Re: new solution for old Apple II rev 0 motherboard issue

["Michael J. Mahon" <mjmahon@aol.com>]

mwillegal wrote:
> For what it is worth, here are a couple of new fixes for a 32 year old
> hardware problem. :-)
>
> http://www.willegal.net/appleii/applesync.htm

The second fix is:

"I found that there is a very simple hack that can also be helpful if 
you experience this problem.  The video signal seems to have a lot of 
noise in it and I believe the distortion is due to the horizontal sync 
lock of your monitor attempting to lock onto this noise.  Solder one 
side of a 10K pot to each side of the 2K resistor in the video driver 
circuit.  Try adjusting the pot, which reduces resistance, to see if you 
can make the distortion disappear.  Reducing this resistance raises the 
sync signal higher above the noise floor.  Also note that it also causes 
your composite video output signal to move further from NTSC standard 
signal levels.  This may degrade the video signal, if carried too far."

The video signal is generated digitally, and should not have any "noise"
on the black level or the sync.

What this mod allows is altering the black level and the amplitude of
the sync.  This will certainly help some monitors.

But a *much* simpler solution is to adjust the monitor's horizontal hold
control to reduce or eliminate the "flagging" at the top of the raster
which results from the absence of horizontal "serrations" in the
vertical sync pulse on rev 0 Apple II's.

This could lead to some flagging on a normal TV signal, but for a
monitor dedicated to an Apple II, who cares?

BTW, some monitors horizontal oscillators drift with temperature
variations (a result of poor design or leaky timing capacitors).
In these cases, it may be necessary to readjust the horizontal hold
as the monitor warms up to prevent the flagging.  This could become
sufficiently annoying to motivate one to apply "fix #1", which is,
of course, the preferred fix.

-michael

NadaNet and AppleCrate II: parallel computing for Apple II computers!
Home page: http://home.comcast.net/~mjmahon

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