Test lamp SCRs with the power off

Steps

• Meter on diode test; the board may stay in the game.
• MCR106-1: black probe on the cathode (outer leg marked C), red on the gate (outer leg G): 0.4 to 0.6 V. Reverse the probes: 1.4 to 1.6 V.
• 2N5060: black on the cathode, red on the centre gate leg: 0.4 to 0.6 V; reversed 1.4 to 1.6 V.
• Replace any SCR that reads outside those ranges.

Systems

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

Related tips

• Substitute SCRs for the lamp driver board
• Lamp driver board layout: SCRs, decoders and connectors
• Lamp driver board: which SCR drives which connector pin
• A controlled lamp that is always on means a shorted SCR
• Prove the lamp wiring by grounding the SCR anode
• Lamp always off: trigger the SCR gate from TP3
• LEDs flicker in controlled lamps on Bally and Stern SCR lamp boards
• 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)