This time for real. The SN75462 replacements got here yesterday and I popped on in IC3’s socket and the machine works. The sensor lock in STOP and the motors are both responsive. Here, see for yourself:
There are a few things I need to figure out for next time I look under the hood:
Does J3/4-6 provide the +24V to the solenoid?
Does K-EDIT deliver exactly the same voltage as what comes from the +24V rail, or is it something like ~25V (the Zener diode “trick” so that it looks like the output of IC3 is ~25V instead of 5V)?
Does the voltage at IC 4 pin 1 equal to the voltage on pin 7? That is, does it work like some kind of a comparator? See previous post and notes I left in the manual.