Prove the lamp wiring by grounding the SCR anode

Steps

• Press the test button once for the lamp test and note which lamps do not flash; find their SCR (Q number) and decoder on the lamp driver schematic. A ** beside the Q marks an MCR106-1, otherwise it is a 2N5060.
• Press again to reach the display test, where the playfield lamps are off.
• Clip a jumper to backbox ground (the bare braid works) and touch the SCR anode: the metal face or tab of an MCR106, or the lower right leg of a 2N5060.
• If the lamp does not light, the fault is not on the lamp driver board: check the wiring, socket and bulb.

This check is done with the game on; keep fingers off the line-voltage wiring and the high-voltage display circuit.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35

Related tips

• Lamp always on: ground the SCR gate from TP2
• Test lamp SCRs with the power off
• Substitute SCRs for the lamp driver board
• Lamp driver board layout: SCRs, decoders and connectors
• Lamp driver board: which SCR drives which connector pin
• LEDs flicker in controlled lamps on Bally and Stern SCR lamp boards
• One controlled lamp dead even after a new SCR: lamp driver board fault
• AS-2518-14 vs AS-2518-23 lamp driver boards

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