This is a reversing variable speed drive for a shunt wound DC motor that moves the carriage on a circuit board plating line. It includes other interlock logic that monitors the speed of the drive and makes sure that it can't reverse while it's in motion.

The interface for this drive meant that it wasn't easy to retrofit something modern.

 

The drive came to me after several attempts to repair it that hadn't lasted long. The fault lay in the burnt contact of the reversing relays that were causing the SCR's to fail. The relays are still similar to ones in current production, but are not available with PCB terminals. On closer inspection, it was clear that I could modify the available types with a little careful sawing and filing to create the same PCB pins from the spade terminals.

 

The customer had no idea that the drive ought to run as quietly as it now does. I guess the relays had been on their way out for years.