A shorted coil or driver also damages the CPU board.

Early driver boards lack isolation diodes

Cause: The first driver boards (Cleopatra, Sinbad and early Joker Poker) have no isolation diodes, so 24 V can feed back to the CPU.

Checks

• Look for four diodes CR2-CR5 right of the A3J1 connector at the top edge and three (CR1, CR6, CR7) below Q32 and Q30 above A3J4. Check any game, since boards get swapped.

Fix

• Add seven 1N4004 diodes for Q25-Q28 and Q30-Q32, on the component or solder side.

Systems

• Gottlieb System 1

Related tips

• Replacing the CPU battery with a remote AA pack
• Clamp diode mod to protect U5
• Chime coils used with System 1 must be 12 ohms or more
• Testing a coil diode on the driver board
• Why WPC coil diodes are on the driver board
• Testing coil diodes on the auxiliary power driver board
• Always check the coil diode when a driver transistor has failed
• Testing a coil diode

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