Making a 25 V DC source for the MPU zero-crossing input

Steps

• Use a 120 V AC to 25 V AC transformer and a bridge rectifier rated around 50 V.
• Connect the pair of black primary leads to a mains cord.
• Connect the two yellow secondary wires to the two AC terminals of the bridge (the unmarked AC terminal is diagonal to the marked one); leave the black centre tap unused.
• Connect the bridge + terminal to MPU TP3 and the - terminal to MPU ground TP4.
• Do not add a filter capacitor: the zero-crossing circuit needs the ripple, and the game's own supply leaves this line unfiltered too.
• Power up; the MPU should now give its final flash on the bench.

The transformer primary is at mains voltage; insulate every connection and mount it on a board.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• What changed on the -133 MPU: zero-cross sensing from 6.3 V AC
• Testing an MPU in the game with only J4 connected
• MPU board test points
• MPU board test point voltages
• Why the MPU has ROM jumpers
• Telling a -133 MPU from a -35 or -17
• Converting a -133 MPU to a -35
• Setting up an MPU for the test EPROM

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Two, Clay Harrell (pinrepair.com)