TECH Q: Pac, no red sprites

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Archived from groups: rec.games.video.arcade.collecting (More info?)

I'm really scratching my head over this one, and I'm looking for help.

_Ms. Pac Man_ motherboard. Problem: No red sprites. That is, you see the
red monster's eyes, but not his body; Ms. Pac doesn't have a ribbon.
Everything else is just fine.

I am certain that the problem is not the character ROMs at 5E and 5F; it is
not the color PROMs at 4A and 7F; and I've swapped out various TTL chips in
the sprite-control region (74LS377, 74LS157, 74LS158) and I don't think it's
caused by the four video RAMs (2115s).

Mowerman's Pac file does cover this sort of thing, but all it says is as
follows:
"Plays but No Red on moving sprites (Ms Pac Bow/ Cherry/ Speedy).

Ram Data Bus D5 high resistance."

I'm not sure exactly what this means. Which RAM bus is he referring to --
the 2114's, or the 2115's? Can anyone be more specific?

I don't think it's an open trace, either, as all these chips are getting
data to them (verified by oscilloscope). I've even replaced the resistor
networks in this area.

Thanks!!!!

Matt J. McCullar

 

[Guest]

Archived from groups: rec.games.video.arcade.collecting (More info?)

Matt,

The schematic included with the operators manual has everything you
need to track down that bus line. High resistance suggests trace
resistance going up due to corrosion etc. So you are looking for a weak
output at point A OR B and then tracking down what is responsible for
it be it chip or trace, socket, etc.

I fixed about 20 of these boards in the past month or so. The only 5D
I found was pin 13 of 1H that is tied to pin 4 of 1F

But I of course suggest you do your own reading.

- Matt

 

[Guest]

Archived from groups: rec.games.video.arcade.collecting (More info?)

Okay, I'll try "ringing out" traces with the ohmmeter. Haven't done that
and it does sound like a good idea. I'll post the results. THANKS!!!!

Matt J. McCullar