[Coco] On the subject of RF output

[Charles Hudson]

My intention has always been to upgrade to the VGA board, however, my 
mobo is Revision E.

On 12/17/2021 10:05 AM, Keith Monahan via Coco wrote:
> I was less precise here than I should have been.
>
> The COMPOSITE mods, like those shown here
>
> https://thezippsterzone.com/2018/10/28/coco2-composite-a-v-modulator-replacements/ 
>
>
> require removal and replacement of the RF modulator.
>
> CoCoVGA, however, can certainly co-exist with the existing RF 
> modulator as described here
>
> http://cocovga.com/documentation/installation/coco-1-rev-f-installation/
>
> Hope this helps,
> Keith
>
> 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-
>>>
>>>
>>
>