Ninja Kid II is an action game from UPL released in 1987. It is also known as Rad Action, JT-104 and NinjaKun Ashura no Shou.
In order to prevent bootlegging UPL decided to use an encrypted sound CPU. The CPU is a NEC MC-8123 which contains a Z80 processor and 8192 bytes of battery backed RAM. When the battery that powers the RAM dies the sound stops working.
At some point in time a few Ninja Kid II PCBs started appearing with a standard Z80 and a decrypted sound program. This is most likely the result of some clever bootleggers. If UPL ran out of MC-8123s they could easily have used a 100% unencrypted sound ROM and a normal Z80 in it's place. The bootlegged sound image had two parts. The bottom half of the image contained the program whilst the top half contained the sound data. The bootleggers connected a wire between pin 27 (M1) on the Z80 and pin 1 (A15) on EPROM 6 so they could bankswitch between the opcodes and sound data. In addition they had to lift pin 1 on EPROM 6 so that there was no connection with the EPROM socket.
However thanks to the talent of Chris Hardy of SEGA Resurrection fame we now have a proper decrypted sound ROM image which requires no board modification at all. In order to fix the sound on your PCB all you need to do is to replace the encrypted CPU with a standard Z80 and burn an EPROM with the decrypted sound image.
The sound image is the same for all versions of the game.
This fix is for the original Ninja Kid II PCB with the encrypted sound CPU. If your PCB already has a Z80 instead of the MC-8123 and the wire modification you must reverse these changes before you can use this fix.
I take no responsibility whatsoever for any damage you might cause to your hardware, yourself or your surroundings when attempting this project.