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

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

Polymorph wrote:
>  To: Michael J. Mahon
> Michael J. Mahon wrote:
>
> > Never worry about this.  If the store is concerned about their
> > transaction costs (which are almost certainly a few tens of cents)
> > then they should just give your the part!  (And some will if they
> > know you.  ;-)
>
>
> OK, but it still feels kinda weird.

And, of course, they wouldn't want to do a credit transaction.  ;-)

Another consideration is *your* transaction costs, which can be
considerable if you have to make a special trip.  But "spending
money to save money" is usually a dubious approach.  ;-)

One thing you might consider, if you have any interest in doing
electronic repairs or experiments in the future, is getting a "kit"
of 200 or so 5% 1/4W resistors with a half-dozen or so of all the
common values.  Such kits are usually about $5, so you are getting
a significant quantity discount, and you'll be prepared for just
about any small resistor needs.  If you ever need another resistor,
the saving in time, trouble, and fuel is easily worth $5.  ;-)

> > The 200 Ohm pot will provide you with complete adjustment flexibility,
> > corresponding to *all* values of resistor, so you don't need another
> > fixed resistor for this.
>
>
> OK, I'll stick with just the 200 Ohm pot then.
>
> > Specifying a video peak-to-peak level of 0.7v and a sync peak-to-peak
> > level of 0.3v corresponds to the standard composite video+sync level
> > of 1v (of which 0.7v is video going positive from black level and 0.3v
> > is sync going negative from black level).
> >
> > It would not be unusual for the internals of the monitor to accept
> > composite sync at the same level and polarity at which it would be
> > found in a composite video signal.
>
>
> Right, that makes sense to me.
>
> > > > I don't have this system connected at this time, sorry. However 
> > > > using the 200 ohm pot as was suggested here should do the trick for 
> > > > you.
> >
> >  >>
> >
> > > No worries. Its good to hear of someone who experienced a similar 
> > > problem as myself using the 1084S with the IIgs. I wonder why most 
> > > people don't have a problem? Did later revisions of the 1084 line 
> > > come with greater tolerance to sync levels? It has me stumped how 
> > > come some 1084 monitors require modification of the sync signal and 
> > > others don't.
> >
> >
> > The sync level tolerance is strongly dependent on the input circuitry
> > of the monitor, and a very minor change would make it quite insensitive
> > to sync level.  It would not be surprising if this were the case in
> > some of the monitors (which may have different manufacturers).
>
>
> I'm just surprised that I have had to drop the sync level to such a low 
> value though when others have not. I mean its gone from ~3.87V to 
> ~0.56V, and it seems that it is still too high! This seems like a pretty 
> big difference. I'm still amazed that later revisions of the 1084 can 
> handle the IIgs' comp sync signal unmodified. It would be interesting to 
> know what they changed to allow for this to be possible (as I'm sure 
> they didn't have the IIgs in mind when making the 1084 monitors ;-).

Adding an input buffer or Schmidt trigger would make the input quite
level-insensitive.  This would add about $0.25 to the cost of the
monitor, but would broaden its specs enough that a single design could
be used in multiple applications without adjustements--a potential win.

-michael

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

"The wastebasket is our most important design
tool--and it's seriously underused."