Gun Fight Repair Log


Gun Fight arrived in the Repair Station in very good condition but blew a fuse upon being turned on. Usually this is indicative of a problem with the power supply board. I removed the board and reflowed a number of bad solder points. I reinstalled the board and again turned the game on. The game blew a fuse immediately. I needed to isolate the problem to a major component so I disconnected the power transformer, power supply board, PCB and chassis. I put in a new fuse, plugged the game in and began reconnecting each component one at a time. Everything was fine util I connected the PCB. The fuse blew as soon as the PCB was connected. Even theugh I was about to run out of fuses, I had at least isolated the problem!




The PCB was diagnosed and two diodes on the daughter board were found to be bad. The diodes were replaced and the PCB was put back in the game. I crossed my fingers as I turned the game on. The game no longer blew a fuse but nothing on the screen was recognizable.

 


I felt the display problem may be caused by ar ROM error so I removed, cleaned and reseated all the ROMs as well as the Intel 8080 CPU. I put the PCB back in the game and once again turned it on....




Success! Sort of. The screen now had a picture but it was noticeably reduced in size. Since I had a display, I was finally able to play the game! The game coined up and worked fine but there was no sound.




Checking the PCB carefully, I noticed that one chip appeared to have been replaced in the past. Figuring this would be a good place to start, I checked it with my logic probe. It was bad!  Since the chip appeared to have gone bad at least once before, I figured I would make it a socketed chip so if it went bad in the future it could be easily replaced. The chip was an SN7404N located at H6. I reinstalled the PCB and tried the game. Yes! I now had sound.

Now it was time to work on the display issue. The monitor was a Motorola XM701, which was
a stalwart of the early B & W games. Checking the schematic indicated that I should have 73VDC on the B+ circuit. I only had 48VDC. Turning the voltage regulator failed to increase the voltage. Studying the schematic, I narrowed the problem down to four components within the power supply circuit. I had planned on buying all new components but cross referencing the original Motorola part numbers proved to be quite a task. I was eventually put in contact with a collector in Kentucky who had the parts that I needed. I installed a 33V, 1W Zener diode  and tested the game. Yipee! Fixed. OK partner, draw!!