The Galaxian Trouble Shooting Manual Part I says:
"When character ROMs are removed the TV-screen will show horizontal bars."
Since there are still letters on your screen, I guess only one of the ROMs is not "seen" by the PCB.
Check out the lower PCB. When you hold the connectors to the top, the two roms are all the way on the bottom side of the PCB, right in the middle.
Look if they are firmly and completely inserted in the sockets. If not carefully press on them to get them back into place. If they don't look to be loose, remove them (lift each side just a tiny bit at a time, switching from side-to-side !), maybe clean the pins/sockets a bit and re-insert.
If this doesn't work you can take the ROM's from your first PCB (the one with the bad RAM) and insert these. Then at least you have one working PCB set.
The code on the chips should be (acc. to the service manual:)
M866L- D2316EC-647 and M866J - D2316EC-648