How the solenoid expander board doubles up coils

From Eight Ball Deluxe on, some games have a solenoid expander board (SEB) under the playfield: a relay that switches certain solenoid driver outputs between two coils each. The relay is not driven by a solenoid transistor but by a lamp driver SCR, so one driver output fires coil "a" with the relay at rest and coil "b" with it pulled in (page 36). It normally handles slow, seldom-used coils such as the outhole kicker and drop target resets.

Systems

• Bally MPU AS-2518-35

Related tips

• Test the coils behind the solenoid expander
• The bulb next to the solenoid expander board must work
• Solenoid expander relay buzzes or does not switch
• Centaur uses its solenoid expander for the ball-to-playfield kicker
• Replacing the hard-to-find 48 V expander relay
• Expander-driven coils fire in pairs: drop target coil fitted without its second diode
• Expander games: check the controlled-lamp power first
• Drop target coils misfire after LEDs or a new lamp driver: expander boards need bulbs

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)