I got a pretty interesting board on the bench this time: Head On N, released by Nintendo, Licensed (Or stolen) from Sega. This is an interesting boardset, as this is for the most part a clone of Sega Vic Dual hardware, except they spread it across two boards and included the sound board on the CPU board. I had already done a deal of work deconverting the board, unfortunately this thing was missing a lot of parts and was modded to play an unknown game (the roms weren't included).
On to the issue. The problem at hand is that there is no detail, just blocks moving around on the screen.
First thing I did was hook it up to my Fluke 9010A to do RAM and ROM checks. The RAM had been previously desoldered by someone else, so I socketed it (4027 DRAM fail pretty often) and fixed some damage traces done by the previous owner. Everything came back as good, so I figured that the issue was after the RAM, since graphics and system RAM are shared. There are no schematics, but since this is so similar to Sega Head On hardware, I just used schematics for that (I actually used the schematics for Tranquilizer Gun, since Head On used a custom IC for video stuff). Looking at it, the first thing that happens is the graphics data gets serialized. From there, it get's processed with sync and color gets overlayed. Also note that on Head On N, the 74ls166 was sometimes substituted with a 74ls198 (as other Nintendo games like Space Fever used that IC at the time).
Looking at the 74ls166 (found at 3C) with my oscilloscope. Looking at all the signals, everything except for the output looked correct. Output was stuck high. I decided to hook it up on my logic analyzer just to make sure that it wasn't some weird timing. However, it just seems that the output was always high despite not supposed to be. Notice that it gets loaded, but the output doesn't change despite being enabled.
Pulling the 74ls166 out of circuit to test in my ABI chipmaster, the chip tested bad!
After replacing the 74LS166, the graphics were back to normal. Next step is going to be testing sound!