Recently got hold of a original galaxian(midway) pcb from a member on here,
i was asked to have a look at it and try to get it fixed.The fault description was that it used to work with issues of sounds missing,now it boots to garbage.Here it is on my work bench :-
ok now lets plug it in and fire it up.
o dear ,yes indeed its garbage on the screen every time i power up so this game is not booting.Scoped all the data+address buses on the z80 cpu and they were present but not stable ,hitting on pot holes 😄.Clock signal was good with correct frequency,halt was high,but when i checked the reset pin it was toggling,which tells me the watchdog is barking.
so the program is not running,so next i removed all the program eproms from the daughter board and did a romident.Checked out all good.
Looking in the schematics there are 2 work rams(2114) located at 7P,7N which are common to failure,luckily these were socketed so i removed them and put them in my pacman pcb to test and all was good.
Decided to randomly scope some decoder chips near the cpu and looking at a 74ls139(8e) pin 5 output looked bad.
and just to make sure the chip was faulty.
removed this bad ls139 and put in a new one,powered on and...............
ok we have some progress,good news is the reset signal on the z80cpu is good and all data,address buses are smooth,we now have a good output on the 74ls139 pin 5.
scoped a 74ls138 at location 8n and pin 13 was floating,replaced chip and powered on.............
o come on 😣,ok looks as though its trying to tell me something as i can just about see a message BAD RAM 2 while the screen garbage is jittering.
looking in the galaxian manual this means bad video ram.
to be continued..............................................
i was asked to have a look at it and try to get it fixed.The fault description was that it used to work with issues of sounds missing,now it boots to garbage.Here it is on my work bench :-
ok now lets plug it in and fire it up.
o dear ,yes indeed its garbage on the screen every time i power up so this game is not booting.Scoped all the data+address buses on the z80 cpu and they were present but not stable ,hitting on pot holes 😄.Clock signal was good with correct frequency,halt was high,but when i checked the reset pin it was toggling,which tells me the watchdog is barking.
so the program is not running,so next i removed all the program eproms from the daughter board and did a romident.Checked out all good.
Looking in the schematics there are 2 work rams(2114) located at 7P,7N which are common to failure,luckily these were socketed so i removed them and put them in my pacman pcb to test and all was good.
Decided to randomly scope some decoder chips near the cpu and looking at a 74ls139(8e) pin 5 output looked bad.
and just to make sure the chip was faulty.
removed this bad ls139 and put in a new one,powered on and...............
ok we have some progress,good news is the reset signal on the z80cpu is good and all data,address buses are smooth,we now have a good output on the 74ls139 pin 5.
scoped a 74ls138 at location 8n and pin 13 was floating,replaced chip and powered on.............
o come on 😣,ok looks as though its trying to tell me something as i can just about see a message BAD RAM 2 while the screen garbage is jittering.
looking in the galaxian manual this means bad video ram.
to be continued..............................................
