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Re: IIgs Monitor Repair
Jerry wrote:
Michael J. Mahon wrote:
Jerry wrote:
I recently bought a Benq FP241W LCD monitor since my old Dell 19" CRT was
dying. This new monitor has all sorts of inputs, including an NTSC RCA
jack, plus individual R, G, and B RCA inputs. I haven't tried my IIgs on
it yet, but an Apple IIc works quite well. There is a very slow roll to
the image, but it's quite usable even in 80-column mode.
What, exactly, is a "very slow roll to the image."
Does the image position change on the monitor? Does a small "ripple"
move through it? I'd like to know more.
I didn't describe it well. It's a very slow ripple in which the picture is
every so slightly shifted to the left or the right, I cannot remember which
direction. It's borderline noticeable; I was looking for such things.
I'll try to take a picture of it soon. When I do, I will let the group
know.
I presume that the "ripple" is horizontal and moves slowly from top to
bottom or bottom to top.
This is most likely a beat between the power line frequency and the
vertical sync frequency of the Apple II. This would not be an issue
unless there is 60Hz power line noise getting into the signal or the
sync circuits of the monitor, causing a slight disruption of the phase
of detected horizontal sync.
You might try plugging both the monitor and the Apple into the same
electrical circuit (for example, on a power strip). You may also
want to check the tightness of the RCA plug outer connector on both
ends of the video cable, or try another cable.
If this does not fix the problem, it may result from power supply ripple
in either the Apple II or the monitor. If the line side rectifier is
half-wave, the ripple frequency would be 60Hz, causing a single area
of distortion per frame. If a full-wave rectifier, then there should
be two areas of distortion per frame, moving together up or down the
raster.
Ironically, the reason that 60Hz was picked for the vertical scan rate
of NTSC (and 50Hz for most PAL systems) was to allow the TV sync to be
locked to the local AC power frequency, causing distortions of the type
you observe to be immobile, and therefore less noticeable. Of course,
this became less useful as nationwide networks became prevalent, while
there is still no nationwide synchronization of several power grids in
the US.
With CRT displays, the biggest likely source of line frequency
interference was the stray magnetic field of the TV's power transformer
or other nearby appliances, but that cause is irrelevant today. ;-)
-michael
NadaPong: Network game demo for Apple II computers!
Home page: http://members.aol.com/MJMahon/
"The wastebasket is our most important design
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