Marshall JCM800 100watt lead schematic.

[DrTurbo]

I've got a Marshall JCM 800 100w lead amp that has an issue with the boost channel dropping the signal.

Does anyone happen to have a schematic for it, I'm really struggling to find one, even on dr tube.
Its a combo and has a head in it labelled as a model 4212, rated output of 50watts, but has four 5881 output tubes, suggesting it is 100watt, like it says on the front?? Strange..

But I'm in the middle of working out if its the CA3046 IC that does the switching, or something else..

If anyone can shed some light on this or has a schematic it'd be appreciated, the only diagrams I can find don't show the pins on the ic very well..

Thanks in advance..

[g1]

If it has 5881 tubes then it's been converted. This schematic should match your preamp I think.

[DrTurbo]

Hi,
Thanks for that.. Yes it is fitted with the 5881's, I'm guessing it was once a 50w then, it has a Marshall sticker inside saying it has the 5881 valves.

I think the most of the trouble is coming from the valve sockets which look like Beltons, as soon as the boost channel is switched in the signal drops right off, which I guess is the ca3046 grounding the signal via C40.

There's definately issues with V1 and V3 as the input will completely go if V1 is touched, and same on V3 will lose the reverb, everything else pretty much tests ok. You do get a small signal through the boost channel as you can hear that full wave rectifier part doing its bit as the volume is adjusted.

Thanks again, once I've got the sockets changed I'll let you know how its going.. she'll sure sing on the first channel, man that thing is loud!

[g1]

That sticker will have been put in by someone, they didn't use 5881's or those stickers til the JCM900 series. (aside from very earliest Marshalls)
Not all that relevant but you know the unit has been modified so there may be other things modified in the preamp as well.

I seem to recall a volume drop being normal for the boost channel, but from your description it is almost unusable? Hopefully the sockets will clear it up. They may just need a good cleaning with deoxit or something.

[Helmholtz]

In the early 90s many Marshall models were OEM equipped with Russian 5881s.
Here is the 1994 conversion manual: http://home.myfairpoint.net/vze1u14s...conversion.pdf

[g1]

In the early 90s many Marshall models were OEM equipped with Russian 5881s.
Here is the 1994 conversion manual: http://home.myfairpoint.net/vze1u14s...conversion.pdf

Yes, that is when the JCM900's came out. No JCM800's came out of the factory with 5881's.
That conversion manual does instruct how to perform the mod on JCM800's.

edit: forgot to mention that during the JCM800 era, the 'alternate' power tube was the 6550.

[DrTurbo]

Thanks again for the input guys, interesting as to whats gone on with it, it all looks pretty factory inside to be honest, apart from the internal wiring all having cable ties apart from the preamp section..

I've got the sockets now so we'll see how it is after they'e been changed, whatever happens they need changing anyway..

[DrTurbo]

In the early 90s many Marshall models were OEM equipped with Russian 5881s.
Here is the 1994 conversion manual: http://home.myfairpoint.net/vze1u14s...conversion.pdf

Thanks for that, appreiated that is..

Something has definately gone on with it, as the chassis is labelled as 4212, 50watts, but has the quad valves and 100w label on the front..

[glebert]

Does it have 100 watt transformers? Does it just have one blue can cap next to the tubes? The other 4212s that come up in GIS have two. Seems like quite an odd amp.

[drewl]

I often find intermittent tube socket connections on these amps, that would explain why touching them causes the signal to drop off.

I've also found several amps with the 3046 bad.
you have a scope to check the chip?

You can make one with transistors on a socket or just buy a 3046.

That's how I got my 2210 head cheap!

[g1]

That is definitely a 100W chassis with 4 stock power tube sockets. Aside from the 5881 mod, everything looks stock inside. (coupling caps C32 & C34 from PI, and screen resistors look to be changed in accordance with conversion pdf maybe a resistor in bias circuit as well).
Attaching schematic for 100W version of split channel (2210 and 4211).

It seems that when they ran out of rear plates they subbed in 50W versions and put stickers over the model number area and fuse holder areas. Probably also did vice versa when they ran out of rear panels for the 50W models.
I think the sticker that went over the model number area got removed on yours.
Here's an example with the model number sticker and you can barely make out the stickers for the fuse holder like yours has.

[DrTurbo]

Thanks for all of the input everyone! No I havent scoped the 3046, but basic diode tests show the transistors working so hopefully ok, hopefully the sockets will clear the issues..If not i'll dig deeper..

[DrTurbo]

Hope all you guys had a good xmas, just to say I'll be changing the bad sockets within the next couple of days, I'll do all five as a matter of coarse.

Another thing that does look to be bad is C48, across pins 2 & 3 the grid and cathode, looks to be open circuit, part of the boost channel.

I'll keep you updated when its back together..

Thanks

[DrTurbo]

Just an update, it did turn out to be the valve sockets, all five pre sockets were replaced as they all felt a bit rubbish, interestingly C48 shown on the diagram as 47pF, was actually a 220pf.
Its fitted with a 47pF at the moment as i had those, when the 220's arrive I'll swap it and see if theres much of a difference.
That boost channel certainly is lively though!

[g1]

interestingly C48 shown on the diagram as 47pF, was actually a 220pf.
Its fitted with a 47pF at the moment as i had those, when the 220's arrive I'll swap it and see if theres much of a difference.

The 47pF is on the 50Wschematic. The 100W schematic in post #11 shows it as 220pF, although it is called C45 there.