Just as with the Extruder Controller, the Thing-O-Matic stepper motor driver boards derive their logic supply from the +12 V line through a 7805 linear regulator. While that works in the ideal case, it makes the logic supply vulnerable to glitches induced by motor current switching.
This modification gives the stepper controller chip a clean +5 V supply from the Thing-O-Matic’s ATX power supply, by the simple expedient of removing the 7805 regulator chip and connecting the +5 V from the power supply Molex-style connector to the circuit pad that was the regulator’s output pin.
This is what the modification looks like on the PCB layout.
Use solder wick and a big soldering iron to de-solder the connections, then yank (gently!) the regulator off the board; you can see the outline printed on the board near the lower-right corner, between the two blue capacitors. This picture is rotated half a turn from the PCB layout shown above.
Connect a jumper from the Molex connector’s +5 V pin to Pin 3 of the 7805 regulator outline. The wire can be any size, because it carries minimal current to the driver chip’s logic circuitry; I used a strand stripped from a ribbon cable.
Put the wire on the bottom of the board, because the connector pin isn’t accessible from the top and the traces at the regulator output pad are on the top where they’ll be easy to solder.
Repeat for all three stepper motor controller boards.
Reinstall in your Thing-O-Matic and rejoice that nothing seems to have changed. This modification should reduce the number of weird motor-control problems, although it will not prevent lost steps due to mechanical overload or excessive traverse speed.