Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!hellgate.utah.edu!helios.ee.lbl.gov!nosc!crash!simpact!jeh
From: jeh@simpact.com
Newsgroups: comp.sys.ibm.pc
Subject: Re: SCARY Monitor problem
Message-ID: <727.25503e21@simpact.com>
Date: 2 Nov 89 20:16:17 GMT
References: <21739@gryphon.COM>
Organization: Simpact Associates, San Diego CA
Lines: 33

In article <21739@gryphon.COM>, lampi@pnet02.gryphon.com (Michael Lampi) writes:
> Wavy monitors tend to be caused by changing magnetic fields near the screen...

Yup.  I recently bought a nice new VGA-plus monitor, checked it and the new
VGA-plus card out at the store, and brought it home.  Worked fine.  Then it 
started to waver... and then it stopped... repeat, ad nauseum (literally).  

Drove me nuts until I happened to make a phone call from an old-style WE 2500
desk phone (which has many coils in a network in the base), which happened to
be newly perched on a shelf near the monitor... and heard AC hum.  It turns out
that the master power cables for the building (a four-unit townhouse) come
straight down through that particular wall, right behind the monitor (judging
by the hum)!  The hum in the phone and the waver in the screen came and went
with varying AC loads in the building.  Moving the phone away from the wall
stopped the hum, and moving the monitor away from the wall stopped the waver. 

I traced a similar problem, also in a VGA-plus system, to ground loops.  There
was a difference of a few tenths of a volt between the chassis ground
potentials of the monitor and the computer (the latter as seen at the end of
the monitor video cable).  This of course caused an current flow in the shield
braid of the monitor cable, and may also have imposed a 60 Hz signal atop all
of the video and sync signals, exactly what they don't need.  Plugging the
monitor into the "monitor power" receptacle on the computer (instead of the
wall socket) helped some. I then got into the video cable and disconnected the
shield from the connector shell at the monitor end, and the waver disappeared
completely. 

I'm sure looking forward to the day when we can get rid of that last vacuum
tube!  

	--- Jamie Hanrahan, Simpact Associates, San Diego CA
Internet:  jeh@simpact.com, or if that fails, jeh@crash.cts.com
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