I wired the CoCo 3's RGB video lines from the connector on the bottom of the computer to the RGB lines in an SCART connector. I connected the audio from the CoCo connector to the left and right audio on the SCART connector. I connected the CoCo 3's composite video to the composite video/sync on the SCART connector and I connected +5V to a 100 ohm resistor connected to the "fast switching" pin on the SCART connector. I then plugged the SCART video connector into my SCART --> HDMI converter, which I plugged into my TV. Viola! Perfect RGB video up scaled to 1080P.
CoCo monitor out pins 1 + 2, ground, to SCART pins 4, 5, 9, 13, 17, and 18
CoCo monitor out pin 3, red video, to SCART pin 15
CoCo monitor out pin 4, green video, to SCART pin 11
CoCo monitor out pin 5, blue video, to SCART pin 7
CoCo monitor out pin 7, audio, to SCART pins 2 and 6
CoCo composite video out center wire to SCART pin 20 composite/sync
+5V connected to a 100 ohm resistor to SCART pin 16
I used an old cell phone charger to supply the +5V. You could also get +5V from the CoCo's joystick port.