Re: Analog NTSC to RGB converter?

[Michael J. Mahon <mjmahon@aol.com>]

Linards Ticmanis <ticmanis@gmx.de> wrote:
> 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'd carefully open the can to get to the internal node. A little delicate
soldering and you've got it. 

> 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.

It can't be usefully paralleled with the trap--you have to parallel it with
the internal capacitor. 

>> 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.

No, a saturated transistor or analog switch will do as well--particularly
with a 470 Ohm resistance to ground. 

-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon