[Coco] On the subject of RF output

[Charles Hudson]

Thank you, Keith, for this information.

-CH-

On 12/17/2021 9:56 AM, Keith Monahan via Coco wrote:
> 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-
>>
>>
>