BIZARRE MPC Live crash! Help?

TL;DR: Today during rehearsal for a gig on Wednesday, my MPC Live locked up three times during playback, when I touched my foot to an expression pedal hooked up to my Hydrasynth.

Long version: We are rehearsing for our first gig, a synth/vox duo. The MPC Live is the heart of the system; I use it to control a Hydrasynth Deluxe, a Pro 3, and an Eventide H9 Pedal, connected via USB through an isolator (noise). I also use the MPC to process vocals from two wireless mics, receivers plugged right into the MPC. Outs 1 and 2 go to a Radial Key Largo mixer, and then through that to my studio mixer (balanced connection). Outs 5 and 6 also go to the studio mixer through an unbalanced connection (an error). The studio mixer stands in for the house mixer and PA. All the items in the performance rig are powered through a Black Lion power conditioner; the studio gear is on a separate Furman power conditioner. The expression pedal is a high-end Mission Engineering pedal, connected through a TRS cable to the Hydra. I had the same behavior with the MPC running on battery and on a supply. It was hard to restart the MPC; simply powering it off did not help, I had to wait a few minutes and then it would come back on its own.

I've used this rig in pretty much this exact configuration for the pas several months, other than the unbalanced connection. But today, when were playing, the MPC locked up three times during the set; and each time it happened exactly when my foot touched the expression pedal. It did not happen every time I touched it, however. I am on carpet and a glass deskmat; rubber-soled shoes and rubber coating on the pedal; humidity is 30%.

The only change I have made recently was to change the ground lift switch on the Key Largo output to Ground (it had been Lift, but I changed it after a gig without the MPC last week - not exactly sure when.) I took the expression pedal out and reprogrammed the Hydra to not depend on it, and the problem seems to have gone away.

So, sorry for the longwinded explanation, but - does anyone have any experience with ground issues causing this kind of wacky behavior in the MPC Live? Any advice at all? (I will miss the expression pedal but not badly... but I would rather fix the grounding issue and see if that solves it.)

The MPC and the Hydra is connected via USB sending MIDI both ways, yes?
What is the expression pedal programmed to do in the Hydra? What type of MIDI messages are created and are these sent to the MPC or only internally to the Hydra?
It’s not creating pitch bend messages by mistake is it? I would guess that such messages sent en masse to the MPC might make it choke and crash, as I’ve seen things like this happen before.

Logrinn wrote:The MPC and the Hydra is connected via USB sending MIDI both ways, yes?
What is the expression pedal programmed to do in the Hydra? What type of MIDI messages are created and are these sent to the MPC or only internally to the Hydra?
It’s not creating pitch bend messages by mistake is it? I would guess that such messages sent en masse to the MPC might make it choke and crash, as I’ve seen things like this happen before.

Yes, a bidirectional USB connection. On most songs I am ignoring the input (monitor set to Off), but I do use the Hydra keyboard to control samples on one song. The keyboard sends expression pedal CC on two channels, and poly aftertouch. I could disable both of those as I am not using them. PB is not being send (glad the MPC has that MIDI monitor).

Poly aftertouch does also gobble up a lot of bandwidth. So if you could disable that, then you could check to see if the crash happens again if you force it or if it now seems ok.

Then again, it could be caused by something completely different, like some ground loop as you have suspected.

Hope you get it fixed.

Well, here's the slightly scary part: I hooked the pedal up again and cannot get it to happen now. No changes otherwise.

So I added a powered hub, and disabled poly AT. And I'm going to cross my fingers.