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The original video-mod was done by analyzing the 6847 and the color signal on the MC1372. This was back in 2009, where I was more interested in "please understand how it works" rather than "cheap, fast and what a horrible person you are". - The Op-Amp design had a lot of responses in the latter form. It is now 2022 and what little information I have kept appears below.
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(brain-dump mode)
1) measure the video at the MC1372 "Video-In" pin-?? same with the Chroma ... the MC6847 shows the voltages and all relevant signals into the MC1372 ... and the PDF datasheet of the MC1372 shows the schematic (minus the lead/lag capacitor values)...
2) design "something simple (easy to analyze by DeVry or ITT-tech students) that is a simple "amplify by -6", do not help the students outside of "here's the op-amp I used, the OPA-2350" - these were in a set of Burr-Brown evaluation from eBay. It has plenty of application notes to reference and even says that it is a "Video" amplifier that runs on a 5-volt supply.
3) discussion item of how to add the chroma (color) signal, which is 3.58MHz and phase-encoded. This is AM into the video signal, then the AM-video + subcarrier chroma + subcarrier audio is sent to the MC1372 RF modulator and modulates the signal AM (amplitude modulation). - - passing note on the VSB (Vestible Sideband Filter), RF Tank, etc and disable of the RF to get the MC1372 to output the Motorla (page-12 of the datasheet below)... old US-TV /NTSC signals and the "skip a channel" used in most cities/markets of the 70's,80's,and90's .. some of the early markets where channels were allocated channel 2,3,4 - maybe a refence to the old Channel-one civil-defense... nah, keep-it-simple...
4) Adam's version 7A ... It did *not* have the "Hairball" in it. - Show the ( -6 gain ) amplifier that is in the MC6847 PDF ( https://archive.org/details/Motorola_MC6847_MOS_Video_Display_Generator_1984_Motorola -- page 12 ) - - Point out the "Devstar" test circuit and video-amp. - Now, build a Devstar amp, ( https://devster.monkeeh.com/ee.html#video and https://homebrewcpu.blogspot.com/2009/02/this-is-mc6847-character-generator.html ) ... point out that the Motorola datasheet uses one PNP and one NPN, Devstar uses only NPN (and an emitter follower uses NPN). - - - add the video subcarrier and output that to a NTSC to HDMI converter ... These are still about $10.00 US today (July-01-2022) https://www.newegg.com/p/2VR-00N7-00002? - It works as well as the op-amp version, and if we complain that the op-amp is expensive, the circuit uses Devstar's 2-transistors, another for the chroma as an emitter-follower and another for the audio as an emitter follower, then add the resistors, capacitors, the A/V RCA connectors, etc and the cost difference becomes minimal. - but the transistorized version is more acceptable to a lot of builders (and also works well).
5) Adam's Version-3 M1000 motherboard. )does not have the "Hairball", ) the video cable from the MC6847 to the MC1372 is on the bottom of the board, this looks to be original. (I have a 3A, where they ran it on top of the board, but had to cut a "notch" in the FCC-Part-15 shielding around the MC6847 to run the cable "on-top" ... I should get a more close-up pix of that sometime.)
A most potent food: Red cabbage cole-slaw with diced eggs, spinach and beans.
first attempt at tinkering with a simple emitter follower circuit on draw.io - - it created a link to some place it stored it online ... and seems to refuse to let me save on my Google-Drive. Saving locally is just an XML with a link to here, there is a URL that Google-Sites does not allow "invalid link"....
https://draw.io ... works, but you have to "search shapes" for "resistor" "NPN", "capacitor", and "coaxial" instead of "cable" or "CoAx" or ... also, the wire from the resistor never matches up with the NPN transistor.
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