All lights connected to the Aux Lamp Driver PCB (A9) do not work. This is most noticeable in the pac-maze; none of the red lights work. I have traced it down to no CLK signal on U1 (A9). This signal is generated on the MPU board, a signal called "Lamp Strobe #2". The machine boots fine, I can even watch the boot sequence (the green led) signal at the CLK signal on U1 (A9) via the o-scope (it's all connected); however, once the boot cycle is over the Lamp Strobe #2 goes low and no activity is observed on the o-scope.

Just to rule out an issue with the 6821 port expander I swapped U10 and U11 on the MPU (A4) PCB. The machine behaves the same. Inspecting pin 39 (CA2) on U11 with the o-scope the signal is inactive. Inspecting the same pin on U10, I see activity. Reading the datasheet for the 6821 I suspect Bally has pin 39 configured as an output and then sometime after boot-up configures the pin 39 (CA2) to mirror pin 25 (E). This is just a theory and would love to know if someone knows this for a fact.

Inspecting U10 pin 25 (E) with the o-scope I see activity. Anyone have any suggestions of things to check?

Thanks for the response. The led on the mpu looks completely off to my eye.

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I should have mentioned that the pictures were with the game opperating in attract mode.

Yes, everything else works normally, including the displays. The LED flashes normally during the self test.

Yes, I can see the pulses on the oscope. I will check for solder bridging tonight with the multimeter. Thanks for the suggestion!