[Coco] [Color Computer] composite video conversion for Coco2

[jdaggett at gate.net]

Steve

Not sure if you got my reply, but you had to disable the oscillator section of the 
MC1372 and pullup pin #12. I think 5VDC will do. Not sure if you erally need 
7.5VDC. The buffer stage following needs proper bias set to give proper levels for 
the particular monitor. Usually 1 V peak to peak in the proper polarity. 

Pin#12 of the MC1372 likes to see a load impedance of about 3900 ohms.

Somehow my emails to the Yahoo group bounce. You have to power pin #13 and 
pin#14 as they are open collector to a diferential pair that forms teh RF oscillator. 
Pin #13 connects internally to the base of a transistor that supplies bias and 
operating point for a dual diferential pair that is the RF modulator. One of the 
collectors of this dual differential pair has an open collector that is tied to pin#12. 
What is happens inthis configuration is that rf modulator is no longer functioning as 
modulator but as a single ended ouput differential amplifier.  

The buffer amp can be a video amp like Neil's circuit or even more sophisticated 
like the video amp NE572. Even some wide band op amps will work if they are kept 
to low gain, say between 2x and 10x. 

james

On 7 Mar 2004 at 17:06, sadolph wrote:

> 
> Well, I must say that was a difficult conversion. Basically, you have
> to rework the MC1372 chip to display composite video rather than act
> as an RF modulator. Also, needed to add an audio buffer like what is
> used in Coco3. Also, needed to add a 7.5 V power supply..
> 
> If anyone is interested to know how this was done, I am considering
> writing it up.
> 
> cheers, steve
> 
> 
> 
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