Galaga motion graphics problem

[channelmanic]

OK,

This Galaga is kicking my tail... The blue color in every character that has blue is missing every other vertical line. This problem is also making the player ship look yellow instead of white.

The white text is OK though... It's only affecting motion sprites and only the blue.

The blue bees have wings that are missing a couple of vertical stripes of blue color.

Any idea as to where to check? I've swapped out the 02xx chip and all the RAM signals look good on the 2147s. The PROM at 1C on the video board had the wrong checksum but burning a replacement had no effect on the problem.

The board worked beautifully until it was burned in for about 4 hours.

 

[cdjump]

i had one do something very similar. i replaced the 2147's with the 2 2149 mod and it took care of it. That's where i'd start looking. also- have you double checked and made sure the right customs are in the right places? Mine had a couple of misplaced customs making it do weird stuff as well.

 

[channelmanic]

Swapped the RAMs out... no change.

Swapped out the LS157 driving the WE* lines... no change. Swapped out the resistor packs on the board... and started swapping out the other ICs... no change yet.

I need to pick up some 74LS161, LS377, and LS298 chips tomorrow.

This is frustrating!

 

[channelmanic]

Galaga is fixed!

FIXED!

WHAT A BITCH!

OK, this thing was a major PITA. After swapping out the motion RAM it had some garbage on the screen that looks like a bad RAM... Hit one of the RAMs swith coolant and it goes away until it warms up.

That's it, right? NOPE. The replacement RAM chip does the same thing.

Swapped out the resistor pack that holds the lines high. Nope. Wasn't it either.

That only leaves the 74LS365 at 5A/6A and the 74LS298 at 5B.

Replaced the 74LS298 at 5B to finish the board.

 

[channelmanic]

F***...

OK, now I have blue dots randomly trailing blue characters... Looks like bad Video RAM, but it's not.

Turns out the 74LS298 was bad and that was what was causing the missing blue lines, but not the blue streaks...

Swapped the 74LS365 chips, that wasn't it either.

Noticed on the 74LS157 at 4A that when I accidentally bridged pin 13 and 14, the streaks disappeared. Checked the 74LS20 at 3A and found that when the logic probe was touched to pin 10, the screen went nuts with streaks. That is the 6MHz* signal.

Swapped out the 07xx IC and no change. Replaced the 74LS368 at 3N to get rid of the streaks.

Damn....

 

[Solder]

What a pain in the butt that board is! Kudos to you for perseverence and fixing the faults though.