Scorpion Maintenance 2022
19/05/2023 - Scorpion game PCB PA20210406 repair
Pre-show testing found that the game PCB displayed no sprites and the PCB was removed
for repair. On the bench, the video probe was setup to locate where there was or wasn't
sprite data. Starting with IC 8E (LS157) outputs from the graphics ROMs found:
- pin 14 - sprite data
- pin 12 - sprite data
- pin 9 - sprite data
- pin 4 - sprite data
All the outputs looked OK, moving forward to IC 8A (LS00) found:
- pin 11 - no sprite data
- pin 12 - sprite data
- pin 13 - no sprite data
This indicated that sprite data from the graphics ROMs was entering the sprite buffer RAM
circuit but not leaving it. Using a scope to map the sprite RAM IC 10B (AM27LS00) found:
| pin 1 | idle lo | pin 16 | idle hi |
| pin 2 | idle lo | pin 15 | active |
| pin 3 | idle lo | pin 14 | idle lo |
| pin 4 | idle lo | pin 13 | active |
| pin 5 | idle lo | pin 12 | clock |
| pin 6 | active | pin 11 | active |
| pin 7 | idle lo | pin 10 | active |
| pin 8 | idle lo | pin 9 | active |
The address input pins 2,3,4,5 were all idle but should have been active. Checking IC 10H
(LS161) outputs were all idle low but it sibling IC 9H (LS161) was active. Trying a
piggyback on IC 10H didn't restore the sprites and a scope showed the outputs clamped.
Replacing IC 10H (LS161) fixed the sprites and the game ran OK on the bench for several hours.
Scorpion Maintenance 2023
prswan@gmail.com
