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Re: Problem Using a Commodore 1084s Monitor on a IIgs
Polymorph wrote:
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
That looks like it--and it's in the right neighborhood on the
PC board.
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.
Great. If you don't have a plastic adjustment tool, be careful. ;-)
<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
Either one would work fine. (The first is a little better enclosed.)
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.
Since most voltmeters will produce an average reading of a varying
waveform, and the open-circuit IIgs output is ~3.8v, that suggests
that the IIgs is outputting TTL level (5vpp) negative sync--meaning
that the sync pulses are negative-going from about ~+5v.
Adding the 75 Ohm resistor is not "terminating" the signal, but
simply attenuating it--in this case to about 0.7vpp (coincidentally
ideal for the monitor ;-).
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?
P-P stands for peak-to-peak.
You are currently supplying just about that level to the sync input.
Nothing states the expected polarity of the sync (or the video, for that
matter), so the conventional polarity is positive for video and negative
for sync. We can hope that that's true for this monitor. (Since others
have had good luck with it connected to a IIgs without using an
inverter, this seems a reasonable assumption.)
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?
I come to the same conclusion.
I think that a little higher sync level than your fixed resistor
combined with an adjustment of the horizontal hold will put things
right.
As always, thanks heaps for your time.
You're welcome--let's get it working for you!
-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."