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Galaxian PCB repair..


andykmv

Question

I recently purchased a Galaxian parts pcb from davefjedi as non-working and decided to resurrect it instead of using it for parts.

 

Symptoms:

No sound, No video.

 

This log is the challenge of bringing one back from the dead. It might cost me more than buying a working one, but thats challenge of restoration and preservation.

 

i made a harness adapter using a finger board, card edge connector and a switch mode psu to convert the Galaxian card edge to a pacman wiring compatible card edge so that I could use my Pacman Test Rig for testing on the work bench. (this pcb's power supply section is not populated, hence the sw mode psu)

 

As i am completely unfamiliar with a galaxians pcb but am now reasonably familiar with spacei invaders architecture and the pacman schematic i figured i would start from the start and work my way through the pcb checking everything. I have got it hooked up now so this job is going to be an inbetweener whilst i am waiting for parts for the pacman #2 pcb.

 

Step 1 - checking the crystal clock oscillator circuit.

It took me a while to find the crystal oscillator circuit in the schematic as it was on the far rhs of the schematic. Hooked up the DSO for this one. the hex inverter used for this is an odd fish as they used a gated inverter ic 74LS368 - the gates are tied low to enable the inverters to function as a non gated hex inverter. following the hex inverter at pin 7 is a pair of flip flops used to divide the 18.432mhz clock down to 6Mhz. the output of pin 7 is showing an unbalanced clock signal, which the flips flops helped to cleanup into a nice uniform square wave clock signal. both the normal and inverted clocks are being produced, but i have yet to check the frequency - need to find the frequency counter for that or use the dso frequency measurement feature.

Edited by andykmv
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Step 2. Mechanical/Physical checkover

 

power decoupling caps are spread around the board, mainly the two sides and a few in the middle.

these are 0.1uF ceramic disk caps.

- a number were missing (broken off or cut off) - about 5

- a small number were broken in half - 3

since i am going to replace the missing and broken caps, i decided to replace these with monobloc ceramic caps so until i get a supply of these with the correct lead spacing, i am steadily removing these.

 

the ULN2975B ic is missing so i have ordered a couple of these. this ic is associated with the Coin Lockout & Coin Counter functions (being a high current darlington type device) therefore for the purposes of repairing this boards video and audio a missing one does not matter - it can wait.

 

the two ROMS, rom 1 & 2 and the end of the board (opposite end to harness edge connector) looked ok initially but for some reason i touched rom 2 and noticed it was loose in the socket. so was rom 1. bu that i mean one side of the socket had absolutely no grip on the pins on one side of the ic and therefore it could freely jiggle up and down!

looks like these are candidates for a new set of ic sockets so thats next on the list to desolder and replace.

 

- - - Updated - - -

 

thanks for those links! useful for anyone interested in galaxians too.

i have a copy of these in the mail

 

Looking forward to this repair log.

 

Edit:

Check out this "Galaxian - Trouble Shooting Logic Board Guide"

 

http://www.arcade-museum.com/manuals-videogames/G/galaxian-ts-p1_2.pdf

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power decoupling caps are spread around the board, mainly the two sides and a few in the middle.

these are 0.1uF ceramic disk caps.

- a number were missing (broken off or cut off) - about 5

- a small number were broken in half - 3

 

I'd hate to get a glimpse of how poorly these games were handled over the years. Poor filtering can cause some games to act up, I had up to 4 of those caps missing recently and replacing those fixed some gfx glitches and random crashes.

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two ROMS, rom 1 & 2

(at the end of the board (opposite end to harness edge connector))

 

removed the two sockets. a bit of messy job as my desolder gun is kaput at the moment so had to chop up the sockets to expose the pins. then i individually removed each pin.

 

rom 1 socket is out and all the holes cleaned out.

rom 2 socket is removed but still have to clean out the holes.

 

time to go and do some paddock fence rebuilding - punching in replacement pine posts at the moment (replacements for the bushfire damaged ones).

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two ROMS, rom 1 & 2 (at the end of the board (opposite end to harness edge connector)):

 

rom 2 holes cleaned out. pcb cleaned around the rom 1,2 sockets area both sides.

 

pcb ic pads and traces inspected on both sides in rom sockets 1 & 2 area, and only minor damage to corner of one ic pad. the copper layer and thru hole plating seems quite strong on this pcb.

 

next is solder in the new dual wipe sockets.

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no life in it yet!

replaced the outside edge disk ceramics (some missing, some broken) with monoblocs from column 1 thru 6, and one disk ceramic in the middle that was broken.

 

started tracking the system clocks starting with the master clock.

master clock looks ok.

then onto horizontal clock divider circuits (74ls161's) and the 1h, 2h, 4h, 8h, 16h, 32h, 64h, 128h and 256h derivatives all look aok.

started poking around at these clocks where they are distributed to end points around the board. no answers as yet.

 

also the audio amp is giving out an occassional (and frequently) low level high pitched police siren noise. found a supply for those amps so have to order some parts and replace this sucker and the related caps..

 

i am seeing similarities with the pacman and mspacman schematics architecture so it is making it easier to read the schematic too.

time to give up for the night.

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