Hello,
I have built a YPbPr (component video) to RGBs (analog, composite sync) transcoder using the LMH1251. This also includes the LMH6739 tripple video buffer.
The probelm is that the screen goes black after just a few seconds, but the image is stable and clear while it is on. Here is some more diagnosis info...
When I tried it out for the first time the image had a rolling black bar that was solid (not waving) and took up half the screen, rolled very slowy. If I touched a grounded area on the pcb the black bar would increase and get full of horizontal lines on the top and bottom of that bar, as if the image was compressed into the center of the screen. Again, this was only when I touched it. When it was 1st plugged in, for a few seconds, the screen was fine then the black bar started. If I picked the pcb board up, not touching ground or anything conductive, and tilted the board, the black bar would consume the screen and become distorted in a simlar way to when I touched ground. The amount of distortion would be directly proportional to the angle and amount of tilt.
Now for the odd part. The next day I pluged it in a went to try it out and experiment. IT WORKED PERFECTLY! I didn't change a thing. I didn't even unhook it. I would leave it on for an hour and worked beautifully. I could touch GND, tilt it and worked 100%. Now, again, it is not working.
This time the screen goes black after about 2 seconds and nothing has changed. Could I have a bad Capacitor? How much grounding is required? I basically have one main vein that runs though the pcb and branches off where needed. When the screen goes black, I need to let it sit for at least 5 seconds before I can turn it on again and get the image.
I am using a LM1881 to get comp sync. Pretty much used the schematic from the National web site with the exception of the LM1881. The screen is PSOne lcd (Playstation 1 LCD). Power supply for the transcoder is an old Zip drive power box 5 Volts 1.0 amp. Power supplu for LCD is from a gutted playstation. I know this is a really general and vague question, but I hope this may "sound" like an obvious problem to someone with more experience. Thanks in advance...
LMH1251 datasheet
Here are some pics of the transcoder: