The RTBB computer grade capacitor is what I use and is mandatory for SYS 80 games.

The +8V DC Test point is the "offset voltage" for the 5V AC from the transformer. It is unusual to see it at 15VDC. Sounds like either R10 is open or the 8.2 V Zener is dead. There's a heap of info on this circuit, and the recommended updates here ;

http://www.pinwiki.com/wiki/index.php?title=Gottlieb_System_80#Recommended_Updates_and_Repairs_for_the_System_80.2F80A_Power_Supply_Board

Of particular note is the update for R10, which allows for a 1 Watt Zener CR7 (more commonly found).

PLEASE ensure you update the power supply and ground mods for your game.