1131 DISK DRIVE RESTORATION
My new approach was to drop the 48V power, which switches on when the motor is activated, using a sequence of 4.7V 3W Zener diodes. The eight diodes bring the 48-52V down to the right level to feed the 7812 voltage regulator. The timer board activates its relay after 110 seconds, which disconnects the logic signal from ground to indicate it is now safe to load the disk heads.
This worked well in practice, so I began to install it into the system. The old relay is back inside the power box, but my wires take the 48V from those connections out to the new box I built. The plastic box contains a board with the zener diodes, the regulator chip and the timer board, hooked to the four wires I routed out of the power box.
NEW KEYPUNCH INTERFACE DEVELOPMENT
I completed the Arduino code and began testing the unit. The first stage was to validate the basic communications link, startup process and first level diagnostic tests. Once done with that, made more difficult by failures of the Java code in the Arduino workbench which left my serial port unusable until the machine was rebooted. The port that failed is the basic link over the USB cable used to program the board, not the one that users will use for commands and data flow.
I now suspect that the USB port can't drive the power that the Arduino plus its two relay boards are drawing, thus it is going offline periodically. My power supply bricks for the Arduinos are in a side shed which would be inconvenient to look through in the dark.