Marshall 2100 JMP issue

[p2pAmps]

Guys,

I had this JMP 2100 combo come in the shop.  Guy wanted the Master volume someone stuck in there pulled out which was 5 minute job.  Another weird issue however.  When you turn the Bass control up al the way the amp goes ultra thin and bass is gone.  Happens around 9.
I also noticed someone put a 22uF cap where a 10uF is suppose to go.  Not sure if that is an issue or not as I haven't had the time yet to dig in. 

The new Shop Dog is causing me to lose some sleep LOL...

~Scaggs

[drgonzonm]

Do you have a schematic? 

The forum does not not have a schematic.   It is allegedly  is  a beefed up version of the Mark II, and the current closest amp in design is a reissue bluesbreaker (62')

 Where on the amp was a 10uf replaced with a 22uf?  Part of the bias circuit? 

Thanks for reading this post.

[p2pAmps]

I do have a schematic on this one that I found online.

So here is what I have found so far. 

Bass pot wrong value (25K)  replaced with a 1M.  Bass pot was grounded - someones mistake...
This amp was converted to EL34s from 6550s - poorly
Added 5.6K grid stoppers as there weren't any
Replaced twi cliff jacks that were bad
Mixing resistors were wrong, fixed that.

Now I have a working amp again that sounds good.  What a dang mess this thing was.

[sluckey]

This is an unusual amp. I'd like to see the schematic. It would be a good addition to Doug's schematic library as well.

This info from Dr. Tube...

2100 JMP Lead & Bass, 50W 2x12" combo

This amp was in production from 1973 up to 1976. It is pretty rare as it was only available through mailorder. This is a two channel amp, one channel is that of an 1987 Lead amp and the other channel is that of an 1986 Bass amp. The 1964 is the head version of this 2100 combo.

[drgonzonm]

Won't argue with Steve or Dr. Tube, but if the 2100 was made before the other amps, wouldn't it make sense, that the bass channel is also found in the 1986 bass amp, and the lead channel is also found in an 1987 amp.  Did not find a 1964 marshall in doug's libary. 

I know Marshall's are great amps, and have stood the test of time but time travel?   

[sluckey]

The JMP 1987 and 1986 were in production from 1967 up to 1981. That is six years BEFORE the JMP 2100 was introduced.

[p2pAmps]

I re-looked at the schematic I had and it is not the right one for this amp.  So---  I still need a 2100 schematic.  I have one last issue to solve and that is the inputs.  Something still is wrong...

[sluckey]

I would think you could look at the schematics for the JMP 1986 and 1987 to resolve any input or preamp issues.

[VMS]

You can find the 1964 schematic that somebody has drawn here:


http://www.marstran.com/SCHEMATICS.html

[sluckey]

Thanks!

[p2pAmps]

You can find the 1964 schematic that somebody has drawn here:


http://www.marstran.com/SCHEMATICS.html

Thanks so much