On 15/09/2007 Jerry Lynds wrote:
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 http://www.national.com/ds/LM/LMH1251.pdf
Here are some pics of the transcoder:
http://i241.photobucket.com/albums/f...s/IMG_0209.jpg
http://i241.photobucket.com/albums/f...s/IMG_0207.jpg
http://i241.photobucket.com/albums/f...s/IMG_0221.jpg
http://i241.photobucket.com/albums/f...s/IMG_0220.jpg
http://i241.photobucket.com/albums/f...s/IMG_0219.jpg
http://i241.photobucket.com/albums/f...s/IMG_0215.jpg
http://i241.photobucket.com/albums/f...s/IMG_0216.jpg
http://i241.photobucket.com/albums/f...s/IMG_0217.jpg
http://i241.photobucket.com/albums/f...s/IMG_0213.jpg
http://i241.photobucket.com/albums/f...s/IMG_0210.jpg
It's probably something as simple as a bad solder joint on one of those
SM packages. As the chip warms up the pin lifts from the pad and
everything stops working.
Been there several times over many years.
--
John B