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?
EDIT: Yes. Look at the schematic. J3/7 is connected to the +24V through the power ON/OFF switch. The other end of the switch goes to J3/4-6. So one end of the solenoids is fed directly from the +24V rail.
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)?
EDIT: The fast answer is that K-EDIT gets exactly what the other end of the solenoid is fed, because of KVL. See here for the longer answer. There’s no zener “trick” so to speak.
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.