Marshall JCM 800, Series 2000, 235 Watt, Pots and Replacements

[TomCarlos]

Team MEF....

A friend brought over a Marshall JCM 800, Series 2000, 235 Watt amp for me to look at. There are a couple issues that I will address in a separate thread. But for now, I would ask your help to identify if the schematic I have is the same as what is in the amp.

Before I pull anything for measuring....

For the preamp, the PC board is the "PBC JM62 202A"

When I check the schematic (from the Service Manual, Page 18), it shows VR8 as a 100K Linear but the stamping on the pot shows 22K.

On the schematic, it shows VR9 as having the center pin and one side of the pot to ground. But looking at the installed pot, neither of these pins connect to ground (and I can check that with my meter).

For VR11, that is the volume pot on Channel B. I need to confirm if this is the correct value and if Linear or Audio.

So my first question on this amp, does anyone know if the Service Manual schematic is accurate?

Next, for VR9, it looks like this was a mod? (I will need to trace this out). I would think Marshall would have used a 3 legged pot soldered into the pc board. Please confirm. The four resistors just below VR9 are the original value (R45=1K, R46=1M, R47=100K, R48=22K).

Lastly, no big deal that VR9 and VR11 are not physically connected to the other pot tabs. I checked that the case of these pots is connected to the chassis. Anything else I should be looking at?

Service Manual - marshall_jcm800_series_service_manual.pdf

Thanks, Tom

[The Dude]

The schematic looks correct and matches what I have. It looks to me like maybe it was repaired at one time and the tech didn't have the correct parts.

[TomCarlos]

Thank you Dude.....

In assessing this amp, I wanted to start with the visual inspection and I noticed the change out of the pots. My friend reported two issues - (1) the touch and feel of the volume pot (VR11) is not the same as on his other Marshall amp. So I was thinking the pot value might be off or a Log pot was changed out for a Linear pot. (2) I will do some more testing, but if you plug directly into the Channel B input, if you dicker with the Channel A Gain control, that affects the volume of Channel B. So I was wondering if any of the mods might have caused this. That will be for another thread after I do some additional testing.

I guess for now, if I had to change out any of the pots, were would be a good source for these? These are the three legged pots, the center pin is staggered forward with round shaft.

[Mars Amp Repair]

Hello Tom,
You should check AmpRepairParts.com. They have all kinds of amp pots that are otherwise difficult to source.

[g1]

For VR8 (Sweep), if they thought it had too much range, they may have reduced the value.
If VR9 does not connect the ground, then I would not expect the tone stack to work.

[TomCarlos]

Thank you Mars and G1.

I can report the following: VR9 has been changed from a 22K pot to a 100K pot. One end of the pot is NOT connected to anything.

UPDATE - The schematic shows VR9 as a 25k Pot. That has been changed in the amp to 100K.

I also noticed that there is nothing in the place where R49 should exist - see my prior photo. And I do not see R49 on the schematic. So maybe that was deleted by Marshall on the schematic and pc board. Other than the alternate wiring on VR9, everything else looks original. And I know that the tone stack must end up with a ground connection - I just cant figure it out (yet).

The tone stack in both Channel A and Channel B are working. The amp has ample gain and volume and tone controls.

But there is something odd happening with no footswitch and using Channel A and Channel B inputs directly. I think I will need to open a separate thread and title for that.

For now, I was hoping someone might shed some light on VR9 and if there were any known mods.

Thank you,

Tom

[g1]

Your statements in post #6 about VR8 and VR9 are different from what you said in the 1st post. Please fix whichever one is wrong.

[TomCarlos]

Fixed....

The schematic shows that VR8 should be 100k. The amp has a 22k pot installed.

The schematic shows that VR9 should be 25K. The amp has a 100k pot installed.

[g1]

Sorry, I didn't realize there were 2 pots that were wrong value. Sounds like they were reversed, maybe from new or during repair. Then, a bad VR9 got replaced with same one that was there. I think you can just swap them around.
One end of VR9 can float disconnected like that. They draw it as if the wiper is shorted to one lug, but that lug does not need to be connected.

[TomCarlos]

Thanks G1.... observation and recommendation makes sense!! I will talk to my friend about making that change.

[The Dude]

FWIW: I would connect as shown and ground both wiper and outside of pot. The reason is that if the wiper connection ever becomes bad, there is still a ground connection for the rest of the tone stack to function.

[TomCarlos]

Warning: Do not proceed with this thread beyond this post !!

Now I understand why the confusion. I absolutely believe there is an error in the Preamp Schematic. In fact, the Preamp Schematics for Version 1, 2, and 3 all have the same error!!!

Looking at the panel and what is written on the PC board, the pots placement vs schematic are correct. That is for Channel A.

The problem is with Channel B. The most obvious error is that the schematic refers to VR8 as the Sweep. But if you look at the PC board and the control on the panel, the pot in position 8 is the Middle Control. An error also appears for VR9. The schematic shows VR9 as the Middle but that one (as per the PC board and panel) is the Sweep. And in doing some tracing of the VR7 to VR11 pots, I see some mistakes.

On thing to note in the photo... This is the dangling pot... in position 9, the Sweep pot. You can see that one leg does not connect to anything. The center pin connects to R44. The other outside edge connects to the junction of C29 and C30. So clearly, the schematic is wrong. And there could be other errors. I will stop here!!

So thank you Mars, G1, and Dude. If I should ever find a corrected schematic, I will post it. For now, the tone stack is doing something so I will leave it alone!!

(From right to left, Vr11, Vr10, Vr09 is dangling behind the PC board).