Midi IN circuit stopped working

[bossredman]

I have a self built midi foot controller powered with a Teensy 3.6.
It has both Midi IN & OUT ports/circuits.
(the midi IN uses a PC900V opto-coupler)

This has worked with no issues for over 2 years now.

Yesterday, the midi appears to have stopped working.
This coincided with me unplugging & reconnecting the midi leads whilst the Teensy was powered up.
I think I have managed to narrow it down to just the midi IN not working.

Is it possibly to "fry" the midi in circuitry by disconnecting/connecting the midi lead whilst powered up?

I'm assuming it is & guessing that the most like culprit would be the opto-coupler.

Would this be correct?

 

[Theremingenieur]

As a guess, yes possible, but highly improbable. On the input side of the optocoupler, there is basically just a LED, nothing really sensitive. That's why I would check the incoming signal with an oscilloscope to be sure. Since you have access to the external series resistor of the LED, you should also be able to see if the current through the internal LED corresponds to the applied voltage.