wavy display on a 3B1.

[John B. Milton]

In article <169 at cjsa.WA.COM> jeff at cjsa.WA.COM (Jeffery Small) writes:
>This is a request for help from some of you UNIX-PC hardware gurus out there.
>
>I have three 3B1 machines which were recently relocated to a new site.  On two
>of these machines, I am now experiencing "screen wave" - a distortion in the
>display which looks like a wave running across the screen.  The frequency
>which I observe this condition has been increasing and now I experience this
>every few minutes typically.

This does not sound like the dirty pots problem, and no, I'm not talking about
bathroom hygiene. I'm talking about the variable resistors in the monitor.
If you take the back off the monitor (two screws), they're on the PC board
poking out the back. The symptom that goes alon with these is "jumping", not
"waving". The procedure for the jumpies is to turn each pot back and forth
a couple of times, then BACK TO WHERE IT WAS. This cleans off oxides that
build up on wiper. DANGER!! WARNING!! There is 14,000 volts in there!

It may sound stupid, but
Wavies are usually caused by the screen refresh rate being out of sync with
the 60Hz line and the power supply being weak. If this is what's going on,
then the rate of wavies will change over the day as the line frequency shifts.

For those of you who don't know: during high load periods on the power grid,
the line frequency drops below 60 Hz by a little bit. This means that all
clocks based on the 60Hz line run slow during the day. To fix this, the
electric utilties run the grid a little over 60 Hz at night, so you clocks
catch up and do not loose time. They run a little slow during the day, a little
fast at night. Since the wavies are caused by slight difference in the screen
refresh rate and the grid, the rate of the wave being the difference, this rate
will be different at different times of the day.

If that's the problem.

A related problem (stupid as it sounds) could be the lighting. If the systems
were moved into a flourecent lit environment, the same reason I just described
could be cause the screen to APPEAR to flicker. I wonder if the one system that
does not have the wavies is facing a window?

Hmm.

John
-- 
John Bly Milton IV, jbm at uncle.UUCP, n8emr!uncle!jbm at osu-cis.cis.ohio-state.edu
(614) h:294-4823, w:764-2933;  Got any good 74LS503 circuits?