Re: Analog NTSC to RGB converter?

[Linards Ticmanis <ticmanis@gmx.de>]

On 10/26/2012 07:04 AM, Michael J. Mahon wrote:

> The chroma trap is, as you've observed, a series-tuned circuit. If there is
> a PAL/NTSC level generated by the decoder IC, you can use it to control a
> relay that would switch in an (adjustable) additional capacitance in
> parallel with the trap capacitor. Hopefully, the signal goes to the
> inductor and the capacitor goes to ground, but it will work either way. 

Well... the trap is a single metal-can component which according to the
schematic contains both inductor and capacitor, so I could only put
something in parallel with the whole thing, which I hope will work too.
It doesn't go directly to ground either, but through a 470 Ohm resistor,
which I hope won't matter.

I've tried with a 27pF adjustable capacitor, but that was clearly too
low a value - adjusting it didn't seem to make any difference
whatsoever. Unfortunately adjustable capacitors seem to have gone out of
style, particularly when you don't want SMD parts. You can still order a
few types on-line but electronics parts stores have stopped carrying them.

> Alternatively, if you don't mind seeing the pixel structure of colors (like
> most RGB monitors), just remove the chroma trap entirely.  That will
> improve the bandwidth even more. (This option won't work if the chroma
> signal is taken from a second winding on the chroma trap.)

Since this monitor doubles as a TV for me, and the color grain in TV
images is pretty strong without the trap, I guess I'll try to find a
switchable solution.

Is the relay necessary? Alex suggested a simple transistor switch with a
standard BC548 or other small NPN.

-- 
Linards Ticmanis