When I powered up the board it was totally dead.
There was no output to the screen, no activity from the ROMs or CPU.
I started at the 'Clock Generator' and all was good there.
Moving up the chip numbers I started to look at the four 74161's at [5] [6] [7] & [8].
Pin 15 had a pulse on [5] and [8] but was stuck low on [7] and [6]
I removed, socketed and replaced [7] with a new 74161 and I now have a title screen, can clock up credits and play a game but have the graphics glitch as shown in the pics below.
After rechecking the four 74161's I found that pin 14 on [6] had no output.
I removed it and put a socket in it's place butI didn't have any spare 74161's left so I desoldered one from a scrap board and tried it out.
The game has a slightly different fault and pin 14 now has activity.
I checked the rest of the pins and everything is as it should be.
Following on from pin 14 at [6] I then looked at what was happening at pins 5 & 6 on chip [1].
There was a pulse to pin 5 but the output at pin 6 was stuck high.
The 7404 chip was removed from [1], socketed and chip replaced with a new one.
Hey presto!!!! Another Space Invaders PCB brought back to life.
A Space War Part Three at that...
I played a few games and P2 fires a missile immediately at the start and won't fire after that.
I've seen this before in a previous repair so should be an easy fix.
On closer inspection of the PCB, I found that Dipswitch 7 was half way between on and off.
I switched it on for 2 Player Cocktail mode and all functions are now working.
There was no output to the screen, no activity from the ROMs or CPU.
I started at the 'Clock Generator' and all was good there.
Moving up the chip numbers I started to look at the four 74161's at [5] [6] [7] & [8].
Pin 15 had a pulse on [5] and [8] but was stuck low on [7] and [6]
I removed, socketed and replaced [7] with a new 74161 and I now have a title screen, can clock up credits and play a game but have the graphics glitch as shown in the pics below.
After rechecking the four 74161's I found that pin 14 on [6] had no output.
I removed it and put a socket in it's place butI didn't have any spare 74161's left so I desoldered one from a scrap board and tried it out.
The game has a slightly different fault and pin 14 now has activity.
I checked the rest of the pins and everything is as it should be.
Following on from pin 14 at [6] I then looked at what was happening at pins 5 & 6 on chip [1].
There was a pulse to pin 5 but the output at pin 6 was stuck high.
The 7404 chip was removed from [1], socketed and chip replaced with a new one.
Hey presto!!!! Another Space Invaders PCB brought back to life.
A Space War Part Three at that...
I played a few games and P2 fires a missile immediately at the start and won't fire after that.
I've seen this before in a previous repair so should be an easy fix.
On closer inspection of the PCB, I found that Dipswitch 7 was half way between on and off.
I switched it on for 2 Player Cocktail mode and all functions are now working.
