[Coco] New thread, cga-rgb->vga convertor GBS-8220

[gene heskett]

On Wednesday, July 27, 2011 09:48:52 PM Mark Marlette did opine:

> Gene,
> 
> There is an adjustment pot that will allow your display to be cleaned up
> with Roy's.
> 
Several in fact, BTDT.

> Just not sure which one it is ATM....Roy?
> 
> Quartus 11.0 is currently working overtime........ :)

Not here.  It will not install, my libxml2.s0 is missing the gzdirect call 
and the installer bails with a lot of whining.  I may yet have to build the 
thing myself.
 
> Regards,
> 
> Mark
> Cloud-9
> 
> 
> 
> ----- Original Message -----
> From: "gene heskett" <gheskett at wdtv.com>
> To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
> Sent: Wednesday, July 27, 2011 3:40:56 PM
> Subject: [Coco] New thread, cga-rgb->vga convertor GBS-8220
> 
> Hi folks;
> 
> I pinged Tim again, and got an answer this time, fairly quickly.
> 
> He said that one user had used a resistor pair, of 100 and 1000 ohms as
> an attenuator, and that this user had tied the H&V syncs together, then
> the 1k from that point to the S input on the left hand 5 post input,
> with the 100 ohm resistor from the S pin to ground.
> 
> But that could be pretty hard on the driving gates, having 2 outputs
> which may be pulling in opposite directions shorted together.
> 
> So I made a combiner with a resistor of 1800 ohms, connected to the near
> end of R6 on the coco's motherboard via a small drop of silver bearing
> solder and butting the side of a shortened end lead against R6 to pick
> up a 5 volt source.
> 
> I connected the other end of this 1800 ohm resistor via a wire to the S
> pin of the P3 header.  This then is the pullup resistor because the
> next piece blocks the logic 1's in between the inverted sync pulses.
> 
> Then I had already added a piggy-backed 74LS04 onto IC15, with its pins
> bent out flat, hooking the HSync output of IC15 (see the schematic) to
> the input pin of the same gate in the added chip, doing the same for
> the VSync signal also.  So I had right side up Vsync on the added chips
> pin 13, and right side up HSync on the added chips pin 11.
> 
> Then the outputs of those two gates were connected via a 1n914 for each
> signal, to the 1800 ohm resistor, with the cathode end of the diode
> facing to the added chips output pin 11 and 13, and the anode end of
> both diodes connected to this pullup resistor and the S pin.  Each sync
> signal can then pull the line feeding the S pin of P3, down to the
> total of the gates VSat, about 80 to 100 millivolts, plus the forward
> conduction voltage of the 1n914, somewhere in the .65 volts range. 
> Schotkey diodes would help, but this worked a buck a diode cheaper.
> 
> So I now have a well combined sync signal, sitting at about 1.5 volts in
> between sync pulses, and is pulled down to about .78 volts during the
> active time of either sync signal.
> 
> This works very well but did take some fiddling with the menus to
> optimize the display, which I found the 640x480 setting the easiest to
> read on my aging touch screen Elo vga monitor.  Its tube is getting a
> bit soft with old age & hours as it sat face up in our weathermans
> composition desk, running at enough brightness to be usable when 300
> ft/candles worth of studio lights were on.  For several years.
> 
> Booting from the HD, even multivue ran albeit on its somewhat smaller
> area screen. And only recognized one mouse (left) button on my seriel
> mouse.  I can recall when all 3 were usable for the icon editor.
> 
> I received the rest of the chips I had ordered from Jameco today, after
> doing this of course (there is a Murphy's Law about such things) and its
> possible that a single chip adder solution can also be done.  Future
> project, would only need one gate of a 74LS00.  ;-)
> 
> The image I am seeing is not quite as noisy as Roy's adapter gives, but
> if you have one of those, I do not see a $50 improvement in the picture
> from my chair.  I do not know what Roy's situation is now, but frankly
> I'd druther see the money go to someone 'in' our community if you need
> something like this.
> 
> I more than likely made more diff in my picture by putting a real,
> honest to goodness ferrite bead across the 2.2 ohm resistor my
> motherboard has in the "FB5" position, which gives my gime nearly 200
> millivolts more Vcc voltage to play with, and my rise and fall times
> are quite noticeably improved now.
> 
> Cheers, gene


Cheers, gene
-- 
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