Galaxian - bootleg

				Symptom: Unsync'd screen. Unable to tell much more Work: Not really much of a fix as such, but worth going over i guess. Bit of an interesting one this. For some reason EVERY ic is socketed (kinda handy!). There is also a very tidy looking hack on this board. As yet no idea what it does or what game this used to be since all the roms and rams are missing. Poplated rom, ram etc and plugged in. Initially just a garbled screen that won't sync. Having not yet come across a galaxian board with a sync problem, just on a hunch i decided to switch my monitor over to positive sync. Bingo. Stable screen of crap! The patch wires appear to end on the sync pad of the edge connector. Removing any trace of patch wires sorted the "problem" and got the board back to negative composite sync. Next up, put it on the fluke. it's reporting tied data bits when i do a bus test. After probing about for a bit i noticed that the sockets at 4K, 4J, 8H & 8F had 74LS138's in them rather than either having 8216's or being empty. I also noticed that 9B had a 244 instead of a 245. Close, but being one out can make a whole heap of difference :) Removing these chips and replacing the 244 with a 245 basically got the board running, but there was no text coming up when i ran the galaxian test rom. Okay. Removed the test rom and put galaxian roms in. Game basically comes up but all the text is doubled up (vertically) and the graphics are a bit garbled. The troubleshooting guide points to the 74LS86 at 3B, and sure enough, this was missing! (Should have spotted that earlier!) Populated that, and that was those problems sorted. At this point the board is pretty much fine. There is just some video corruption on the outer columns of galaxians and in the bottom left and bottom right of the screen. Using the fluke gives inconsistent ram checks on the video ram. Swapping the program ram with the video ram confirms that the code no longer runs. Fluke now gives a decode error on address bit 7. The 2114 at 7N handles the higher four bits of the ram address bus and replacing this chip fixed the board. Due to lack of spares, a single rom hack was then done on this board. The 2101's had to be removed and put back on the board they were borrowed from. Parts Used: 1 x 74LS245 1 x 74LS86 2 x 2716 1 x 27128 Parts Salvaged: 1 x 74LS163 1 x 74LS244 3 x 74LS138