[Coco] On the subject of RF output

[Keith Monahan]

CH,

The CoCos use a Motorola MC1372, which is a Color TV Video Modulator 
chip. It's a 14-pin DIP chip.

Please find some links here:

https://console5.com/techwiki/images/f/f8/MC1372.pdf
https://www.allaboutcircuits.com/tools/tank-circuit-resonance-calculator/
https://www.rfwireless-world.com/Tutorials/TV-channel-frequencies.html

While my knowledge of all this stuff is weak, one of the coils is part 
of the RF Tank Circuit (you'll find it connected to pins 13 and 14 on 
the MC1372).

This LC tank circuit would generate the main picture carrier oscillation 
frequency, which is 1.25mhz above the bottom of the band. This datasheet 
says the coil(approx 0.1uH) in combination with different value 
capacitors should produce the necessary carrier frequencies.

The datasheet says 75pf, for channel 3, claiming a target frequency of 
61.25mhz. Using the calculator above, however, this doesn't make sense 
to me. I think the right capacitor value would be around 67pf.

The datasheet says 56pf, for channel 4, with a target frequency of 67.25mhz.

The CoCo 2 has a physical switch on the mobo to switch the channel. Not 
sure what the 1 has.  Maybe that's the whole idea of an adjustable coil 
-- but then why have a switch?

My speculation is the other coil is for generating the sound carrier 
frequency in a similar fashion, which is 4.5mhz above the bottom band.

Even after adjusting, try different RF modulators, etc -- I was never 
able to get what I consider clean video out of the CoCo2. It just looked 
like crap. Text was blurry, dark sections ran into light sections to the 
right of it.

I highly recommend the EXCELLENT cocovga, produced by Brendan Donahe and 
Ed Snider, and I now have perfectly crisp clear video, as it should be. 
It involves removing the RF modulator, which is not a task for the 
inexperienced. I don't have the right tools, like a vacuum desoldering 
station, hot air, etc etc.

I barely know what I'm talking about here, so YMMV, but hopefully those 
links will point you in the right direction.

Thanks,
Keith

On 12/17/2021 8:44 AM, Charles Hudson wrote:
> My CoCo 1 is being output to a 20" Toshiba LCD TV of a certain age, from 
> the RF output on the CoCo through a cable and a coax converter plug into 
> the TV's RF input.  The TV is plenty stable and sharp but the output 
> from the CoCo is a mess of banding, video "noise" and herringbone.
> 
> Yesterday out of frustration I "tweaked" the two accessible coils in the 
> RF section just to see if I could make things better. (Using anti-static 
> plastic tools)  I did, sorta; the text was in sharper focus and the 
> v-noise abated somewhat.  I was working without the RF shield on the CPU 
> so probably things could have been better, but I wanted to ask:
> 
> What are the two coils controlling?  If I turned one of them too far I 
> lost the color signal.  I also noticed I could produce noise.  So I 
> guess one of the coils acts as a signal mixer for the final output? 
> What's the other one for?  Where might I find documentation on this topic?
> 
> Thanks for your replies.
> 
> -CH-
> 
>