System 80 schematics' 'offset' voltages are DC bias for the display filaments
Where Gottlieb System 80 schematics mark a supply as an 'offset' voltage (for example 5 V or 8 V offset), it is a DC bias added to the AC filament supply of the vacuum fluorescent displays. It keeps the filaments always positive so the grids can switch the digits off. Measure it with the meter on DC. The displays also use high voltage, so never work on them with the power on.
System 80 displays run on high voltage; switch the game off before touching the display boards.
Systems
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Source: Pinball Repair help group!! (Facebook group)