One lamp output stays dark (or lights in the wrong test) with the test EPROM.
An LED stays dark on a lamp header during the test
Cause: A bad 2N5060 lamp SCR, a bad driver chip U15, U16 or U17, the resistor between them, or the PIA.
Checks
- Remove the LED strip and probe the center leg of that SCR with a logic probe: pulses there mean the SCR is bad.
- No pulse: check the driving output on U15/U16/U17 (see the schematic), then the resistor between chip and SCR.
Fix
- Replace the SCR, the driver chip or the resistor as found.
Systems
Related tips
- Lamp driver SCRs on the CPU board
- Lamp driver SCR blew apart with a crack
- Lamp matrix still wrong after replacing transistors: upstream parts
- Test a 2N5060 lamp matrix SCR with a meter
- Lamp driver board layout: SCRs, decoders and connectors
- Lamp and switch matrix transistor positions on the CPU board
- Lamp row still dead after a new TIP122
- How the CPU keeps lamp phases in step
Source: Repairing Bally 6803 Pinball Games from 1985 to 1989, Clay Harrell (pinrepair.com)