My 11 year old son and I are long time Mac afficionados. Guy Kawasaki
(author of the Macintosh Way) actually once gave a friend of mine and
I a couple of state of the art (at the time) Performas for originally
introducing him to this new-fangled thing called "the World Wide Web."
I HAVE run across this problem on a vintage 6500 before. Yes, this
would be classified as a "vintage" computer as computer years are like
dog years, i.e., 1 computer year is 7 years of "real" time.
After verifying the monitor was fine, we replaced our monitor out card
and also had the same problem remain. Our conclusion was that either
we got another bad video card (we bought it second hand as a "pull" on
eBay with no guarantees) or there was a logic board problem. Though
we're still not absolutely, positively sure that the second card was
bad, we eventually (and sadly) set aside "the beast" for parts.
For more information, you may want to refer to the actual service
manual for this machine. The best site I know for service manuals for
Macs (personal knowledge) is at:
The specific Service Manual for PowerMac 6400, 6500, Performa 6400,
6410, 6420 is at:
This is the manual used by a Mac hardware specialist in order to solve
problems, upgrade, etc. It has troubleshooting charts, exploded
views, guides on how to take apart things, etc. It is surprisingly
easy to use for the non-technically savvy, and it is awesome.
In the 6500 Service Manual see:
There are several comments regarding video problems that refer to
problems with the logic board (ouch!). Also some of the
troubleshooting hints refer to zapping the PRAM (I can't remember how
to do it without consulting one of my old Mac Bibles or PowerMac
Secrets books - if you need that information just ask and I'll get it
for you if it exists in my print sources).
The general troubleshooting for NO video says:
1 Check monitor connection.
2 Try known-good monitor.
3 Replace monitor-out card.
4 Remove all DRAM DIMMs and try replacing them one at a time
to test. Replace any bad DIMMs.
5 Replace logic board. Retain customers DIMMs.
6 Replace power supply.
These would thus be all the places to look for fixes and indicates
that video problems can be affected by memory chip problems and the
logic board. I still can't figure out why "Retain customer's DIMMs" is
there as it is in almost all of these Mac service manuals. Tech
If you can locate a known working monitor-out card and swap it in to
your machine, that would definitively tell you if it's the logic board
or if the second card is bad also. If it's not the card, 90 percent
chance the logic board is hosed. I KNOW it's unlikely you can find
someone who just happens to have a 6500 who would lend you a good
video card, but that's the problem with "vintage" computers!
The 6500 service manual also refers to an additional section for video
problems you may want to check out:
6500 Distinguishing Video Jitter ...
Though excellent, such manuals can never detail every problem that
occurs, but this is the best approach I've found yet.
It would be nice to say something like "just turn the little screw
beneath the case 1/4 turn to the left and it'll work perfectly," but
that's unlikely. If you download the service manual (it's about 5 mb
of PDF file) you may find some other possibilities as well.
Good luck and Apple rules!