[Coco] video composite on korean coco 26-3134A

[John W. Linville]

NTSC CoCo2's use a modulator that does _not_ take a composite video
input.

The 6847 outputs signals called Y, phiA, and phiB.  These are (very)
roughly analogous to the YIQ signals that form a composite video signal.
A Motorola 1372 chip is used to combine those signals into something
useful.  The 1372 chip can produce either a baseband composite video
signal or (by use of an RF tank circuit) a TV-compatible radio signal.

On the Coco1, the 1372 only produced a composite video signal that
was then fed (along with the audio signal) to an off the shelf RF
modulator.  On the CoCo2, the 1372 chip is only included as part of
the RF modulator itself.  The Y, phiA, and phiB signals go straight to
the modulator where the RF-equipped 1372 turns them into a TV signal.

I'm not sure if there is a pristine composite video signal available
anywhere inside the RF modulator on a CoCo2.  I was never able to
find one, FWIW.  You probably need to pull the modulator and build
a new 1372-based circuit to replace it.

Good luck!

John

P.S.  It sounds like the PAL CoCo2 might be designed more like the
CoCo1 in this regard.  Of course, that only helps you if you are
using a PAL CoCo2... :-)

On Thu, Oct 02, 2014 at 09:56:31PM +0200, didier derny wrote:
> Hi,
> 
> I tried but no success...
> 
> I don’t have any image on tv (connected to the antenna)
> No idea if the coco 2 is broken or if my tv is unable to process the signal
> 
> With the oscilloscope I've not been a able to find a composite signal 
> Inside the modulator...
> 
> Any idea ?
> 
> Thanks
> 
> 
> --
> didier
> 
> -----Message d'origine-----
> De : Coco [mailto:coco-bounces at maltedmedia.com] De la part de Simon Jonassen
> Envoyé : samedi 27 septembre 2014 16:26
> À : 'CoCoList for Color Computer Enthusiasts'
> Objet : Re: [Coco] video composite on korean coco 26-3134A
> 
> Hey....
> 
> I did mine by tapping into the feeds that go directly to the rf
> modulator....
> 
> A couple of shielded wires soldered on to the pins and 2 rca jacks on the
> rear of the case (audio/video)
> 
> Works fine... No extra components needed (PAL machine)... 
> 
> /Simon :-)
> 
> 
> -----Oprindelig meddelelse-----
> Fra: Coco [mailto:coco-bounces at maltedmedia.com] På vegne af didier derny
> Sendt: 27. september 2014 13:23
> Til: 'CoCoList for Color Computer Enthusiasts'
> Emne: [Coco] video composite on korean coco 26-3134A
> 
> Hi
> 
> Is there an easy way to get a video composite signal on this machine ?
> 
> I guess a video composite signal or something approaching should be found
> Somewhere near the modulator or inside the modulator ?
> 
> 
> Thanks
> 
> 
> 
> 
> 
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-- 
John W. Linville		Someday the world will need a hero, and you
linville at tuxdriver.com			might be all we have.  Be ready.