Space Invaders Part II

[Jamtex]

Probably an older game that people are likely to play but meh...

Game would work, but after a while the game would start flickering like a mad man, however it would flicker at different speeds depending on what was on the screen.

First thing that comes to mind is that the monitor is losing sync. So attempt to adjust that, but nothing seems to fix it. A close look at the game running, shows that the game is running much faster, replaced the crystal as a matter of course as it was the original crystal and looked a bit old. This did seem to make the game better but the flickering would still come back after 2 to 3 hours, so it was time to break out the schematics, but I found this document (http://arcarc.xmission.com/PDF_Misc/Mid ... 20Test.pdf") which was more useful as a number of games by Midway and Taito used the same 8080 board set up, most of the infomation in here would work on most boards including Space Invaders Part II which had a colour generator shoe horned on top of it.

Like most games you have a crystal which pumps out a certain frequency and then it generally uses a number of counters to generate other clock signals. First chip to check was the 9310 synchronous decade counter (aka 74160) traced from the crystal this generates a 10Mhz (for the Sound Generation and video )and 2Mhz signle for the 8080A. A frequency counter showed that it was pumping out 10Mhz and 2Mhz.

The 10Mhz signal then is fed into a 74LS74 a dual D type flip flop, now if you have an old 70s or early 80s boards you may notice that a number of them (especially Defender) have the 7474 chips socketed this was generally as a large number of Texas Instruments chips where steel legged chips which tended to rust and make the chip useless, TI were not the only company to save money by not plating the legs with Zinc, so it can be easier to replace rusty chips then to try and diagnose problems. There were two of these on the board and I replaced them both as they did show tiny signs of rust.

The problem was still there and there were 4 9316 synchronous binary counter (aka 74161), I tried to measure each clock on the chip especially the last 74161 chip which did generate the 60hz clock, but when this did go flaky and increase to near 120Hz, I did replace it. However this did not seem to solve the problem, so rather then try and trace each clock back to the chip, I replaced them all and that seemed to fix it.

[davewellington]

Top job, thanks for posting