After rebuilding the power supply board and fitting a custom-made replacement transformer, every supply works except the 12 V, which gives only a little over 7 V with no load, and the adjustment pot barely changes it.
12 V supply only gives about 7 V: replacement transformer wound with the wrong secondary
Cause: The new transformer's secondary had been built as 13 V with a centre tap, instead of 13-0-13 V (26 V across the outer leads) as the circuit needs.
Checks
- Measure the AC across the two outer leads of that winding and from each to the centre tap; you should see about 26 V and 13 V.
- Check the DC at the regulator input before suspecting the regulator, pot or resistors.
Fix
- Replace the transformer with one that has the correct 13-0-13 V secondary.
Transformer primaries carry line voltage; measure secondaries carefully with the game plugged in and keep clear of the primary side.
Games
Source: Pinball Repair help group!! (Facebook group)