Marshall JCM 800, Series 2000, Cross Channel Interaction

[TomCarlos]

This is a sister post to my prior thread on the JCM 800 “Pots and Replacements.” Different problem, different thread.

For this amp, there is something odd happening with the controls as they relate to operating Channels A and B. I cannot find a similar post or explanation anywhere.


The footswitch:

The footswitch contains two SPST switches with a TRS plug. I tested the switches and they work fine. (It would have been nice to have LEDs on the footswitch).

Test and operation with footswitch:

As I discovered, there is NO WAY to easily switch between Channel A and B. You must do that in two steps- Ugggg.

If I insert the guitar into the “Mix A & B” input, the amp is working as designed (and expected – I guess).

The starting state, with the footswitch buttons not depressed, neither channel lights up or works – I get that.

If you click on the “left” button, Channel B engages and the controls work fine - perfect

Click off of “B” and then click on “A” - Channel A works fine - perfect

If you engage both buttons, both Channels A and B turn on and they work together – perfect.

But as I stated, you cannot switch between Channel A and B.

Test and operate without the footswitch:


I plug directly into Channel A (and only the Channel A LED is on), the controls for “A” work fine. While still plugged into A, if I change the Channel B controls, they do not impact what is happening with Channel A. All good.

The problem – If I plug directly into Channel B (and only the Channel B LED is on), the controls for B work fine. BUT… if I change the Gain level on the Channel A, that changes the volume on what is happening with Channel B. So there is some type of cross interference here between the channels.

I am looking at the jacks inside the chassis. Channel A and B are mono jacks. The Footswitch is a stereo jack (and there are 6 pins going into the PC board as designed). The Input Mix A & B is a stereo jack. And from the looks of the schematic, one side of the jack is grounded while the tip is the only one that gets the guitar signal? And this is where the footswitch engages A, or B, or both?

I am stumped over the problem of going directly into Channel B but the Gain control in A changes the volume of B.

Any ideas on what to look for and thanks in advance!

[g1]

The mix jack that is installed has 3 normally closed switches. The schematic does not show it like that. It could be just sloppiness in the drawing.
(edit: seems it is just poor drawing)

[The Dude]

The relays just ground the output of each channel depending on which is used. Look just above LED 1 and LED 2 for the relay contacts. I think you have to use the Mix input for proper channel switching.

Edit: From your description, I believe it is working as designed.

Edit (again): Just re-read your post. I don't think channel A gain should have an effect when channel B input is used. The switching jack on input Ch. B should remove signal from channel A in when a plug is inserted. Maybe the channel B jack is the wrong type?

[g1]

Doesn't the fault (crosstalk) also happen when using the mix jack and switching to Ch.B only? It must.

[The Dude]

There is a 15V supply going through R7 & R8. Each resistor then goes to the sleeve of jacks Input A and Input B respectively. That line also goes to the channel indicator diodes LED1 and LED2. The sleeves of the jacks are switch grounded. If input A is used, Channel B LED should not be able to come on and visa versa. If Mix input is used, both LED's anodes are ungrounded and can illuminate. So,....do the LED's work correctly for each give input?

[TomCarlos]

G1 - The problem ONLY happens when I am NOT using the footswitch, it is unplugged. That is why I am puzzled. When the footswitch IS plugged in and only Channel B is active, the Channel A Gain pot will affect the volume in B.

Dude - Yes, the LEDs are working correctly. And as per my reply here to G1, the fault is ONLY when B is the only channel selected. The Gain control in Channel A will change the volume level on Channel B.

I will turn the amp upside down (guts side up) to see what is happening with the relays. But it appears they are working correctly when the footswitch is engaged. I need to figure out why the problem exists when not using the footswitch.

Thanks guys...

[g1]

Try shorting out R22 while the fault is occurring. Does that remove the ch.A gain interaction?

[TomCarlos]

Try shorting out R22 while the fault is occurring. Does that remove the ch.A gain interaction?

G1 - Yes it does !!!

So again, footswitch removed, I am plugged into the Channel B input, jumper across R22 (15K), and the gain control of Channel A does not affect Channel B.

So does that tell us there is something going on with the Send and Return jacks for Channel 1? I cleaned those jacks before I did any testing.

Perhaps I should put a small patch cable between them and retest?

[g1]

That means the relay 1 is not turning on. Whether it is supposed to or not I'm not sure. R7 and R8 appear to be for the relays when the individual jacks are in use (no footswitch).
But a better question may be why there is even signal getting to ch.A circuit from the Ch.B jack.
This gets back to post #2 and whether the mix jack is the correct type.

[g1]

Looking again, R7 and R8 are for the LED's and not the relay. This is as The Dude mentioned earlier, the relays are not in use when using the individual channel inputs.
Verify that Tip to Tip switch (leaf) contacts of Ch.A jack are 0 ohms when nothing plugged into A jack.

[TomCarlos]

That means the relay 1 is not turning on. Whether it is supposed to or not I'm not sure. R7 and R8 appear to be for the relays when the individual jacks are in use (no footswitch).
But a better question may be why there is even signal getting to ch.A circuit from the Ch.B jack.
This gets back to post #2 and whether the mix jack is the correct type.

Thanks again G1 - I will pull that PC board tonight and take a closer look at the traces. From the schematic, it looks like the mix jack should be just like the Channel A and Channel B inputs. And intuitively, why would you have a stereo jack when you have a mono guitar plug!!! And now that I look at this closer, I see some "goop" around the mix jack (on the pc board). So maybe someone dickered with the jack.

[g1]

The jack as drawn would be a very uncommon cliff jack, with 2 N.C. switches and 1 N.O. switch. The N.O. (normally open) would be at the tip contact. It would be rare enough that someone who did not know better would just replace it with the standard type that has 3 NC switches.
I'm struggling to try and visualize how it would work as drawn.

[TomCarlos]

G1 -

Excellent reply on this pot - I will try to investigate further.

I joined the Marshall Amp Forum just now and opened a post. Let's see if we get anything from that group. If yes, I will post results here.

In the meantime, as you can see, the amp currently has a stereo NC NC NC jack. Yes, someone replaced this jack somewhere along the way and used some bus wire to make connections to the traces. I am also attaching a couple photos of the PC board.

Now that I have the PC board pulled, I am also going to look at the pots that are mentioned in the sister thread.

[The Dude]

...... And intuitively, why would you have a stereo jack when you have a mono guitar plug!!! ......

The ring connection and associated switch contact are used only as a switch so the amp knows which input is being used.

[The Dude]

The jack as drawn would be a very uncommon cliff jack, with 2 N.C. switches and 1 N.O. switch. The N.O. (normally open) would be at the tip contact. It would be rare enough that someone who did not know better would just replace it with the standard type that has 3 NC switches.
I'm struggling to try and visualize how it would work as drawn......

The Mix jack tip and switch contacts are NO. This is so that when the jack is not used, signal is not coupled between Input A and Input B- separating the 2 inputs. Then, when the Mix jack is used, the switch closes tying Input A and Input B together so that both inputs are fed signal. Channel switching is then decided by the relays. If a normally closed tip/switch Mix jack were installed it could certainly cause the problem described because it would work in reverse and both inputs would be tied together when the Mix jack is not used.