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... :/ 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 Viewhttp://www.grand.com.tw/pctv_handview.html
If anyone have an idea, please, tell me quick ^^
Cheers (and sorry for the long post)