Tech : New Pac PROM 7F has "dull" colors

[Guest]

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

This is a new one on me...

I was testing out replacement chips
for the 82s123 Pac Prom at location 7F
and came across something weird.

The chips I'm using are National Semiconductor DM74S288
which are functionally the same as the 82s123.

When I use the original 82s123, the colors are bright and clear.
When I use the 74S288, the colors are correct,
but look dull and washed-out. I need to do some
significant adjustment on the monitor to increase
the brightness and contrast to get the correct look.

NB. The colors ARE correct, they just look dull.

This puzzles me since, as far as I knew,
a PROM is a PROM, and it works or it doesn't.

The only explanation I can possibly think of
is that perhaps the chip is not "driving" the colors
as strongly as the 82s123 (...is that even possible
in TTL logic ? ) and needs to be compensated
by monitor adjustments .

Anyone else see this before ?

Any explanation as to why ?

Curious....

Thanks,
Steph
www.HobbyRoms.com

 

[Guest]

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

>The only explanation I can possibly think of
>is that perhaps the chip is not "driving" the colors
>as strongly as the 82s123 (...is that even possible
>in TTL logic ? )
You bet... I would take a look at the data sheets and compare driving
current or fanout specs. I'm betting the replacement parts you're
using have lower fanout. You could compensate by lowering the pullup
value on the output of the PROM on the pac pcb itself. It's been a
while since I repaired a pac board but I seem to recall RGB pullups
right on the PCB... I may be wrong.

You could also piggy back 2 of the proms thereby doubling the fanout.
-Adam
=================
www.onecircuit.com
=================