Mystery JCM800 Mod

Flanging_Fred

My Marshall JCM800 4010 combo was sounding a bit “off” at a gig at weekend so I bought it home to take a look at. Anyway, I’ve owned this amp for about 15 years but I just realised that it looks like it’s  been modded. There seems to be an unmarked extra socket on the back plate and I have no idea what it is for.  Anyone hazard a guess what this might be for? I’m a bit unsure if it’s an output, input or even for a foot switch. 

Any ideas how/if I could test what it does without opening up the chassis and trying to figure out the circuit (or blowing myself or the amp up).

The socket on the far right.

ICBM

You need to open up the chassis and take a pic. If you do it should be simple to identify what it does.

If you’ve already taken the back panel off it’s just another four bolts.

Nerine

Get it out for the lads! Let’s have a look! 

adamino

It could be an FX loop mod.I have one of these amps and there is no FX loop however one can buy the kit and have it installed.

ICBM

Given that the hole is drilled badly out of position, I would want to have a look anyway, to make sure there isn’t other poor workmanship inside. I know it may sound fussy, but it’s always a bad sign when someone can’t even mark and drill a hole correctly.

Nerine

adamino said:

It could be an FX loop mod.I have one of these amps and there is no FX loop however one can buy the kit and have it installed.

Perhaps the DI out has been repurposed as the other Jack for the loop. 

Flanging_Fred

Well, I got the chassis out and it looks to my untrained eyes that it’s an extra speaker output. It’s wired to the ohm selector and one of the original speaker outputs by the look of it. Please correct me if I’m wrong!

The question is why anyone would bother with an extra speaker output? I’m sure there must be a reason…

Neilybob

Extra speaker output= more speakers= more dbs! 

ICBM

Looks like it's wired as a headphone output, with no pad resistor and interrupting the speaker connection, which is extremely stupid. Dangerous to the headphones, will sound shit even if you don't blow them, and introduces a potential failure point into the speaker path so you could blow the output transformer.

Remove it and wire the speaker jacks directly to the selector as they should be.

Then think of something useful to do with the extra hole...

Flanging_Fred

Thanks @ICBM.  It’s good to know what it was intended for. Next time I get it serviced, I’ll request that it be disconnected.