Re: Problem Using a Commodore 1084s Monitor on a IIgs

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

Polymorph wrote:
> On Jan 25, 9:43 am, Polymorph <polymorp...@hotmail.com> wrote:
>
> > On Jan 25, 12:02 am, "Jeff Blakeney"
> >
> > <jeff.blake...@a2central.com.remove-sd5-this> wrote:
> >
> > > Sorry I haven't jumped in sooner but I fell behind on my newsgroup
> > > reading this week.  If I remember correctly, I had added the 75 Ohm
> > > resistor to try to match the termination expected by the1084S.  After
> > > posting about my wiring someone else mentioned that they either didn't
> > > need it for theirs or didn't think it was required and sure enough, my
> > > current IIgs1084Smonitor cable no longer has a 75 Ohm resistor on it
> > > and it works fine.
>
>
> I've been really busy the last few weeks and had put this project on
> hold, however, I finally got around to trying Jeff's suggestion from
> 1995 and have put a 75 Ohm resistor between composite synch and ground
> and he was definately onto something. Instead of seeing a fast
> scrolling mess, it was possible to fine tune the vertical hold to
> produce the pictures below:
>
> http://apple2.sytes.net/DSC00042.JPG
> http://apple2.sytes.net/DSC00046.JPG

So it appears that the IIgs was overdriving the 1084's sync input.

> Now, as you can see from the above images, it doesn't appear to be
> fully synched. All the characters are leaning to the right (like
> everything is in italics), and you can see the black area on the RHS
> of the screen which is wobbling around a bit.

This means that the horizontal sync is not quite right, either.
You could adjust the horizontal sync on the monitor to get it
closer, or....

> I'm thinking that in my case, 75 Ohms is not quite right, but
> something in the vacinity would be right. Does this sound plausible?
> Do you recommend that I buy a variable resistor and try and find the
> optimum value? Or should I being trying something else.

Exactly.  Try lowering the value (or, better, adding a 75 to 150 Ohm
pot with the IIgs driving the "high" end and the output to the monitor
coming from the center terminal) until you can find where the sync
drops out because the level is too *low*.  Then increase it until good
lock is achieved.

You may still find it worthwhile to adjust the vertical and horizontal
hold controls to the middle of their stable ranges.  It seems that
this monitor's settings may have drifted a bit.

> With the results I've achieved thus far, it would seem that this is
> going to work with a bit more messing around.
>
> What would you guys recommend as the next course of action? All
> comments welcome.

You're almost there!

-michael

NadaPong: Network game demo for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

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