On power up – black screen, no sign of life at all. I removed the 68000 CPU and fitted a socket, so I could use the Fluke on it. This revealed that the main program ram ($10C000-$10FFFF) was totally dead. Replacing the two ram chips IC25 and IC26 solved this problem.
Board now started up, and displayed corrupt graphics with a lot of video noise in the background. An ABI Boardmaster was used to test several of the TTL chips, and it quickly revealed that the 74LS157 at IC62 was faulty. Replacing this restored the background to a clean black, but the sprites were blocky, and the background characters were unrecognisable.
Further tests using the ABI revealed that all of the 74LS373’s were corrupt. These (IC12,IC16,IC18 & IC29) which restored both the sprites and characters to normallity.
Now that the screen could be seen, it was obvious that there were interruptions in the scrolling. Since the game would pause for up to a couple of seconds, but then carry on normally, we traced the interrupt lines from the CPU. These went to a 74LS273 at IC32. Repeated testing with the ABI revealed an intermittent fault on one of the outputs. replacing this chip solved the problem.
Now the game was running correctly, but had no sound, and the colours were wrong. much testing using the ABI did not reveal any possible candidates so I switched to using the trusty logic probe. This identified that the chips used to multiplex the sprite and background colours always had low outputs, but the chips themselves were fine. The selection was driven from the priority PAL at IC33, and this had tested out OK in another board. I checked out the various signals to the bank of 74LS157’s and discovered that one of them had a floating pin. This pin was the enable signal, and if it were the same as the other chips in the bank should be permanently low. I added a wire link, and the colours were restored.
The game was now running correctly, but without sound. Probing around the sound section revealed that the Z80 was not running correctly, so the ram for this was replaced. Now the Z80 was running, and the YM2151 was outputting a signal that changed in response to screen activity, but still no sound. Turning the volume up, and applying a signal to the output amp gave a suitable squeal, so I started tracking back from there. The op amp would give a noise when I fed a signal to two of the inputs, but the other two had no effect. Replacing this chip (IC67) returned the sound to normal.
No Sync – no output from IC33 – a custom programmed PAL B22-06. I managed to re-create the required programming, so if you need one, get it here.
On power up, yellow screen. Socketed 68000, using fluke discovered main ram OK, but could not see shared ram used by C Chip ($800000-$8007ff). Further investigations revealed only one of the three interrupt lines was changing – replacing IC32 (as before) solved this problem, and the board now boots up normally.