The bulb next to the solenoid expander board must work

The SEB relay is switched by a MOC3011 opto-isolator, which does not draw enough current to keep the lamp driver SCR latched. A #555 bulb on the same SCR beside the board supplies that load, so a burned-out bulb there stops the expander working.

Systems

• Bally MPU AS-2518-35

Related tips

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