We are having a problem with a Quadra 950 that has stumped a couple of
techs sent to us by the vendor, as well as us :-)
This Quadra is 'stuffed' with 64Mb RAM, 2Mb VRAM and some large disk (400Mb
I think). What happens is that after a certain variable interval of time
the video circuitry 'loses' vertical sync. It is quite apparent that the
Mac CPU is still ok, and the problem really seems to lie with the video
circuitry, since you can still save your documents, quit your aplpications,
even pull down menus etc. (only the display is scrolling lik crazy, so it's
pretty hard to hit menu items ;-)
We had the original logic board replaced twice now, and the same problem
has been occurring on all boards although the inteval between occurrences has
varied. The first board had this problem every about 1hr, more often if the
cache was turned OFF (yes OFF).  This is the only symptom that is not
obviously related to the video circuitry (after all the cache is on the CPU
right?). The second board had the problem every about 10 minutes (we did
not keep it, rather we put the original back), the current board we have
has this problem a lot less often (has gone about a week without doing it,
but did it twice within an hour yesterday).
The technicians that have come out to try to figure it out seemed pretty
sharp and tried a number of sensible things (most of which we had
previously tried ourselves anyway). These included:
-swapping the RAM
-swapping the VRAM
-swapping monitors (3 different 13" Apple RGB and 2 16" Apple RGB)
-swapping cables
-disconnecting HD and booting from floppy
-sacrificing various small mammals
Here are some other facts:
-when we put a RasterOps ProColor 32 on the NuBus, the problem is fixed
 (it does not occur with the internal video or the ProColor video out)
-the vendor cannot reproduce the problem back in their shop on the boards
 that exhibit it in our lab
-Apple is said to be getting a little bit pissed off at shipping us boards
 that they cannot find anything wrong with, and getting them shipped back as
 faulty according to us.
Here are some speculations:
-I bet that neither Apple nor the vendor has tested these boards with that
 much RAM and VRAM on them simultaneaously, which may have something to do
 with their finding it hard to reproduce the problem (though we can
 reproduce it with just 4Mb of RAM, so this is probably a red herring).
-the vendor's representative blames 'metal in your walls' - claims that our
 walls have metal in them which interferes with the Mac's operation. We
 have another 950 in the lab which works like a charm (though it's not in
 the exact same spot....) Please don't tell me this is probably it, I do
 not want to believe it.
-it's probably not poltergeists, since they do not have badges to get past
 security at the entrance to our site.

Has anyone any ideas? Please? Please email as well as posting cause I seem
to be spending a lot of time rebooting my mac lately and have a lot less
time available to read news! :-)
Thank you in advance.
Carlo Tiana
NASA Ames Research Center
carlo@vision.arc.nasa.gov
