marshall jcm 800 model 2203 blown

[swirlo]

I started out with my 1 amp fuses blowing alot in my jcm 800 model 2203, then recently when I switched
from standby to play my amp and a strange hum came through my cabinet then a flash of light from the back
of the head and now I have no power at all. Does anyone know if this is serious or fixable????

[Robert M. Martinelli]

Hi and Welcome to the forum!

Serious? Hmmm...probably yes....

Fixable? Yes, but I have to warn you against the risk of tampering with tube amps without having the necessary expertise. Tube amps operate at very high voltages and can deliver enough current to KILL you if you don't know what you're doing (80 mAmps are usually enough to cause death).

Having said that, the first thing to learn is that fuses blow for a reason, so replacing them one after another without knowing why they blow can lead to a more serious damage. The rule is "replace a fuse only after the reason for it to blow has been identified and removed".

The loud hum you hear before the fuse blows could be due to an abnormally high current demand (eventually blowing the fuse). If you're not keen on tube amps my advice would be to bring the thing to a qualified tube amp tech.

Hope this helps

Best regards

Bob

[Chuck H]

+1
I realy only posted to "track" this thead via my "subscribed" threads to see what/if anything happens.

The flash was probably the power tubes going kerplooie. The hum was probably related to that, probably excessive current as mentioned. I'm going with a power tube failure (which probably telegraphed itself and should have been paid attention to) or bias supply failure. No doubt the amp needs new electrolytic caps AND even if new filters were installed I'll bet the electrolytic caps in the bias supply were ignored (and consequently failed). I guess most techs are lazy as this happens all the time.

If it's strictly a power tube failure or a bias supply failure odds are good that you have some circuit components blown. Not because they were bad but because shorts in the power tubes cause extreme voltage or current to reach circuits they were never intended to be on. Odds are good that some $2 components and new power tubes (not $2 components) are all that is needed in the hands of a COMPETENT tech. If you are due for new filters that will add to the expense of the repair. If you try to fix it yourself, as a complete novice you will likely spend more time and money on the project than if you bring it to a reputable GUITAR AMP repair man. Unless your interested in learning about guitar amp repair you should not try to fix this amp yourself. And under no circumstances should you consider doing the repair yourself for the sole purpose of saving money. You will cause more grief and expense for yourself.

If you are interested in learning about tube guitar amps... Buy a digital multi meter, a soldering iron and a couple of books on the subject. When your comfotable with safety procedures and some basic terminology we can guid you through the repair.

[Liam]

What Chuck said. But from my experience with these amps the filter caps often get changed unnecessarily. Get the amp to a decent tech - it's most likely a shorted power tube - but sometimes it's just because the bias was incorrectly set. I've serviced a whole bunch of these amps with the same problem, and the worst I've had to do is change screen resistors and filter caps. Most of them it's just been tubes and a rebias. They're pretty robust old things.

Liam