[Coco] Coco3 to Scart to HDMI

[Steve C.]

I've finally found time to revisit this, and I'm glad to report this time I
was succesful.  It seems that the type of adapter I tried the fist time
does NOT convert rgb to HDMI, just composite, so that is where It went
wrong with my first attempt. DON'T BUY THIS ONE or others that are visually
similar:

https://www.amazon.com/ELUTENG-Adapter-Digital-Converter-Support/dp/B076C527Q6/

One thing I did differently this time was to adapt the 10-pin port for
simpler conversion, by using the unused pin 10 on the header for the
composite sync signal, and, here's the tricky part, adding the missing pin
6 to supply 5V through an 100Ohm resistor to trigger RGB mode on the Scart
port without needing any mods to the converter or external power supplies.

The HDMI converter itself is externally powered... it *would* run from CoCo
power but when the CoCoSDC was installed the picture would start tearing,
so it's very probably a bad idea to draw too much from the CoCo's internal
supply.

One issue I have now is the letter contrast is very poor in SDXC explorer,
it is very hard to read the menu items now, and I really had to lower the
brightness and contrast to read it at all, so I'm thinking the RGB signals
in the CoCo3 might need to be attenuated.