Galaga is a difficult board to debug, but here's some helpful information I found.
RAM Error/Replace chip on video board
Also for RAM errors, try reseating the chips, and verify that your voltage is +5VDC (measure on the board, not at the power supply).
From Chris Cope:
1) RAM OK message appears. Stars in background look okay, but are stationary. Game freezes here.
FIX: replaced Z-80 at 4J
2) Background is solid light blue. Solid pink horizontal band at top. Another solid band at bottom. Text appears as solid yellow bands.
FIX: 02xx OR ROM at 4L are bad or not socketed properly
3) Horizontal lines across entire screen. Background stars are visable moving "below them" and foreground graphics are visable "above them".
FIX: no friggin' clue. Someone please help.
4) Foreground sprites are doubled vertically. The ship and all enemies are split apart. Example: the player ship is made up of pixels. Every "even" column of ship pixels will be in the right place. Every "odd" column will be vertically shifted up by about one-half of the screen height. The enemy ships do this as well.
FIX: No idea. I'm assuming that it's an address latch somewhere. I don't think it has to do with ROM 4D, which handles the contents of the sprites, since the data is there, just in the wrong place.
5) Missing half of green or blue coloring in sprites. Example: The big enemies that have the tractor beam have green wings after they're shot once. Every even column of green pixels will be visable. Every odd column will be gone. When it happens to blue instead of green, it's more bothersome, since blue is used in more types of enemies.
FIX: No idea. I tried 05xx, which is the final chip the video goes through, but that's not it.
EPROM functions on video board: 4D : sprite definitions. W/o this, all foreground graphics are big blocks.
4F: this seems to handle enemy explosions. If this isn't there, a big white block appears where you've just shot an enemy.
4L: background. w/o this, the screen looks like #2 above.