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

["Polymorph" <polymorph@a2central.com.remove-lmc-this>]

 To: Michael J. Mahon
Michael J. Mahon wrote:
> > Michael, as per all of the IIgs-->1084 FAQs suggest, I've only
> > connected the composite sync from the IIgs to the composite sync of
> > the 1084. Also note that in the 1084 the composite sync and horizontal
> > sync are tied together (the 1084S-D schematic I found confirms this).
> > I have left the vertical sync not connected as per the Apple II FAQ.
>
> I wasn't referring to the horizontal sync input, but to the horizontal
> sync (or "hold") adjustment on the monitor.  This may be internal, or
> accessible only through use of an "alignment tool" through a hole in
> the back or bottom of the case.  Ideally, it would be an external
> control like the vertical hold control.

OK, understood. After considerable snooping around on the circuit board, 
I think I've found the horizontal hold control. It's internal, and not 
very obvious to find - it's well away from all of the other internal 
adjustment controls and is significantly smaller. Here is a picture:

http://apple2.sytes.net/DSC00022.jpg

As you can see, I will have to slide out the main circuit board to 
adjust this - definitely not the easiest of adjustments, however the 
circuit board is free to slide out, so it shouldn't be too hard.


<snip my ramblings>

> A 200 Ohm pot is perfect.  Actually, I hope it's a carbon pot, not
> a wirewound pot.  Low resistance pots may be wirewound, and the
> inductance of the winding can distort high frequency pulse waveforms.

How about one of these:
http://www.jaycar.com.au/productView.asp?ID=RT4350

OR

http://www.jaycar.com.au/productView.asp?ID=RT4304

> In my "better" scenario, the pot is connected like a volume control
> between the IIgs composite sync output and the monitor sync input.
> A pot is a 3-terminal device--a resistor with a sliding tap.  The
> two outside terminals are the "ends" of the resistor, and the middle
> terminal is the sliding tap.
>
> You would connect the counterclockwise terminal (the "end" terminal
> nearest the sliding tap when the pot is adjusted fully counterclockwise)
> to ground (common to both the monitor and the IIgs).  The other "end"
> terminal (the clockwise terminal) would connect to the IIgs composite
> sync output.  The middle terminal (the sliding tap) would connect to
> the monitor sync input.
>
> Then you can adjust the sync input to the monitor from zero (fully
> counterclockwise) to maximum (fully clockwise).

Thanks Michael, that explanation cleared it up nicely.

> > The vertical hold was very difficult to adjust to get the image to
> > stop rolling - i.e. there is a *very* small stable range for the
> > vertical hold. But it was possible, as the images I posted previously
> > confirm. I am guessing with a different resistor value this problem
> > will rectify itself. This problem with vertical hold was only present
> > using the RGB port however, when I hooked a DVD player up to it via
> > the composite ports I had no such problem.
>
> Interesting.  I wonder what the sync output spec on the IIgs is?
> Could it be TTL levels: 5v peak-to-peak?  If that's the case, no
> wonder the level needs to be reduced, since most monitors would
> expect 1v or less peak-to-peak.

Well, I just tested the voltage on my IIgs' composite sync output, and 
it measured ~3.87 Volts. With the 75 Ohm resistor wired up between 
composite sync and ground (Jeff's suggestion), the voltage was pulled 
down to ~0.56 Volts.

I dug up some info on the 1084 monitors here:
http://www.amiga-hardware.com/showhardware.cgi?HARDID=849

and here:
http://kmt.hku.nl/~pieter/HARD/MON/commodore1084.html

I'm not sure, but those sites seem to suggest that the input should be 
0.7 P-P. What does P-P stand for? I assume 0.7 is the voltage level?

> Hopefully, you will find a setting (guessing around 20%) that will
> sync stably for both horizontal and vertical scan.
>
> There is a possibility that the monitor expects the sync signal to
> be opposite in polarity to what the IIgs outputs.  If that is the
> case, an inverter before the pot will straighten things out.
>
> Needless to say, some definitive specs for the sync signal that
> the IIgs produces and what the monitor expects would answer all
> these questions.

As there are so many people successfully using 1084 monitors on a IIgs, 
and nobody has mentioned the need for an inverter, I would guess that 
this is not the case - but I could be wrong. I wouldn't think that even 
if certain models were more tolerant of sync voltages, that they would 
invert the signals for a particular model, as it would break 
compatibility wouldn't it?

As always, thanks heaps for your time.

Cheers,
Mike
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