Started out with a dead board. No clock signal at all.
Tested the clock pin (2) with the logic probe on the IC 74LS161's at 3A, 4A, 5A, and 6A. 3A and 4A were floating, 5 and 6 were stuck low. Replaced the IC 74LS368 at 1E and the crystal (18.432mhz).
Got clock activity on 3A and 4A, 5 and 6 still stuck low.
I ended up replacing the 74LS161's at 5A, and 6A, as they were terribly corroded as well, they probably were not bad, but, better to replace now than wait for failure.
Cleaned the corrosion around a lot of the other chips, and reflowed solder, did not make any difference.
Tested IC74LS74 at D2, getting no signal on pin 1 input. Signal should be coming from the IC's at A1 and B1 74 LS164's pin 9 as per the schematics. Both of those had no signal as well. Used the schematic and traced the line back to IC 74259 at 9N pin 9, that pin had a signal. 9N is on the top board, A1, and B1 are on the bottom board, traced both sides back to the ribbon cable for continuity and they checked out. Replaced the ribbon cable as it visually looked awful, had definite corrosion. Also cleaned the pins on the headers, and reflowed the solder on the pins.
The IC's at 1A and 1B started to get proper signal. Noticed the board had a lot more life at this point, but still no stable picture.
Next step was to check the XOR gates at 3,4,5, and 6B, 6B tested bad, replaced (several of the IC's in that area were bad, ended up replacing a lot of them proactively after this)
Still getting garbage on screen...
Something clearly wrong in the sync area. Back to the schematics.
Pin 12 on IC 7400 at 8L is floating, traced back to IC7404 at 6E pin 6, which was not floating. Again on opposite boards, traced back to ribbon cable again, good continuity, tested the ribbon cable, tested fine, so I reflowed the solder on the headers on both boards, and cleaned the pins, and got good continuity afterward.
Plugged it in, and now get this:
Definitely making progress now.
Replaced so far:
74LS161's at 4A, 5A, and 6A (corrosion).
74LS368 at 1E (bad)
18.422 Xtal at 1F
74LS86 at 6B (bad)
Ribbon cable (bad)
What's left to do?
Starfield background is moving way too fast.
Fix the foldover issue.
Try to install the FourPlay or Video Eight Mods from Macros-Arcade
More to come...
