Testing C23 for AC ripple

Steps

• Unplug MPU connector J4 (and the sound board's large connector).
• Power on and put the meter, on low AC volts, across the two leads of C23.
• Under 0.25 V AC is acceptable; anything over 0.20 V means the cap is wearing out. At 0.25 V or more, stop and replace C23 before powering the MPU.
• Switch to DC and measure from GND to TP1 or TP3: 4.9 to 5.3 V. Fix any problem before going on.
• Power off, plug J4 back in, power on and measure the ripple again under load; it should still be under 0.25 V AC.

Systems

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

Related tips

• MPU locks with LED on because of C23 or Q20 on the solenoid driver board
• Solenoid driver TP5 under 13 V
• Solenoid driver TP1 below 5 V with TP5 normal
• Only about 1.6 V at solenoid driver TP1
• Route of the +12 V supply to the MPU and 5 V regulator
• Logic 5 V reads over 5.25 V on Bally/Stern: the R49 boost resistor
• Adding a ground jumper at C23
• MPU shuts down or resets: broken C23 ground return wire

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