Making the test LED for the test EPROM

Steps

• Solder a 150 ohm resistor to the flat-side (cathode) leg of an LED.
• Clip the resistor end to ground (TP4).
• Clip the other LED leg to U9 pin 15 (address line 6), or to MPU connector J5 pin 16.
• The board LED cannot be used without U11 fitted, so this LED shows whether the test program is running: it should flash about once a second.

Systems

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

Related tips

• Add a blanking LED to a driver board
• Build a tester LED for PIA and driver checks
• Using the test EPROM memory test
• Setting up an MPU for the test EPROM
• U9 signals with the test EPROM running
• U6 select signals with the test EPROM
• Checking the PIA outputs with the test EPROM
• Never use a TMS2716 EPROM

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