Beast Busters Repair Logs

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Repairer: Apocalypse
Source: Beast Busters - SNK 1989 (repair log)

A very good game, the last one SNK made before the launch of the Neo Geo. Hardware has a lot of similarities with its successor: 68k for main CPU, Z80 + YM2610 for audio, etc. Even the test menu looks like the one of MVS slots.

1) Getting the game to boot

After a first inspection I noticed someone previously worked on the board as two program ROMs have been replaced. Also one of the ribbon cables connecting the top board and the bottom board (there's a middle board too so a total of 3) was missing. I made one with old parts I had in my stock:
ApocBB 1.JPG

Then I powered the board and it got stuck on a PROGRAM ROM ERROR.
ApocBB 2.JPG

2) Fixing the program ROM error

Looking carefully at the two program ROMs that have been replaced I discovered they were 27C010 type (JEDEC) when the board needs 27C301 type (non-JEDEC). Out of curiosity I dumped them and they contained the right data. Only pinout was the matter. I burnt two fresh 27C100 ROMs (27C301 compatible pinout) and tried to boot the game again. This time self-test reported everything good:
ApocBB 3.JPG

But then got stuck on a black screen: this is typical of a failed gun calibration or corrupted calibration data.

3) Gun calibration

I followed the procedure that has been added at the end of the manual you can find online.
ApocBB 4.JPG

If you don't have the original positional guns you can hack a joystick.
For each player there's a 6 pin JST NH connector with the following pinout:
1 = GND
2 = X DATA
3 = +5V
4 = GND
5 = Y DATA
6 = +5V
Original potentiometers are 5k so depending of the ones in the joystick you plan to use it may not work.
Gun coordinates depend of the voltage on pins 2 (X DATA) & 5 (Y DATA) :
0V = most left/up
2.5V = middle/centre
5V = most right/bottom
An other solution is to desolder the 28C04 EEPROM located @ K7 on the top board and burn the file named bbusters-eeprom.bin included in the MAME romset on it.
After doing a proper calibration game booted:
ApocBB 5.JPG

But... Some sprites had jailbars over them and sound was crackling.
ApocBB 6.JPG

4) Fixing the sprite issue

With the help of the test menu I learned only "FONT 1" sprites (probably a misspelling of FRONT 1 by opposition to background) were affected.
The game shows 4 sprites in a loop, each being stored in a different ROM. So problem was probably downward in the video processing as it seemed unlikely the 4 ROMS were bad with the same jailbars.
ApocBB 7.JPG
ApocBB 8.JPG

According to MAME those sprites are stored in ROMs 11, 12, 13 and 14. They are located on the middle board so I looked carefully at it for broken traces when I noticed a factory defect: RAM @ C14 had its pin 15 not soldered at all. It was probably not a problem when board was new but with oxidisation and dust accumulated over the years connection became poor.
ApocBB 9.JPG

I resoldered it and sprites were restored:
ApocBB 10.JPG
ApocBB 11.JPG

5) Fixing the sound issue

Continuing playing with MAME I was able to reproduce the exact same crackling sound by blanking the sample ROM @ L5. However by using the sound test menu I discovered some samples/tunes were played perfectly while others were totally crappy, and after probing both PCM samples ROMs it revealed sound was perfect when accessing only to the PCM A ROM @ L5. So I burnt a replacement for the PCM B ROM @ L3 in a 27C400 EPROM and tried to piggyback it: that fixed the problem!
I dumped the faulty ROM and compared its content with the MAME file:
File:ApocBB 12.JPG

As you can see bit 8 is stuck low.

Game fixed.