Started off as a physical problem with corroded legs on MASKROM’s giving corrupt graphics on sprites.
Three legs were corroded on MASKROM T44.00, which is an NEC 23C1000. After a little asking about, I was told the EPROM equivalent was a 27C301 (non JEDEC).
I obtained a 27C301 eprom and burned a T44.00 from MAME. Pretty much all the graphics were back but there were some white blocks around the ship sprite and certain enemy bullet sprites. Found a broken track from pin 20 (chip enable). Repaired this and the white boxes are now gone from the ship sprite. There were still white boxes around the projectiles. I have now confirmed that with an original ROM the game works fine.
I took a closer look at the pinouts from a 23C1000 MASK ROM and a 27C301 EPROM. MASKROM’s have less pins than their EPROM equivalent mainly due to not have programming pins but also this ones only has one enable pin (pin 20) and the EPROM has separate /OE and /CE pins. I lifted pin 2 and soldered a wire from this to pin 22 on the EPROM and fired the game up, all the graphics are now 100% working. I initially thought this corruption was down to the speed of the EPROM but now I know otherwise.
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