Marshall JCM800 combo problem

[ChrisF]

I'm new to this very informative site so Hello-and thanks- to all. I wondered if anyone could help with a problem I've just experienced with my jcm 800 2X12 100W reverb combo. Clean channel working OK. But, when you switch to the "boost" channel, suddenly instead of a lift in volume and some nice crunchy overdrive, you get a muted mush of reverb (which is actually switched off!!) and as much distortion as a Dalek speaking through a fuzz box. I should say this doesn't happen immediately- the amp needs to have been on a while for it to occur.

I've had the amp about 14 years- but I haven't owned it from new. It's had only occasional use for many years but now I need its services again on a regular basis, it seems it is not equal to the challenge. The output valves were all changed together ages ago (to Siemens- I didn't have it re-biased, I know naughty naughty-but it's always worked ok) but all the pre-amp valves are of unknown age and origin. I have already cleaned the effects loop sockets as these have caused probems in the past, and I've re-seated and swapped round the valves, but to no avail. Does any one have any ideas where I might start looking-new valves, for example? All suggestions/ideas welcome. Ta

[Ray Ivers]

Chris,

I'm assuming your amp is a model 4211 (it's stamped on the back panel) ? There were two quite different versions of the 50W 1-12 (4210) channel-switching model, so it might well be the same on yours. A quick-and-dirty check is to see if the front-panel knobs have set screws (as opposed to push-on), but a more definitive one would be to pull the chassis and look for wiring running from the pots to the PCB, instead of the pots being mounted directly on the PCB as in later versions - also, the early version will have a dual-pot master-volume control.

If it's the earlier version, it could be a shorted switching transistor; in the later model it's more likely a bad switching chip (but it could be any number of things).

Does the boost channel's LED light when that channel is engaged?

Ray

[Amp Kat]

A faulty footswitch jack can cause some freaky problems like this also but it does sound like a switching problem or isolation breakdown

[ChrisF]

Thanks Ray and Amp Kat for your responses. Yes, it's a 4211 and it has push-on knobs. And the LED does light up as normal. I'm using a mono lead only connected to the footswitch (stereo-to-stereo leads seem unobtainable, short of making one myself, which I haven't bothered to do as I don't need to mess with the reverb at all), if that's significant.

ChrisF

[Ray Ivers]

Chris,

OK - assuming that you've already tried swapping preamp tubes around to eliminate that possibility, and that you don't have an oscilloscope, I would suggest removing IC1 (CA3046 transistor array; Mouser has the $5 NTE912 equivalent, perhaps you can find a genuine CA3046 if you hunt around long enough) and replacing it with a socket and new chip. If this doesn't help, at least you've narrowed down the problem area greatly. If you've never removed a 14-pin chip before, you may want to take it to a tech so as not to damage the PCB.

If you open the amp up - which you'll have to do anyway at some point - you may luck out and find the problem on a visual inspection, who knows.

Ray