Marshall JCM 800, Series 2000, Cross Channel Interaction

[TomCarlos]

Ack... so confusing. Thank you Dude. So it looks like you confirmed G1's assessment?

Tip = Normally Open
Ring = Normally Closed
Sleeve = Normally Closed

Is that correct?

[The Dude]

Yes. That's correct. The jack circuit is a bit confusing because there's lots of switching going on, both for LED's and Channels. The designer must have been good with the Rubik's Cube.

[The Dude]

I confess, I've never had to find a Cliff S2/BBM. So, out of curiosity, I did some searching for a source. Couldn't find one anywhere- not even on the Cliff website. I wonder if it's discontinued. You might have to make one by modifying/bending the switch contact. I suspect that's why the wrong jack was installed in the first place. The previous tech probably couldn't find one.

The guy in this thread modified a jack to work (with pictures).

https://www.thefretboard.co.uk/discu...ff-jack-socket

[TomCarlos]

I confess, I've never had to find a Cliff S2/BBM. So, out of curiosity, I did some searching for a source. Couldn't find one anywhere- not even on the Cliff website. I wonder if it's discontinued. You might have to make one by modifying/bending the switch contact. I suspect that's why the wrong jack was installed in the first place. The previous tech probably couldn't find one.

The guy in this thread modified a jack to work (with pictures).

https://www.thefretboard.co.uk/discu...ff-jack-socket

Way too funny Dude.... I found that same link not more than 1 hour ago. I was searching to see if I could find this jack... no such luck.

Depending on if this amp is something my friend will use often, I am guessing he will primarily be using Channel B. So if that is the case, I would jump some points on the PC board and leave the Tip open. That will allow him to go into Channel B directly and thus eliminate A from the picture.

I will go through my box of stuff tonight, to see if by some miracle I have such a jack or see if I can make one!!

[g1]

Then, when the Mix jack is used, the switch closes tying Input A and Input B together so that both inputs are fed signal.

This switch is redundant and I guess a fail-safe in case either of the tip switches at A or B inputs don't close.
I've been trying to figure too many scenarios for both how it should be, and what is happening with the wrong jack, thus my confusion. Thanks for the extra insight.

Still trying to figure out why with wrong mix jack, using Ch.A input, Ch.B controls are not affecting anything.

[g1]

Still trying to figure out why with wrong mix jack, using Ch.A input, Ch.B controls are not affecting anything.

I think Ch.A jack tip switch is not closing when nothing plugged in (post #10). Fixing this may solve the problem even with wrong Mix jack.
(edit: no, this can't be true as Ch.A would not work when using Mix input as currently configured)

[The Dude]

Well, crap. I just accidentally deleted that post and I'm not typing the whole thing again! Basically, I was saying that the Mix input won't work properly without the tip and switch connected when the jack is inserted.
Edit: That is probably why the previous tech jumpered those traces together. Aside from "building" a jack, I think this is probably the best option (leave the jumper). Just use the Mix jack and the channel switch. At least you can use both channels with that arrangement. Ignore the other 2 inputs.

[g1]

Edit: That is probably why the previous tech jumpered those traces together.

Did I miss something? I saw a few traces repaired, but no jumpering together. If that happened, it might explain some things. It also seems neither tip contact could be getting grounded at any time, or they both would be, and then there would be no sound.

[TomCarlos]

Did I miss something? I saw a few traces repaired, but no jumpering together. If that happened, it might explain some things. It also seems neither tip contact could be getting grounded at any time, or they both would be, and then there would be no sound.

I do not believe that traces were "jumped." It appears that during a previous repair, traces were damaged as well as the holes for the jack to be soldered. So what you saw in my photo was buss wires used to make connections (from a previous tech). I will double check this work an d connections before I reassemble the amp for testing.

I have an extra jack that I modified and will insert into the amp tomorrow. It is similar to what Dude found on the net. I bent out the metal on the Tip to create an "Open" state but it will short when the plug is inserted. I need to let the Superglue dry!! But I hope this will do the trick.

Yes, there are other workarounds but it would be great to restore the amp to how it should work. But for me, the lack of a single button to go from A to B is a deal breaker. My friend suggested using an A/B box in reverse. One line into the A/B, one line to the Channel A input and another to the Channel B input. But I explained, it doesn't work that way!!! So we will move on.

More to come tomorrow.

[TomCarlos]

Update ... disappointing news...

I checked and rechecked the continuity on the modified jack before installing into the amp. I made sure the Tip connectors would be shorted ONLY when the Plug in installed. I installed the Jack on into the pc board and made the connections to the traces and bus wires that were there. Yes, the bus wires follow what is left of the existing traces.

Unfortunately, I have the same problem as before. With no footswitch, I plug into Channel B and the Gain control of Channel A affects B.

If I add the footswitch and plug into the Mix A & B, I am now getting a very loud hum. So I don't know if this is because of the footswitch or if a problem previous existed but was masked by the improper jack. I can enable the LEDs for A and B but there is no way the amp can be used in this state.

So maybe this is my next step.... Just in case there is a problem with the Jack itself, REMOVE the jack but wire as if the jack were there? So I would tack a wire across the Ring and Sleeve pins but leave the Tip connectors open. Would this make sense or can you recommend something else for me to try first?

[g1]

You can just put some tape or paper between your new modified switch contacts so they do not close when you plug in to Mix. Verify with your meter that they are not closed when plug inserted..
Also verify that all the other switch contacts are closing when they should (A,B, and Mix jacks).. Measure on the solder side so you are not putting pressure on the top 'leaf' arm.

[TomCarlos]

G1 - Using my moded jack, I made sure the contacts are behaving as expected. The connections across the Tip are open, the connections across the Ring and Sleeve are closed. But I want to skip dealing with the Mix system for now.

In a way, I am back to square 1, and then a new issue has risen if I try to use the Mix input. But let's forget that for now.

I still have my original problem.... If I plug into Channel A, the amp is fine. If I plug into Channel B, Channel A's Gain will change the volume level of B.

Now then, if I plug my guitar cable into "Mix A & B" and and no footswitch, both LEDs are on and I get a very loud hum. And there are other problems if I try to use the footswitch.

So to try and simplify and solve the original problem, I will NOT plug anything into the Mix or Footswitch.

Next, going back to what G1 had me do before, I plug into B and then put a jumper across R22. Yes, this fixes the issue.

So, R22 is now connected to ground. The ground connection then goes through the Channel 1 Send and Return jacks and that goes to one side of the R204 a 10K resistor. So that part of the signal path is now grounded. BUT, the signal coming through Channel B makes it's way through R203 (another 10K resistor) and that then goes through the Mix Send and Return which then goes through the 27K resistor and onto V3a. is that how it works and is that what we want, Is that correct??

So without that jumper across R22, are we relying on Relay 1 to be functioning correctly so that Channel 2 is muted? Maybe that Relay is NOT (normally) closed and that is why we have this issue? And vice versa, if Relay 2 was bad, I would expect that Channel B would bleed into Channel A.

[Greg Robinson]

This jack isn't a drop-in replacement, but it IS a source of a proper "make" terminal. Cliff jacks can be disassembled with some difficulty, stripping the contacts out to be rebuilt. The way I'd approach this is to source this Dunlop S4 style BMB replacement jack (the only source of Cliff "make" style contacts I know of), strip out the contacts (jewelers screwdrivers and a small vice prove useful here), and do the same with the tip contact of an S2 jack, replacing them with the parts from the Dunlop jack.
Hope that makes sense?

If that's not workable for any reason, Neutrik also make clones of Cliff S2/S4 style jacks which are easier to disassemble, and they do have some available options with "make" contacts, I've purchased them before from Mouser. Don't have a part number to hand and not sure they are a direct drop-in either, but I can look further into it if you need.

[Greg Robinson]

Ok, just did some looking. Mouser has Neutrik S4 style with a make contact: NMJ6HCD2-SM (SM on the end of the part number indicates "sleeve make", there are also RM "ring make" which are slightly more expensive for whatever reason, I've not seen TM "tip make" available anywhere) which you could salvage contacts from.
You'd then need a Neutrik S2 style jack to install them into: NMJ6HFD2.
Now, the Neutrik jacks are slightly different from Cliff - the spacing side-to-side between the pins is slightly narrower on the Neutrik at 16.23mm vs 16.5mm for the genuine Cliff, but the largish PCB through holes should account for that without issue.
The other problem is the distance from the "Front Mounting Face"/FMF as Cliff calls it, which is shorter on the Neutriks vs the Cliff PCC style pins you need, at 4.35mm for Neutrik and 4.8mm for Cliff. The barrel on the Neutrik is also slightly longer than the Cliff though, so an extra fiber washer should take up the slack. Cliff ones are 1.6mm, Neutrik are 1.2mm, both thicker than the 0.45mm we need. There are thinner fiber washers produced by Cliff, but I don't recall anyone that stocks them right now. A few seconds on a belt sander should thin one out for you easily enough though.

[TomCarlos]

Ok, just did some looking.

Thanks Greg Robinson.... I am looking at all options. But first, I want to resolve the problem of using the amp with the non Mix inputs and switch. I think I have some ideas on "maybe" how this can be done.