Overvoltage protection on the logic supplies

+5 V and -12 V each have a crowbar: SCR101/SCR201 (S107Y1) with zeners CR101/CR201 (1N4734/1N4743). If the output passes about 5.6 V or 13 V, the SCR fires and shorts the output to blow the fuse.

Systems

• Gottlieb System 1

Related tips

• Power supply voltage drifts or overvoltage trips
• R21 value on early power supplies
• Upgrade the logic rectifier diodes CR1 and CR2
• Logic and display reference section of the power supply
• System 1 power supply shows about 110 V where 4 V and 8 V belong: wrong ground
• Quick checks on the similar System 1 power supply
• Spare diodes, capacitors and other parts for System 1
• System 1 power supply outputs

Source: Gottlieb System 1 Pinball Repair from 1977 to 1980, Clay Harrell (pinrepair.com)