Set the display high voltage to about 170 V

Steps

• Find the HV trim pot at the upper left of the solenoid driver board, below C26 and left of C23.
• With the game on, measure from TP2 to the GND test point.
• Bally specifies 190 V, but set it to about 170 V (as low as 155 V is acceptable): turn it down until a display just stops lighting fully, then raise it a few volts above that.
• Brand-new display glasses may need the full 190 V for their first few hours; turn it down once they are seasoned.

The solenoid driver / regulator board makes about 190 V DC for the score displays (around 230 V on some Stern boards); a shock from it hurts. Work with the game unplugged and let the filter capacitors discharge, and use one hand and insulated probes for live checks.

Systems

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

Related tips

• Where the display high voltage enters and leaves the solenoid driver board
• Power up and set a rebuilt high-voltage section
• Rebuild the solenoid driver board high-voltage section
• High voltage still too high or too low after a rebuild
• All displays dark after fitting a new power supply: blown solenoid driver board fuse
• No score displays light at all
• The solenoid driver board also carries the 190 V display supply
• First checks before repairing Bally score displays

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