This information is from a newsgroup post by Geoff Gunn"  geoff_gunn@hotmail.com

No ship fire or explosion.
All outputs of 4006 @ IC45 stuck high. Replacing IC fixed the problem.

Ship explosion too short in duration.
C25 weak. Replacing the capacitor fixed the problem.

Intro music clipped / distorted.
Melody generator chip, MN6221 @ IC49 faulty. Replacing this IC fixed
the problem.

No fire or ship explosion sound.
Output of 555 timer @ IC42 stuck high. Replacing IC fixed the problem.

Game displays garbage at power on. By manually resetting the processor
the game started fine.
Electrolytic capacitor C2 was broken thus not resetting the game at
power on. The capacitor forms a power on reset function. Replacing the
capacitor cured the problem.

Black screen. Sync was present but there were no colour outputs at the
edge connector.
All outputs of 7407 @ IC47 stuck low. Replacing the IC cured the
problem.

Game resets at attract mode. Text fine until birds appear on score
table and then the game resets.
Bad EPROMS @ IC39 & IC40. Blowing new EPROMs fixed the problem.

Game displays OK but it won't coin up.
R17 missing. Replacing the resistor fixed the problem.

Ship moves to right without any joystick action.
Bad 74LS157 @ IC12. Replacing IC fixed the problem.

Excess noise. Lots of background noise.
100nF capacitor (on back of board) between pins 2 & 3 of LM380 @ IC43
missing. Replacing the cap fixed the problem.

Game (at switch on) coins itself up and starts to play. The ship fires
but doesn't recognise any controls.
Faulty 8085 processor. Replacing the processor fixed the problem.

Game starts itself as soon as credits are inserted.
Faulty 74LS244 @ IC4. Replacing IC fixed the problem.

No ship fire or explosion noises.
Faulty LM 324 @ IC46. Replacing IC fixed the problem.

Incorrect alien swoop noise.
Bad capacitor @ C22. Replacing capacitor fixed the problem.

No sound.
C35 broken. Replacing the capacitor fixed the problem.