Optocouplers can definitely fail for weird reasons, they’re pretty fragile. No idea if that’s what happened to yours, but it has happened to me a couple of times. Once all it took was putting a piece of gear with an acrylic enclosure into a new case that apparently had a lot of static charge in the foam lining from the factory. MIDI input still sort of worked, but it depended on what the source was - with some controllers it wouldn’t work at all, with others it was inconsistent and had things like wrong o skipped notes, completely random velovities, etc. Passing any MIDI through an original MIDIpal first made it work nrmally. Eventually figured out the optocoupler had been damaged just enough that it became extremely sensitive to the leading slope of incoming pulses, so it needed an extremely clean MIDI signal to work. Swapping in a new one fixed it and it’s still working find 6 years later. The moral is if you’re repairing, designing or building something, always socket the optocoupler! I’ve only had to replace two or three but that’s still more than any other component except power supply capacitors (and patch memory batteries but that’s not a repair, that’s routine maintenance).
Something like that seems really likely in this case. Is the OT optocoupler socketed? If it is there’s no harm in swapping it if you can get one easily (I still have about half a dozens so I haven’t had to buy one since the supply chain got bad).
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