A controlled lamp that is always on means a shorted SCR
Locked-on lamps are almost always an SCR that has failed shorted; replace it. If 4, 8 or 12 lamps fed by one decoder are dead, suspect that 4514 decoder instead (page 31).
Systems
Related tips
- Lamp always off: trigger the SCR gate from TP3
- Lamp always on: ground the SCR gate from TP2
- LEDs flicker in controlled lamps on Bally and Stern SCR lamp boards
- Test lamp SCRs with the power off
- Substitute SCRs for the lamp driver board
- How Bally controlled lamps are powered and switched
- Lamp driver board layout: SCRs, decoders and connectors
- One controlled lamp does not light
Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)