GCX + GCP + Marshall JCM2000 DSL100 channel switch issue

[Checco83]

Hi to all, this is my first post, but I didn’t find anything in the forum.

I have a Marshall JCM2000 DSL100 connected on loop #4 to the channel switch.
I also have on loop #1-3 three pedal (ehx hot tube and big muff).

I have this issue.

With loop #4 i can switch the green (off)/lead (on) channel of the JCM2000 but if I have loop #4 on (lead) and loop #3 off (big muff), when I turn on loop #3 the JCM2000 switch automatically to green channel and GCX and GCP show loop #4 on... but the amp switch channel...

Can anyone help me to find a Solution?

The footswitch of the JCM2000, for change channel, connect tip to Ground.... also try with a “y cable” thai connect top to Ground...

Thanks to all for your help!

[JohnClark]

Does Loop 4 have any other cable connected to it besides the one leading to the footswitch jack on the amp?

Did you get this GCX new?

[Checco83]

Whow, thanks for your super rapid reply.
No, it’s a used GCX.
And no, there isn’t any cable that connect any other loop to the loop #4. Only the cable for the channel switch in the amp.

Initially I used balanced cable (I had only that) and I was thinking about the cable. But now the patch cables are all unbalanced (mono).

Do you think about some modification in the GCX?
You think it has to work with my JCM2000?

Thanks.

[JohnClark]

The behavior suggests that the loops are connected together somehow. If there is no other cable connected to Loop 4, and this was purchased used, then it would seem likely that someone had this GCX modified to series connect the loops internally. It isn't an uncommon mod but it severely limits the GCX's normal flexibility to use any combination of the eight loops for any switching task.

Feel free to give us a call or write to info@voodoolab.com and we can arrange for an inspection if you would like.

[Checco83]

Hi John,
I’ve done some homework...

I recable my GCX:

Loop #1: switch channel green/lead JCM2000
Loop #2: EHX Hot Tube
Loop #3: fuzz
Loop #4: Big Muff

Now the switch channel of the JCM2000 work very well!!! But...

It seem that when Loop #3 and #4 are “On”, if I switch Loop #3 to “Off” also Loop #4 is turning to “Off” (dis-engaged, I hear the 2 relay “clicks toghether”), but the led on the front panel and on the GCP, remain “On”.

It seem to be a mod of my used GCX. I think I have to try to open it and verify... do you have any advice on where you think I should look to find the problem???

Thank you very much!!!

[JohnClark]

Based on years of seeing modded GCXs coming in to be put back to stock, the mod will likely stand out quite clearly once you remove the top cover. Take pictures of the inside and send them to info@voodoolab.com (The forum has limited bandwidth for good pictures, you can email them at full resolution)

It could be that someone tied the control of Loops 3 and 4 together... You say you hear a click when switching Loop 3, but does it click when switching just Loop 4?

[Checco83]

Thanks for your reply.

Loop #4 is indipendent (On / Off with GCP) and it work alone (I hear only “one” click of the relay #4).

Loop #3 is also indipendent (if loop #4 is off) but when (and only when) the Loop #4 is on, turning off loop #3 also switch off loop #4 (but only “internally”... the led on front panel and on GCP remain on!!!)
I also think to hear 2 click in this case.

Try to take a picture... thanks!

[JohnClark]

We received the pictures, but I don't see any signs of any mods or problems that would explain this behavior.

How exactly are you powering this GCX?

[Checco83]

The gcx is powered by a power supply 9vdc 1000mA.

Yesterday I tried with a tester the gcx, powered on. When I test the resistance of the Loop #4 with the tester to see if some current (12v I see) goes to the switcher #4, when I connect only the first pin of the tester I hear the click of the switcher... release the pin and connect again, another click of the switcher... when I try the same thing on the other loop, nothing happens (in effect I only touch the resistance with only one pin of the tester..).

Do you think some component gone???

[JohnClark]

The GCX was designed to be powered from a 9VAC power supply. But, it does have a full wave rectifier circuit so it will also accept DC power. However, when using a DC source it should be run at 12V.

9VDC will not reliably switch the relays... that is likely what you are actually seeing as the problem here if you are indeed using a 9VDC power supply.

p.s. We use 9VAC mostly due to the ability to provide Phantom Power over the MIDI Cable with minimal loss. If you will be using Phantom Power, I would suggest swapping to a 9VAC supply if your MIDI controller is compatible.