Ill try soldering 100nf then between pins 5 and 8 should be easy to do on the reverse of the board.
Woo Hoo
Added the cap no difference, postman calls and some 137s arrive. Popped one in and bingo all working.
Just waiting for the hex trigger to arrive to finish off 
very happy
Matt
And I used a 74C14N and that worked fine driving the midi thru and LED circuit.
The two are quite different internally:
Fairchild datasheets quote electrical characteristics with a load resistor of 350 Ī© for the 6N137 versus 1.9k for the 6N136. In this module a 10k load resistor is specified! Iāve seen values of 1k to 10k with the 6N137 in MIDI circuits Iāve found online, I donāt know why they use such high values. (I went with 3.9k in my MIDI to CV, somewhat cargo cultishly, but at least thatās within the 350 Ī© to 4k range shown in the datasheet plots.)
With the stated loads, the 6N136 has propagation delay times of 300 to 450 ns, compared to 40 ns for 6N137. Low-to-high time for the latter is much longer with a 4k load and I can only guess it might be around 200 ns with 10k, while high-to-low time is insensitive to RL at least up through 4k. Both times get longer with larger RL for the 6N136 so might be up around 600 to 700 ns at 10k.
Iāve read the 6N136 is actually too slow for the MIDI spec. But youāre probably not pushing the MIDI speed limits here, and with a 10k load the 6N137 isnāt a lot faster. (No, itās the 6N138 thatās too slow, with propagation delay times of 1.5 and 7 Āµs.) So Iām kind of surprised it works and the other doesnāt.
Hmm, I seem to have a dead short between +12 and ground. Canāt spot it as yet and think I am going to have to get a thermal camera. 
Rob
Ouchā¦ did not realise thermal cameras were so expensive.
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Well, I pulled all the ICās and desolderd the regulator, 2n3904, electrolyticsā¦ And still a dead short, went over with a big magnifier and no joy. Started unsoldering legs of 100nf and efter 4 found one that cured the short and low and behold the 100nf was 0Ohms resistance or close enough.
Re-assembeld it and tested ebfore chips inā¦ No short
Chips in and more smokle from the 10R resistoe. chips out and then back in one by one. until I found the offending socketā¦ Ahhh, chips oriented correctly to the socket, socket is oriented like all the others but the footprint is actualy the other way up 180 out compared to all the others.
Anyway itās all back togheter and I have modded the code to do a ālamp checkā on start upā¦
Hopefully get it tested tomorrow
update as i cant post againā¦
So itās all tested and working spot on. I have done some updates to the code which I will publish, mostly at the moment to do a POST so it triggers all the outputs on boot to confirm the hardware is working.
Update 13/7/23
Since I canāt post again update here
The second video of the diagnostic and test . Quality is terrible for some reason and the mic cuts out
And my slightly revised code with the lamp check.
Rob
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