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

[sadolph]

James, thanks for the information.  I've gotten some of this right so far-- 
(the datasheet is pretty good for this part)

-  pins 13/14 are biased up from 5V with a diode drop to separate them
-  pin 12 has only 1kohm to +5V right now (i took out my 7.5V supply..not
needed as you point out).  pin 12 is taking about 13 mA, which if biased
through 3900 ohms would need much more V than is there-- should I change to
3900 ohms?  This would seem too high, and would bias down to near ground if
sourced from +5V--unless output current will drop accordingly.
- I'm using the emitter follower amp from Neil's site to buffer the
composite signal out to the RCA jack.
- the original audio circuit I assume was modulating the audio and mixing it
in according to TV audio channel specs.  I changed that circuit to be a
simple emitter follower essentially the same as what is used in Coco3.

It's working not too bad as it is, but I think the levels and/or bias are
off.  I don't have a scope to check it out.  I know that after the 22 ohm
resistor, the voltage level is around 2.2 VDC.

If you have time for more advice on how to deal with the pin 12 resistance
that would be great!


thanks, Steve


----- Original Message ----- 
From: <jdaggett at gate.net>
To: "sadolph" <sadolph at magma.ca>
Sent: Sunday, March 07, 2004 6:49 PM
Subject: Re: [Coco] [Color Computer] composite video conversion for Coco2


> 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 so here is my second reply to
your
> problem. 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
>
> first message.
> Steve
>
> Is there circuitry connected to pin #13 and #14 of the MC1372?
>
> If so then what you are getting out of Neil's circuit is double sideband
amplitude
> modualted carrier at either channel 3 or 4. You are going to need to do
some
> modifications to the MC1372 to generate solely composite bassband video.
>
> 1) remove all components that is connected to pins #13 and 14.
> 2) from Pin#14 to pin#13 wire in a 1N4001 diode or equivalent with the
anode
> end to pin #14 and cathode end to pin #13.
> 3) wire in a 360 ohm resistor from pin #14 to +5 VDC
> 4) wire a 3.9K ohm resitor from pin #12 to + 5VDC
> 5) wire in Neil's circuit.
>
> 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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