RE: Pac repair

Well
        I know you were all sitting on the edge of you seats wondering if I
managed to track down the sound problem on my pac board. Did it last night.
The symtoms were a buzz and a click whenever a sound was to be played. I
had the board in test (by grounding pin K) and another ground on M to give
the self test sounds.

        Scope showed a small distorted square wave thith a spike when the
sound should come out. I consulted Kev's most excellent page and
concentrated on the logic portion. I did not see anything too obvious,
butpin 7 of 1K looked odd. It is driven from pin 2 of 1L. Given the inputs
to 1L, on pin 3, pin 2 should have looked better than it did. I swapped 1L,
but to no avail. I swapped 1K and she roared to life.
        I did try the comparitor on these chips, and it did indicate a
problem with 1K, but due to corrosion on the pins, I did not trust the
readings.
        Another working one for the pile! Two left to go. One of them has
a really bad hum visible on the scope with the speaker hooked up, but the
scope shows nothing with the speaker disconnected. I swapped all the pieces
that make the feedback, but she no work. I have pulled the amp and see a
nice signal going in, and am awaiting a care package from Bob Roberts to see
if the amp was the culprit. The other has some serious overheating going on
in the power supply when I inject the 5 volts right on to the rail. This
should not happen, so I want to fix that first before I start looking at the
logic.

        A note on clay's replacments. One of the benfits of these that was
overlooked in our discussion, is that they clear up some room, and allow me
to get the logic probe on some pins that I could not normally reach.

        Oh, and I played some omega race, piled up a couple of defender
boardsets I want to sell, and fried up some bipolars (pac 4a, 7f) for a
friend.

        A note on desoldering. For years I have used the little solder pump
in conjunction with my soldering iron with much success. I tried out thew
big blue soldaput pump here at work the other day, and it does a much
cleaner job. I also, recently picked up one of the radio shack red bulb
desoldering irons. I have found that the little pump works well for me, but
is more cumbersome to use. If the chip in question is not bad, it is clean
enought to stuff back in the socket. The RS red thingy, is way easier to
use, but it does not have enough suction to clean the chip up well. The big
blue, seems to me to be the best of three, while more cumbersome, cleans up
the chip the best for re-use, should my trouble shooting technique fail me
and I pull a good chip. I would really like to get a profesional desoldering
system, but I'm to cheap :-)

That's all for now

Mike

> -----Original Message-----
> From: mike_ranger@dofasco.ca [SMTP:mike_ranger@dofasco.ca]
> Sent: Monday, December 11, 2000 9:47 AM
> To: rasterlist@synthcom.com; TechToolsList@flippers.com
> Cc: Paul.Tonizzo@sybase.com
> Subject: 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
>
> To UNSUBSCRIBE from techtoolslist, send a message with "UNSUBSCRIBE" in
> the
> message body to: techtoolslist-request@flippers.com. Please direct other
> questions, comments, or problems to jrr@flippers.com.
---------------------------------------------------------------------------
** To UNSUBSCRIBE from rasterlist, send a message with "UNSUBSCRIBE" in the
** message body to rasterlist-request@synthcom.com. Please direct other
** questions, comments, or problems to neil@synthcom.com.
Received on Wed Dec 13 08:15:18 2000