[Coco] CoCo 1 composite

[Gene Heskett]

On Thursday 04 January 2018 12:10:04 Richard Lorbieski wrote:

> I found a source for the RF modulators:
>
> http://leftturnonly.info
>
> I order a couple to restore a couple of CoCo1s.
>
> -----Original Message-----
>
> With the recent post asking about CoCo 1 composite video, I decided to
> build the circuit shown on Hack-a-Day. I have a CoCo 1 with RF video
> that doesn't work.
>
> I'm pretty certain I built it correctly, alas, I wasn't able to get
> any video.
>
> So either, I built it wrong, the circuit has an issue, a part(s) is
> bad or the CoCo I'm working on has some issue, hence the RF problem. I
> did try swapping out the MC1372.
>
> Has anyone built this circuit?
> https://hackaday.io/project/7613-coco-1-composite-video-output

This circuit would work as well with just one good transistor. Its an 
emitter follower, driveing another emitter follower. Operating properly 
it should not present a detectable load to the signal point its 
connected to.

Be aware too that vendors often will call an off speck nearly  junk 
transistor a 2n2222, and it could fall out of that bag of 20 with some 
being npn's, and some being pnp's. I have such a bag, from Radio Shack.

And it has a gain of not more than .95. No gain, but a small loss.

The monitor if it worked with your old vcr's composite output, expects 
about a .7 p-p volt video, with about .3 volts of synch on the bottom of 
that. But the coco's synch is a long way from meeting ntsc synch specs 
as it not a truly interlaced format.

What I am saying is that more modern digitally based monitors may not 
work because it is not a valid signal to them.  Only an oscilloscope can 
show you how much signal you actually have.

If the monitor is a "TTL" input monitor, this circuit is incapable of 
generating a high enough signal to be seen by the monitor. TTL input 
monitors need a voltage swing from below .4 volts to around 3.5 volts or 
more to work well. EGA is a color variation of this, needing this higher 
drive level for all 3 colors.

So all in all, I can think of several reasons this circuit won't work for 
your gear on your table. The circuit could be working well while the 
monitor is ignoring it. Only a scope can answer that.

Cheers, Gene Heskett
-- 
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