With the test EPROM in U3 and bench power on, the CPU LED does not start fast blinking.

Test EPROM fitted but the LED does not blink

Cause: Bad LED driver, or the 6803 is not running (no reset, no clock or a bad address/data line).

Checks

• With a logic probe look for activity at U1 pin 10, which drives the LED through Q1 and three resistors; if present, Q1 or the LED is bad.
• Pull the test EPROM and any U2 ROM, then check U1 pins 4, 5 and 6 for +5 V.

Fix

• Replace Q1 or the LED if pin 10 is active; otherwise go on to the reset, clock and bus checks.

Systems

• Bally 6803

Related tips

• Test EPROM socket signals at U3
• Bench-test a CPU board with the 6803 test EPROM
• Power a 6803 CPU board on the bench with a PC supply
• Test EPROM memory test does not pass
• PIA outputs not toggling under the test EPROM
• CPU board landmarks: battery, LED, relay and ROMs
• CPU jumpers JW1-JW6 for game EPROM size
• Find a bad 6803 or a shorted bus line

Source: Repairing Bally 6803 Pinball Games from 1985 to 1989, Clay Harrell (pinrepair.com)