These fixes posted on RGVAC by phil@morris0.fsnet.co.uk (Phil)

Symptom: Permanently scrolling starfield on screen that is also out of
sync. No game play. No other graphics.
Cure: replaced LS161AN at 3A

Symptom: Colours were inverted, ie red was white (and vice versa), etc
- ROMs 1H and 1K were in the wrong sockets!
Cure: swapped 1H and 1K ROMs around

Symptom: Namco name is displayed on the attract screen, even though
Midway program ROMs are being used
Cure: swap the ROMs at 1H and 1K for some by Midway (ie the Namco name
is stored in these ROMs/EPROMs)

Symptom: No stars, no explosion sound when ship is destroyed
Cure: Track to pins 2 (and 1) of 74LS164 at 1A was broken - repaired

Symptom: No stars
Cure: pin 2 of 6M (74LS273N) was stuck low - replaced IC

Symptom: No background drone, no fire sound, no explosion sound, weak
'jingle' at start of game
Cure: On the solder side of the board, pin 4, 5 and 6 of IC 9L (an
LS259N) were 'smudged' together due to something rubbing hard on that
part of the board. Removed the solder from between these pins and all
the sounds then worked again

Symptom: Top rows of aliens all the wrong colour, ship (and spare
ships) wrong colour, diving aliens sometimes flickered off, also
sometimes as an alien dived the complete row it was diving through
would temporarily change to the correct colour
Cure: Pin 8 of 3N (an LS20) was stuck low, replaced IC and fixed
problem

Symptom: Board kept blowing fuses
Diagnosis: Did a continuity check between +5v line and Ground - this
was shorted. Tested out various components and it turned out that the
diode D8 had shorted, so shorting together +5 volts and Ground. 
Cure: Replaced faulty diode (a 1N4736A)

Symptom: Weak sound
Diagnosis: As part of an AC to DC mod the two diodes at D6 and D7 had
been removed and *not* replaced by wire links (as should be the case)
therefore the 12 volts wasn't getting through
Cure: Put in wire links where the diodes should have been

Symptom: Starfield was scrolling erratically, sometimes stopping
completely, sometimes very jerky, sometimes going up instead of down!
Diagnosis: Flexing the board seemed to affect the scrolling, but after
much prodding of the board I couldn't see a definite area that
reproduced the fault consistently. After a posting to RGVAC, Mark
Spaeth suggested that I check out the 74LS107 at location 1D. I took
some measurements with my logic probe and found that prodding some
pins affected the scrolling of the stars. Prodding pin 2 either caused
the screen to go blank or the board to reset.
Cure: Replaced 74LS107 at 1D

Symptom: No sound from a 'clone' board (looks almost identical to an
official Midway Galaxian board, but uses an MB3712 amplifier)
Diagnosis: When comparing to an official Midway Galaxian board I
noticed that pin V was unconnected on the board. Grounding this with a
wire caused the sound to start. 
Cure: Connect pin V on the board to Ground

Symptom: Second line of aliens from the bottom lost their bottom half
after a few minutes
Diagnosis: Replaced RAM chips one by one
Cure: Replaced 2101 RAM chip at 4F

Symptom: Wings on diving blue aliens sometimes disappeared
Diagnosis: Video RAMs from 1N to 1T were all socketed - gently
pressing on the ICs and/or rocking them a very small amount caused the
problem to come and go on the RAM at location 1N
Cure: Replaced socket for 27LS00 at 1N

Symptom: Game immediately starting if set to Freeplay mode or, if set
to require credits, on coining up
Cure: Replaced 0.1uf ceramic disc capacitor that connects to pin L
(Player 1 Start)

Symptom: On powering up, display constantly redraws and if looking
carefully you can just see the message BAD RAM 2
Diagnosis: Swapped out the 2114 RAMs at 3F and 3H to no avail. Then
tried replacing the 74LS245 at 5J and that fixed the problem
Cure: Replaced 74LS245 at 5J