Midway - Space Invaders memory problems... 

(Why do I keep getting these letters? ;)

>    HELP! I've got an space invaders deluxe upright that has got me
> baffled! It apparently sounds as though it has similar symptoms to many
> others that have asked for tech help on the r.g.v.a.c. The game powers up
> fine but the monitor is coming up all scrambled except for the "credits"
> at the bottom of the screen the vertical & horizontal hold is ok, I've
> check the anti-slam switch in the door and taken off / cleaned with
> contact cleaner & put back on the 4 barrel clips to the boards. Guess
> what.... no improvement! Although if I do slam the door (not hard) the
> "high score <1> score high score<2>" line appears for a couple of seconds
> clear as day. The rest of the monitor is putting up partial space invaders
> (attract mode I would guess) and a couple of letters of the high scores
> (like *F*or *E*) . Of course besides the monitor being scrambled, I can't
> get the actual game to start either (I assume that this is a related
> problem?).  From what has been recommended to me, you are the man to ask.
> When you have a couple of minutes could you perhaps point me in the right
> direction? This is my 1st anything with circuit boards (and should perhaps
> be my last!) I'm more used to the upkeep on my EM pinball games (at least
> there the problem is usually obvious) Thanks in advance for any
> suggestions that you might have
...

Sounds like a ram problem. To fix Space Invaders family of game
boards, first remove ALL the game ROMs, then turn game on. You should
see a repeating pattern of a thin and fat  lines across the screen.
Next close the Slam switch for a moment, this pattern should change, but
still be solid lines (thin or fat doesn't matter, but they repeat). Now
if any lines are flickering, or there are dots on the screen that are
sometimes covered by lines but reappear when you "Slam" the switch, then
you defiantly have ram problems. Next you try momentarily shorting 
pin #7 of each Ram chip to ground until you find the one(pair) that when 
shorted either make solid the flickering line or cover the dots. Now you just 
have two suspects to replace. Sub one at a time, and alternate, it is only one
of the two Ram chips that share the pin #7 output that should be bad.