Stargate: opto pairs and their interface board pins
Each opto pair has a 4-pin plug: 1 transmitter, 3 transmitter ground, 2 receiver, 4 receiver ground. They run to opto interface board A25 J2: top left upkicker (A9J17) transmitter 1A25J2-2 and receiver 1A25J2-6; lower left ramp (A9J18) transmitter 1A25J2-9 and receiver 1A25J2-1; top left ramp (A9J19) transmitter 1A25J2-10 and receiver 1A25J2-4; top right ramp (A9J20) transmitter 1A25J2-3 and receiver 1A25J2-5; top pyramid trough transmitter 2A25J2-1 and receiver 2A25J2-2. The grounds share 1A25J2-7 (transmitter) and 1A25J2-8 (receiver), and 2A25J2-4 and 5 on the second board.
Systems
Games
Source: Repairing Gottlieb System 3 Electronic Pinball Games from 1989 to 1996, Clay Harrell (pinrepair.com)