About to buy a (broken?) TriAxis. Help requested!

I'm glad to have found this board! I would really appreciate a little help from experts like you good people on this legendary rack unit.

My roommate has left for the holidays and is planning to sell the remainder of his old and unused setup in 2020. I called dibs on his TriAxis, so he left that out for me to try while he's away. The thing is, I don't know if this piece of equipment is working properly.

It hasn't been touched in many years, but it did fire right up. So far, so good and I like a lot of the sounds I'm getting. What concerns me is that when I'm using the MODE button to cycle through the different sounds, there's a very loud and annoying sound when going from LD2 red back to RHY green.
It's as if the preamp has to go from all the gain to none of it, but isn't capable of an instant switch. It's only for about half a second, but still very annoying and very loud.

This only happens when going from LD2 red to RHY green, not between any of the other modes in the order that the MODE button allows. I don't have a midi board to try and switch between modes in a different order.

Does anybody recognise this problem? Is this particular unit broken, or is it a problem with the MODE button that's the same for every TriAxis?

Anybody? Please?

It sounds like a problem with one of the optocouplers, I doubt it's the mode button since it communicates with the digital logic of the machine only and not directly to the optocouplers.

Using a midi foot controller would definitely help to locate which LDR is the culprit. I also suspect it's not one of the LDRs in the tone stack since both RHY and LEAD2 share the same one.