[Coco] SVGA monitor

Steven Hirsch snhirsch at gmail.com
Sun Oct 25 16:09:44 EDT 2009

On Sat, 24 Oct 2009, Steven Hirsch wrote:

> On Sat, 24 Oct 2009, RJRTTY at aol.com wrote:
>>> I have questions about that.  I bought one of these boards on eBay and
>> the
>>> unit I received is behaving very poorly.  It resets  randomly and will not
>>> display the CoCo output.
>>> Color me  unimpressed.
>> It is likely the output of the coco3 is overdriving the input of
>> your converter.    I have run into this problem with  other
>> commercial converters.    Try putting a 75 ohm  resistor
>> in series in each of the RGB lines and see if that helps.
> That's possible, but I think the real issue is that this unit doesn't support 
> 15.75Khz. RGB with separate V & H sync.  When I connect my Amiga using the 
> composite sync output it seems fine.

And that turned out to the be problem!  I breadboarded a simple H+V --> 
Composite TTL adapter and presto...  Nice stable display from the CoCo :-)

The circuit I used was found at:


There are two variants shown there; one uses a pair of discrete 
transistors to drive a true 75-ohm input and the other provides standard 
+5V TTL.  The latter of these worked fine for me since the converter is 
not terminated.

Parts complement is an SN7486 quad xor-gate, two 2k2 resistors and two 
22ufd/10v electrolytic caps.  It's sitting on my trusty Heathkit 
proto-board at the moment, but I'll end up building it on perfboard and 
mounting the entire getup in a little project box.

I'm going to suggest that the manufacturer support 15Khz. RGB w/ separate 
sync, but who knows if they'll care enough to listen.



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