[tech] space invaders - dotty lines

[LukeWells]

tt> I would appreciate any help anyone can offer with the following
problem I am having: -

I have a space invaders (midway 8080) that was not booting and
displaying garbage.

I removed the roms to display the lines on screen, which is the usual
starting point to check the rams.

Instead of displaying solid lines, it is displaying lines of dots,
like every other pixel is missing.

All address lines appear to be active to the rams

I tried grounding the output pin on the rams, which would usually
display a solid line, but that too displays a dotty line.

Oh and the clock driver is a tested working one.

Thanks in advance for any help you can offer

Luke

[/tt]

 

[andrewhannay]

[tt] Oooo, I nice technical one.
Right!
I assume the video display is synced, and that you haven't just got a very
fast rolling display.
The onlt thing between the RAM and the output is the parallel to serial
shift register IC4 which is then mixed with the sync pulse. Is there a
problem with the shift register or CAP.
More likely, is the Sync pulse circuit generating extra pulses which is
mixing with the video. The sync generation circuit takes all of the clocks
and sticks them through a load of gates, so if one of the gates in the chain
is faulty, then you might be getting an extra clock signal in with the sync.
Looks like it might be running at half the line frequency if you are getting
every other pixel missing.
Andrew

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[LukeWells]

tt> Well that's a very knowledgeable reply

Yes, video is synced and stable.

I assume that the problems you are thinking of would also cause
problems for the program? As the game was fully working, now does not
run, the video timing problems would stop the cpu from being able to
read/write to the ram properly?

One point I didn't mention before is that I tested the ram with my
fluke and it seems to be able to read/write ok to all ram between
2000 and 3FFF

I'm going to try and find a decent schematic pdf and then I can start
looking in the areas you suggest, the one I have is hard to read to
say the least.

Thanks

Luke

 

[andrewhannay]

[tt] I've got schematics in TIFF format (1.5M) and pdf (475k) that I downloaded
from somewhere, I can email them to you if you like.
Basically you are looking at everything to the left of F4-F7.
I love these boards.
Andrew

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