This is the easy way to convert a Galaxians from AC to DC. It leaves all of the components that will no longer be used on the PCB, and may actually damage them, preventing it from being converted back to AC should the need arise.

The wire link used should be of a decent thickness, 1mm or more, since it will be carrying all of the power for the PCB.

Around about position 11N on the PCB, are two large diodes, labeled D4 and D5. Near these, by the heatsink, is a capacitor, C37. We wish to make two links from the ends of the two diodes across to the track that the +ve end of the capacitor is connected to. (see picture for details)

 

If you do this from the bottom instead, you can easily get away with a single wire! (for the extremely lazy!) - This wire picks up from the same location on the diodes but underneath the board, and connects to the track where there is a large bunch of through holes. The picture should help you identify the location. n.b. Make sure you use decent wire as all of the power will be going through this.

 

For reference, here are the pinouts for both the original AC supply and the DC 
converted supply:-  A to Z are marked on Namco PCB's - but it loses some letters, so until I know which ones, only the ones printed on the PCB are marked.

AC SUPPLY 

Components Solder
Ground Z 22 Ground
Ground   Ground
12v AC - 12v AC -
12v AC + W 12v AC +
Speaker - Speaker +
Video Blue Video Sync
Video Red Video Green
Ground S Video Ground
Table NC
Player 2 Left Player 2 Right
Player 1 Fire Player 2 Fire
Player 1 Left Player 1 Right
Start 1 L Start 2
Test Service
Coin 1 Coin 2
Coin Lockout Coin Counter
1 Play Lamp 2 Play Lamp
7v DC out E 7v DC out
7v AC - 7v AC -
7v AC + 7v AC +
Ground Ground
Ground A Ground

 

DC SUPPLY 

Components Solder
Ground Z 22 Ground
Ground Ground
+12v  +12v 
+12v  W +12v 
Speaker - Speaker +
Video Blue Video Sync
Video Red Video Green
Ground S Video Ground
Table NC
Player 2 Left Player 2 Right
Player 1 Fire Player 2 Fire
Player 1 Left Player 1 Right
Start 1 L Start 2
Test Service
Coin 1 Coin 2
Coin Lockout Coin Counter
1 Play Lamp 2 Play Lamp
NC E NC
+5v +5v
+5v +5v
Ground Ground
Ground A 1 Ground

Note the yellow 12v wire on the left of the picture, 5v red wire on right (pins C and D)