The MPU usually locks up at power-on, showing 0 (sometimes 4) on its diagnostic display; occasionally it boots, or boots and then goes blank. The reset line goes high normally, but the clock on the CPU's clock pins is very weak on a scope.

MPU boots only sometimes, weak clock signal: failed 6800 CPU chip

Cause: The 6800 CPU itself had failed. A new crystal did not restore a healthy clock; a new CPU did.

Checks

• Reseat or replace suspect sockets and connectors first (a loose 40-pin CPU socket was found here).
• Check the reset line goes high after power-up.
• Look at the clock on the CPU's clock pins with a scope; a weak or sickly waveform after a new crystal points at the CPU.

Fix

• Replace the 6800 CPU (in a good socket) and confirm the clock is now strong and the board boots every time.

Systems

• Williams System 7

Games

• Black Knight

Related tips

• Solenoid fuse F2 blows at power-up as every coil fires: CPU blanking circuit
• Re-jumper a System 7 CPU from Defender/Hyperball/Star Light to a standard game
• Player 3 and match digits show wrong numbers: wire loose in the IDC plug at 1J5
• CPU locked: check the EPROM chip select
• Jungle Lord System 7 CPU won't run a test ROM: open data line D4 to the buffer
• Check the blanking and IRQ signals on a booting CPU
• CPU locked: check the R/W and VMA lines
• Good reset and clock but LEDs locked: check IC17 and its bus lines

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