Pac repair using a chip comparitor

Hi all
        I was working in the shop last night on a pac that has been giving
me fits. I tried the "staring at it for hours" approach first, but that did
nothing to fix it :-0

        Weird symptoms. With a standard set up, I was getting a screen full
of slashes and zeros, flipping every now and again. I replaced the vram
with one of Clay's most excellent products, and the board now went through
self test over and over. up to, but not including the cross hatch. Self
test showed nothing. Removing the two middle roms changed nothing as well.
(that was a clue). I tried a special daughter card I have that runs MS pac
with no roms on row 6. booted to currupt screen images, so I felt there was
a problem in adressing the roms, or ram.

        Last night, on a whim, I decided to try out my new HP 10592 logic
comparitor. I set it up to check 8k becuase I noticed the IRQ was not
firing. The comparitor said it was fine. Since it was my first crack at
using the tool, I was not convinced. Going back to the adressing issue, I
decided to check the ROM selct lines, by 7N chip, 74ls42. The comparitor
showed 3 bad outputs on this. Swapped it and the board came to life. Even
works with the original vram, so the bad chip could have been messing with
the RD signal. No sound yet, but I'll get that working :-)

        On the Logic comparitor. The HP is a nice little unit, but you must
get one with the zif card. This was an add on produced in later runs. It
allows you to configure your chip with dip switches, where the original unit
shipped with cards you had to phsically alter to match the chip
characteristics.

        Others in the comunity feel he BUGTRAP device is a better
comparitor, but I have no experience with it, so if you are looking for one,
you should investigate those as well.

        The logic comparitor will be pretty much useless on any tri-stated
device. It will not make a newbie a guru, and like any test equipment, you
have to understand the it's limitations to use it effectively.

        Too bad I go sniped on that fluke model 90 z-80 thingy last friday
:-( Gusee I'm going to have to save up for one of them nifty 9010A's

Mike

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Received on Mon Dec 11 07:41:07 2000