One of the problems that seems to occur on Galaxian PCB's is failure of the sprite ram used. These static ram chips, 27LS00 run particularly hot and fail frequently. Add to this the fact that different makes of these 256 x 1 bit chips seem to require pull ups / no pull ups / different value pull ups makes replacement somewhat trickier than it should be.
Because of this, and the fact that I had an area of PCB spare on an order, I designed a small PCB that allows you to replace four of the normal sprite rams with a fast 2101 ram instead. (just leaving one fitted at the end)
It fits Midway and Namco pcb's and many bootlegs, but not all, as some have different spacing between the sprite rams and a few have totally different sprite ram chips!
You can grab a copy of the layout for Eagle PCB here. As with all of my projects, there is no guarantee that it will work for you - I have used them many times, and several other people have used them successfully.
Sprite RAM PCB fitted to Namco galaxian.
n.b. as you can see in the picture, you do not need to fit all of the pins - just the ones that are used!