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

[mwillegal <mike@willegal.net>]

On Apr 19, 1:46 am, "Michael J. Mahon" <mjma...@aol.com> wrote:
> 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
>
> "The wastebasket is our most important design
> tool--and it's seriously underused."

Hi Michael,

I'll have to take a look again with the composite video cable
disconnected, but there was some noise on the composite signal on my
test set up.  It appears during the final stage on the output of the
2n3904.  It's actually quite amazing how much poorer the signal
looked, after passing through this one transistor.   Perhaps the
composite video signal is not well terminated on the TV I was using.

I tried another fix that also worked with this one set up that I could
see the problem with.  That was to add a small amount of capacitance
(I used about 35pF) between the output of the 2n3904 and ground.  My
opinion is that this worked by dampening the noise enough that it
didn't interfere with horizontal lock.

I didn't try it myself, but I know that for another person, the
partial fix of adding horizontal serrations to the vertical sync
didn't work by itself.   In addition to adding horizontal serrations,
the fix that Apple introduced on the rev 1 motherboard also reduced
the vertical sync from 8 cycles to 4 cycles.

Many TVs and monitors do not have horizontal lock controls, including
the TV that I saw the problem with.

Regards,
Mike Willegal