Capcom Buster Bros / Pang arcade pcb repair
An original Capcom/Mitchell Buster Bros pcb booted to a solid blue-green screen. This is one of the infamous suicide battery pcb’s, where a custom Z80 CPU contains a decryption table in volatile RAM powered by a battery. When the battery dies the decryption table is lost and the program code cannot be run. This pcb was an untouched original with a 29 year old battery, so safe to say the battery was stone dead.
Full info on restoring these dead CPU’s can be found here – http://www.arcadecollecting.com/dead/pre-cps.html
One of the original tricks of this encryption is that program opcodes (instructions) are treated differently from operands (data). This means that even if you understand the decryption and have the key you can’t just burn a new ROM in place as each byte the ROM will have two values depending if the CPU accesses it as data or instruction. The solution is to decrypt the ROM for both data & instruction separately and burn to a new double size ROM with data in top half, instructions in lower half. The CPU pin that controls whether instructions or data is being fetched is then used to select the upper address line on the ROM.
So the desuicide process is fairly simple – use 27c020 and 27c512 eproms to replace the originals, and run a wire to control the top address bit. I’m pleased to say it booted up first time when I tried it.
But.. pretty obvious there was no red color channel. The palette RAM would be the first port of call in this situation, but a logic probe showed neither of the two chips had any stuck bits. I probed down stream of the RAM and there are 3 LS367 chips. One each for red, green and blue. These are the last TTL chips before the signal is converted to the analog output and they control the blanking by cutting out the signal during h-blank and v-blank events. Shorting pins 2 & 3 on the red channel chip fixed the problem and isolated the fault to that TTL chip (which is a strange type of failure but there you go).