VGA Signal Fading

Hahoy there

I have a bit of a problem here. I just finished soldering a really simple VGA signal fader, to be able to fade a VGA signal to black without losing sync signals and having the beamer or screen telling "cable unplugged". Every cable is directly wired from the IN connector to the OUT connector, except the 3 lines for R, G and B signals going through potentiometers.

I have a pb though. RGB signals (pin 1, 2 and 3 respectively) are just analog lines varying from 0V to 0.7V in theory. With my voltmeter on the vga output of my macbook, if I send a full red image to the second screen, I get 0.5V on the red line, and 0.01V if I send full black. The problem comes when I wire the potentiometers for fading the 3 colors.

When the pot is fully open, I still have 0.5V, but when it's closed, I have 0.10V (ten times more). My pots are 1k linears. Ok, with 0.10V on the red line, I don't see any red color on the screen, so this is not a big deal. But when I turn the other pots, something strange happens.

In fact, I can turn 2 pots completely off, but as soon as I touch the third, the sync signal is lost... :confused: The wiring is really simple : Pin 1 to the ground, pin 2 to the output signal, pin 3 to the input signal. I tried adding 1N4148 diodes on the input signal, but strangely it never works when they are plugged (computer don't even recognize there's a second screen plugged). I also tried without wiring the Pin 1 to ground. Whatever pot I turn last (R, G or B), I get the same behavior. I does the same thing when I turn the 3 pots approximately 3/4 of the course. It's like it's flipping off when there's too much resistance on the signal lines.

I checked checked checked the soldering job and stuff, nothing innapropriate.

One precision : as I have only 1 VGA cable, I used it to plug the computer to the fader, and the VGA output goes to a TV using a scan converter like this one.

GrandTec Grand Hand View
http://www.grand.com.tw/pctv_handview.html

If anyone have an idea, please, tell me quick ^^

Cheers (and sorry for the long post)

Some VGA monitors require the sync pulses on the green channel. It could be you are missing that.

The other thing is that the pots are putting a high impedance in the line, the video is only supposed to be 75R (Ohm), by increasing this you are messing up the termination and causing reflections (ghosting). Also there are automatic gain controls that could be kicking in in your monitor.

As you have found TV isn't as simple as you would like it to be.

I don't think that the sync signal on green channel is the cause, because I can turn the pot fully off on the green channel and still get in sync. In fact I can do it with any channel, if I let at least one fully open.

Does the pot influence on the impedance is still valid if it is fully open(zero ohms resistance) ? I want to be able to fade video out only if there's a computer crash, like a emergency button, so ghosting is not a problem for me.

I'm gonna try it again tonight without the scan converter.