It is my fault that I did not record of the defective optocouples.
No big deal, hegefer:
-Just hook the (normally open) push-button between the opto-couplers Pin 2 to ground (instead of as per the schematic).
Then the +voltage will feed the opto-transistor at all times, or rather as long as the +supply is on,
but the button will draw current through the coupler's LED only when pushed.
Thus you check that:
1) The transistor won't lead when the internal LED is off (No push: External LED OFF),
2) That it'll lead when, and only when PB is pushed (Push and hold: LED lights up), and
3) That the transistor also shuts off when PB is released (Release PB: LED goes off,
thus eliminating that no stray, internal current keeps it ON,
an indeed very rare, but still possible fault mode)
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