I have 3D modeled a 3D-printable box that mounts a DB-9 connector, which mounts under the Coco-3, which may help in constructing a SCART cable. The idea is to make a cable in such a way that the 10-pin IDC connector and the Composite connector stays in place, and a old VGA cable takes the brunt of the disconnections and strain.

The box mounts to the bottom of the Coco-3, either with double-sided foam tape, or by (carefully) threading the 2 vestigial bosses on the bottom of the computer.

The box lid is to be threaded 3mm, while the DB-9 connector should be threaded #4-40 UNC. During final assembly, a drop of thread-locker should keep the #4 studs tight.

I've decided to line the inside of the box with copper foil, and ground that to minimize interference. The box may also hold the capacitors, as mentioned in this thread.

I may do a rudimentary Kicad schematic of the setup variation.

https://www.thingiverse.com/thing:5528298

@ Barry Nelson,

Do the capacitors need to be as large as 220uf?
Do they just couple the video--without DC coming through?
(Thank You)
--BrendaEM

EDIT Solved: Caution! One existing Coco3 Bottom jack pinout is wrong, as mentioned in my video: https://www.youtube.com/watch?v=UyD2w1Z5R-E&t=53s

This box works pretty good: https://www.amazon.com/gp/product/B00MUNIVRO/
For the SCART connector, I hacked this cable: https://www.amazon.com/gp/product/B01DYELZXE

Any fully standards compliant SCART converter will work. Either your SCART converter does not support RGB and is a cheap composite converter with a SCART connector, or you are not supplying the proper  voltage at pin 16 to put the converter into RGB mode. There should be 1 to 3 volts from this pin to ground. Also make sure you have all the ground leads including the one from the +5V power source connected together.

It has been called to my attention that the circuit works better with the fix outlined below, courtesy of David Ladd.

Additional parts needed…

180 Ohm resistors (3)

220uF electrolytic capacitors (3)

Additional circuit…

                      CoCo 3                         to SCART

Existing circuit R line —\/\/\/\/\———|(—

Existing circuit G line —\/\/\/\/\———|(—

Existing circuit B line —\/\/\/\/\———|(—

                                      ^180 Ohm    ^220uF

                                                          <+side

Could you clarify what voltage the 220uF electrolytic capacitors are to be rated?

These caps have less than 3V across them so almost any 220uF cap should work.

Thank you Mr. Nelson!

Feedback from Travis Poppe

Thought I'd share my experience with three different SCART to HDMI adapters
so that others may avoid the trial and error I have.

This is for use with Barry Nelson's RGB to SCART cable (
https://hackaday.io/project/7366-coco-3-rgb-to-scart-to-hdmi-cable).

This converter only seems to do composite; RGB will not work:
https://www.amazon.com/ELUTENG-Adapter-Digital-Converter-Support/dp/B076C527Q6

This one works with RGB, but for me at least, the video stutters left to
right in most video modes, and is incredibly distracting:
https://www.amazon.com/ELUTENG-Scaler-Booster-Converter-Blue-Ray/dp/B077F5J2NJ

Finally, and unfortunately the most expensive, this one works perfectly for
me:
https://www.amazon.com/CiBest-Converter-Adapter-Support-Set-top/dp/B06Y43RVLH

Hope that saves some time and frustration for others.
-- Travis Poppe

The last one looks exactly like the one I used when developing this. It is typically $30-$40, I think I paid $35 for mine.

Hi Barry, thank you for this article. I was having the same problem as many here after building the adapter. I have connected the negative from my power supply to the GND, and the +5v to the Pin16. I just don't understand how the pin16 should have 2 or 3V if the input is 5v. Can you elaborate how the connection should be considering I have a cell phone adapter with two wires: 5v and gnd?

The 100 ohm resistor reduces the voltage applied to the 75 ohm impedance on pin 16 to about 2.14 volts. The gnd or negative connection of the cell phone charger connects to the gnd connection on the schematic and the +5V is connected to the resistor.

OK - so I re-checked the resistor and it was too high. I replaced it with a 100 ohm as required and now we are good!!! Perfect.

Hi - I appreciate your guide to making this cable for the Coco3. I have a problem. I have wired it all up. I used a USB cable for the 5v power. It seems the picture I get from the cable is the same regardless of if I plug in the USB for 5v power or not. So probably just composite coming out not RGB? Any ideas why this might be? I have the ground connect to the ground of the Scart/Coco cable. I am reading 5.11v across pin 16 and 18.

With my video issues, could the scart signal be the JP21 standard as opposed to the EU standard? 

Very odd. In RGB mode it should not matter. Double check the wiring. Perhaps the converter you got is defective?

I picked up an scart to hdmi converter recently, connected it as per the instructions with this project, as dnb the resulting image is in black and white with vertical black bars across the screen. Is there a fix for this or is the converter not correctly converting the signal? 

Upon further investigation I have black and white but no color. 

Hi Barry, I just ordered the parts to build this from ebay.  The converter I ordered is: http://www.ebay.com/itm/272499378560?_trksid=p2060353.m2749.l2648&var=571556433381&ssPageName=STRK:MEBIDX:IT  I hope it is the same one you used.

Glad you got it working Randy! Can you send me a pic or two showing the video output on your monitor?

Looks like that was my problem... your diagram says to ground pin 18 on the scart to pins 1 & 2 on the coco... I needed to connect the negative poll from the cell phone charger to 18...now it works fine!  thanks!

where do I wire with negative post on the cell charger?  Is it in RGB mode when that's plugged in or unplugged?

The negative wire from the cell charger goes to common ground labeled GND on the schematic.

I tried to wire this up but it looks like all I'm getting is the composite signal... the 5v adapter seems to have no effect..?  what did I do wrong?

This is mostly likely caused by not having the proper voltage (between 2 and 3 volts) on pin 16. Check the connections to both sides of your 5V source.

If you have a CoCo 1 or CoCo 2, check out my other project: https://hackaday.io/project/7613-coco-1-composite-video-output