When it comes to Stern MPU-200 troubleshooting, intermittent issues are the most frustrating to diagnose. This week, I had a board on the bench that gave me a real headache with ghost errors..

The board would sometimes boot up, but other times the diagnostic LED would just flash twice and lock up. On this era of Stern/Bally systems, two flashes clearly indicate a failure in the 5101 RAM (U8). However, after replacing the RAM, the issue persisted intermittently—sometimes it would boot perfectly and play, and other times it would refuse to start.

The real culprit turned out to be something highly unusual: the main microprocessor, the MC6800P CPU. Usually, when a 6800 chip fails, the board is completely dead (the LED stays locked on without flashing). But this specific chip was failing in a way that caused data bus instability, tricking the system into thinking the RAM was bad.

The Fix:

I replaced the MC6800P processor with a fresh one. Testing the original PCD5101P RAM out of the circuit confirmed it was actually faulty as well. With a new CPU and a fresh RAM chip, this MPU-200 now boots flawlessly every single time, the flashes are solid, and the game is 100% stable. Just goes to show—you can’t always blindly trust the error codes when the main processor is acting up!

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