Thanks for the welcome and for all the help so far. The more we delve into it, the more I admire the way these EMs are designed - it's very cool nutting out how the stepper units work and actually seeing switching operate etc.

A couple of things to help us along if possible:

1. Does any one have schematics or other diagrams for a Williams EM of this vintage? There are a few labels missing from relays and other units that are making it a bit difficult to check off the pinrepair startup sequence step by step.
2. Can anyone confirm if our understanding of the credit unit function (see photo with the coloured arrows) is correct...and if it's meant to work that way?

I have a feeling Gavin (Retropin) has a Gulf Stream. I don't have a Williams EM within reach to provide you with the info - so maybe he will "chime" in and provide a photo. sounds like an easy fix.