What changed on the -133 MPU: zero-cross sensing from 6.3 V AC

The MPU times lamp and coil switching to the AC zero crossing. Bally boards up to 1983 sense it from the unfiltered 43 V solenoid supply through 2K resistor R113; the -133 (Baby Pac-Man, Granny and the Gators, Grand Slam) senses the 6.3 V AC lamp supply instead, with a 1N4148 diode in place of R113. Knowing the AC phase lets one SCR drive two lamps wired in series, each with a reversed diode across it, doubling the controlled lamps.

Systems

• Bally MPU AS-2518-35

Related tips

• Making a 25 V DC source for the MPU zero-crossing input
• Telling a -133 MPU from a -35 or -17
• Converting a -133 MPU to a -35
• -17, -35 and -133 MPU boards compared
• Convert a -133 MPU to a -35
• Tell a -35/-133 MPU from a -17 by counting J5 pins
• Jumpers for a -35/-133 MPU running two 2732 EPROMs (U2 and U6)
• Bench-testing a -133 MPU: swap the R113 diode for a resistor

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