The game won't boot. The MPU LED flashes twice at power-up instead of completing its normal seven flashes, even with a new 6800 CPU and new 6821 PIAs fitted and good supply voltages.

MPU LED gives only two flashes at power-up: replace the 5101 RAM chips

Cause: Two flashes in the MPU's start-up sequence point to a 5101 RAM failure.

Checks

• Confirm the 5 V and other supply test points are in range.
• Watch the LED closely: a single flicker at switch-on is normal, the count that matters is the distinct flashes after it.
• Boot with only the essential power connector on the MPU, unplugging the others (including the sound board cables), to rule out outside loads.

Fix

• Replace both 5101 RAM chips (socket them), then reconnect the other boards one at a time.

Systems

• Stern MPU-200

Games

• Flight 2000

Related tips

• Keep spare 6800, 6821 and 5101 chips
• Stern MPU-200: memory won't save and displays show garbage: two 5101 RAMs
• All lamps strobe after a replacement CPU board was fitted: check its M-200 jumpers
• Using a PC power supply as an MPU bench supply: which wires
• MPU LED never lights at all
• Power and ground pins of the main MPU chips
• Two flashes only: U8 5101 RAM
• 5101 RAM speed grades and which MPUs need what

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