[Coco] RGB2VGA converter setup issue

[Sean]

FINALLY got the time to hook up the CoCo and try out the RGB2VGA converter
I got from Zippster's run back in September.
So far all the monitor does is say "not supported".  So it's definitely
giving a signal, because when I unplug power to it, the monitor goes back
to 'no signal'.

I'm using an 1080P HDTV with the VGA input.

Should I try turning the potentiometer?  Or could this be the Pin 9 topic
everyone has mentioned?  I've only tried one VGA cable so far, but I know
it's a good cable.

Sean