Problem

Graphical corruption.

Diagnosis

Chequered Flag uses a similar set of Konami customs to those of Ajax. While Ajax has one rotate/zoom layer and a regular tilemap layer, Chequered Flag has two ROZ layers. Both of these layers appeared to be corrupted.

Each ROZ layer is controlled by a 051316 ‘PSAC’ custom, of which there are two:

Many Konami games are engineered so that the video ROMs (and later, the sound ROMs) can be accessed and thus checked by a CPU through an interface on their respective custom IC. The game’s service mode has comprehensive ROM tests for both program and video ROMs:

Between the two PSACs, three ROMs had tested bad. It was unlikely that these were defective or that both PSACs had failed. Instead, there was probably a fault with a signal or bus common to both PSACs.

I couldn’t find any schematics for Chequered Flag (if you have them please let me know!) but the PSAC pinout is readily obtained from the Ajax schematics:

Probing around with the oscilloscope showed that A1 (pin 37) was constantly high, while the rest of the address lines were toggling. I traced A1 to Q3 (pin 10) of a 74LS174 hex flip-flop at O12. There was activity on the D3 input, so I opted to replace the chip. Doing so fixed the graphical corruption and now all the ROMs test good in service mode.

Fix

Replace 74LS174 at O12.

Note that the game uses analog controls, which I did not hook-up. Here I’m using a finger ran back and forth along the analog inputs connector to drive…

Philip Bennett