R21 value on early power supplies
Boards made before June 1978 have R21 as 680 ohms in the +8 V filament bias circuit; later boards use 100 ohms, and 100 ohms is the value to fit.
Systems
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Source: Gottlieb System 1 Pinball Repair from 1977 to 1980, Clay Harrell (pinrepair.com)