This may be of interest to only a tiny corner of the hobby but I thought I'd share. I've often noticed patterns in the wire colors used in Gottlieb EMs but only recently dug in to understand how the wires were specified. It turns out that there were guidelines on wire colors and how they were used. I've cobbled together what I've found at:

https://www.funwithpinball.com/resources/Gottlieb-Wire-Colors

For example, ever wonder why a schematic might specify BL-WH and BL+WH?
WirePairssm (resized).jpg

/Mark

Thanks for the kind words everyone. I'm glad some find it interesting.

I find the link to the telephone industry interesting. I looked for a reference to their color scheme but only found the links above which date from the 1950s and later.

Part of my schtick at shows and workshops points out that a lot of the technology used in EM pinball (switches, relays, steppers, motors, etc.) also ran the telephone system well into the 1980s. If you haven't seen a step by step switch in action have a look at some of the youtube videos like this one:

They also built computers with relays and switches but that's another story.