When a driver fails, replace its pre-driver as well
Most TIP122/102 drivers have a 2N4401 (NTE123AP) pre-driver next to them; the schematic shows which. Each TIP36c on the APDB is driven in turn by a TIP122/102 on the CPU board and that one by a 2N4401, so both count as its pre-drivers. A failed driver has often stressed its pre-drivers, so replace them too, or at least test them.
Systems
Related tips
- What the System 11 auxiliary power driver board does
- What the auxiliary power driver board holds
- CPU and driver board transistors by position
- Testing driver transistors in diode mode (board out of game)
- Coil stays on after the CPU-side driver parts were replaced
- Quick check of all TIP122/102 drivers before first power-on
- Firing a coil or flasher from its TIP122/102 driver tab
- Lock diverter locks on at power-up and gets hot: shorted driver transistor Q22
Source: Repairing Williams System 11 Pinball 1986 to 1990, Deel 1, Clay Harrell (pinrepair.com)