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If it is receiving midi signals and responding as it should then the
opto is fine
If you have an oscilloscope check for data pulses at pin 11 of IC22.
If ok, check if you get an inverted set of pulses on pin 10 of IC22.
If ok check for inverted pulses on each side of R18
R17 should have approx 5 volts on each leg ( just a current limiting
resistor for the midi supply)
Now if all that is good, you either have a midi socket with poor
connectors or dry joints in that
chain above.

Not much special to midi thru. IC22 is a 74LS06 TTL hex invertor - just
buffers and inverts the
output of the midi in opto