With a diagnostic test EPROM fitted, the CPU's LED does not flash and the displays stay blank, although clock, reset and 5 V are fine and the address lines toggle.

Jungle Lord System 7 CPU won't run a test ROM: open data line D4 to the buffer

Cause: The track carrying data bit D4 from the ROM socket at IC17 to the data buffer was open, so the processor always read that bit as 0.

Checks

• Check the ROM contents are correct.
• Probe each data line for one that is stuck high or low.
• Check continuity of each data line from the ROM socket to the data buffer chip.

Fix

• Repair the open D4 connection between IC17 and the data buffer.

Systems

• Williams System 7

Games

• Jungle Lord

Related tips

• System 7 diagnostic display stays lit on a working game
• Leon's test EPROM fitted but the LEDs do not start flashing
• Board boots and passes SW1, but no displays or CPU lamps
• Good reset and clock but LEDs locked: check IC17 and its bus lines
• CPU locked: check the EPROM chip select
• Leon code running but LEDs dark: bad CPU PIA IC18
• Address buffer check: 8T97s at IC3, IC4 and IC8
• MPU boots only sometimes, weak clock signal: failed 6800 CPU chip

Source: Pinball Repair help group!! (Facebook group)