View Full Version : Galaxian repair

29th March 2009, 06:22 AM

I own a galaxian pcb for some time. I got it with a cocktail table. I have decided to take some time to look why the game isn't working.

I first converted it to accept DC instead of AC. Then I made a jamma adapter for it, wiring only the 5V, 12V, GND and video (rgbs).

At power on it displays nothing and appears to be dead.

Checking with a probe, I could notice that all chips were correctly connected to 5V. The main clock is also working.

After investigating a bit, based on the schematics, I discovered that the clock used for the video circuits was always to 0. Checking how this clock signal was made, I found that the vertical sync signal wasn't pulsing. the vertical sync is generated from the vertical counter based on two 74LS161 at 5A and 6A.
While 5A was working, 6A didn't. It was getting good inputs, but bad outputs.

Replacing the faulty 74LS161 at 6A and the video was back :) However, the monitor could not sync with the video signals because I inverted Green and Sync on the jamma adapter.

Fixing green and sync on the jamma adapter made the video appear correctly.

Here is now what happens: The screen displays the caracter 0 on three lines, then a screen full of caracters appears quickly and then back to the first screen.
Looking more closely, I can see "Bad ram 2d".

Inverting rams at 3H and 3F changed the display a bit. Inverting rams at 4F and 5F changed nothing.

Does it mean one or the two rams at 3F and 3H are faulty ? Since I don't have spares for now, it is difficult to say...

I have also checked all eproms with my eprom reader, and they are all good.

Checking on the net for "bad ram 2d" I found that the 74LS245 at 5J may be in cause. However, there is no chip at 5J on my pcb. Should it be there ?

thank you!

29th March 2009, 07:03 AM
Sounds like you need get the galaxian repair log and schematics :)

If 5J isn't fitted, there should be 2 chips at 4K/4J to replace it. They had either at the time I guess to due availability/cost. I'd guess you have the latter if it's getting into the ram test.

Does it change if you remove the rom board?

Got a screen shot?

29th March 2009, 07:21 AM
I am lucky ^^ I have found two 2114 on a fully socketed bagman pcb from valadon automation.

Replacing both 2114 at 3H and 3F made galaxian work perfectly :)

I then replaced one of the original 2114 and I found that one was bad.

Here is the summary of the repair:

Replaced faulty 74LS161 at 6A and a faulty 2114 at 3H.

Tomorrow, I will hook sound and controls to see if all work ok.

Arcade King
29th March 2009, 09:22 AM
well done, moved to the repair log section.
Galaxian is one of the easiest boards to work on because there's so much documentation available.

29th March 2009, 04:42 PM
thank you, sorry to have posted in the wrong section :redface

Arcade King
29th March 2009, 04:44 PM
no you didnt post in the wrong section i just moved it here because you fix it yourself and the thread could be useful to others. :)

30th March 2009, 03:38 AM
some news. I have hooked the controls and the sound and everything is working ok :)

30th March 2009, 06:22 AM
Now to do the fourplay mod to it