In attract mode the Black Jet was erratically flying backwards, not something I'd seen before with this board. When you started a game the Black Jet carried on flying backwards and the left/right rotate controls we not working for either Jet.
I started probing around the vertical and horizontal motion counters to see if I could track down the issue, there are four signal J1C0-J1C3 which are derived from a 64-bit RAM IC, probing these for the Black Jet circuit I found that they were all stick high and not pulsing as I would have expected.
This board is quite nice as the Black & White Jets have pretty much identical circuitry so it was easy to compare one with the other.
Checking out the other pins on the RAM, I found that the W/E pin was being held high, I traced this back to an LS7432 (2-Input OR gate) and confirmed that the output was stuck high (confirmed against the White Jet circuitry). I cut the output pin on the 7432 and was still stuck high and not pulsing away. Once swapped out with a replacement, The game is fully working again.
|Schematics showing the area of the fault this time.|