 'ole man wilder!
Incidentally - 2 stories from Marshall - 1. - set the bias voltage to 49V - lot of fvckin good if the circuit is modded to take el34's rather than 5881's.
2. take the fuse out and measure across the fuseholder - each side will give you the sum of 2 valves - again, lot of good with unmatched valves.
Listen to 'ole man Wilder.