Re: IIgs Analog Video Circuitry

[Michael J. Mahon <mjmahon@aol.com>]

Kevin Dady <kevin@hackaday.com> wrote:
> On Sep 28, 10:31 am, Daniel Kruszyna <d...@krue.net> wrote:
>> MdntTrain <j...@cimmeri.com> wrote:
>>> I don't have my schematic handy, but two types of chips (post the VGC)
>>> contribute to the rear port signals: there is the RGB-> NTSC converter
>>> chip around there (I believe it's the MC1377 or similar), and then
>>> there's the video D->A converters (DACs).  I'm not sure whether the
>>> DACs are off shelf.
>> 
>> Each 4 bit component of the VGC output is first connected to a smaller SIP,
>> which I believe is an R/2R ladder DAC. From memory, the DACs are marked H2, H3,
>> and H4. It's the analog outputs of these three DACs which are then input to the
>> module in my original question. Apologies for not providing this detail
>> earlier.
>> 
>> -- Daniel
> 
> its probably some form of summer, you generally cant take a bunch of
> voltages and simply wire them together and expect them to work
> (correctly), and yes if its a epoxy blob sip IC chances are its
> custom, even if its not good luck finding it unless its a scavenged /
> nos part...
> 
> Is yours broken?

In that case, it's likely to be a resistor network that mixes (or
"matrixes") R, G, and B into I (in-phase), Q (quadrature), and Y
(luminance) signals prior to the modulator chip that converts them to NTSC
color. 

-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon