Re: IIgs RGB to VGA experiment

["js@cimmeri.com" <js@cimmeri.com>]

Ok.... more news.

I managed to get the converter working fairly well with its J01 RGBS
input by changing the power supply to a lab grade adjustable one, and
adjusting the voltage very carefully.

The picture is steady, 80char text is quite legible (but not 100%
perfect), colors are fairly good though with some visible noise
patterns on some shades, and contrast is almost as good as my IIgs RGB.
 The results are not perfect, but are the best I've been able to get
having only tested several composite-> vga converters in the past
(which I found completely unacceptable).

I have yet to compare with the Commodore 1084 monitor used in direct
RGB mode, so will do that soon.

At 5V, the board did not put out a good picture on my Multisync...
somewhat shaky as noted in my previous email.   But by lowerering the
voltage to somewhere around 4.1v, there's a "magic spot" where suddenly
the board gets very stable.

Since I replicated the shimmy with the lab adjustable supply, I think
my first power supply (regulated linear 5v supply) is fine, but just
not putting out the magic voltage number.   I suppose a more permanent
install could tap 5V off the IIgs from somewhere (anyone know where a
good spot is?), and since the wattage is fairly low (the board draws
.31A at ~4.1V, one could just rig up a variable resistor between the
tap and board.

Now, the problem still exists that the converter is going from
15.734khz -> 30.9khz... the Multisync reports 31khz x 60hz.   I think
we would much rather have 31.5khz so as to be compatible with ALL VGA
monitors -- not the select few that can sync down to 31khz.

My next test will be to try using the J02 input -- which is the RGBHV
one -- because of it's advertised much greater flexibility (should do
15.75khz->31.5khz).  First, I will see if it will sync up only using
the H line with the IIgs' composite sync... as many monitors do.  If
that doesn't work, I'll have to build a gizmo using the LM3881N chip to
give me the pseudo HV signals to feed the input.  I think this small
chip can easily be piggybacked or flown over the converter board
somewhere.

Thank you all -- Michael, David, Roger -- for your very supportive,
ongoing comments.