I'm having a tough time trying to figure out what is wrong here, and
if anyone has any tips or pointers I would appreciate it. The tv is a
1992 model, Sylvania SSC3184 202. The original problem was that the
raster was all red, traced it to the LA7672 video processor and
replaced it with an NTE7148. That cured the video, then the
horizontal sweep went dead a while later. That was caused by cold
solder joints on the supply resistor for the horizontal drive
transformer; resoldered and that problem was fixed. Now that those
problems are fixed, I am observing a weird distortion of the raster.
When turning the set on, everything is fine. I can set the sub
brightness, brightness and picture at max and the raster will scan ok.
However, after a few minutes, a bright vertical line begins to appear
slightly more than halfway across the screen. As time goes on, the
image will pull in where the line is. Then, line width will vary
every few lines, forming a sawtooth pattern at the right side of the
screen. Eventually, the whole raster will pull in horizontally, and
the line will turn into about 2" of foldover in the middle of the
screen; the sawtooth pattern goes away at that point. If the image
decreases in brightness (due to me decreasing the sub brite, pix or
the signal's brightness goes down) the raster will get better. The
dimmer the image, the better it will appear, although the problem
never goes away totally. Also, if I turn down the width using the pot
on the pincushion correction board, the problem improves somewhat (but
does not go away totally). If I set the sub brightness for a dim
raster and turn the width down to minimum, it will run indefinitely
without collapsing.
I posted some images of the problem at
http://public.fotki.com/rpm1200/sylvania/. Please let me know if the
photo album is not accessible.
There are no other problems with the chassis that would indicate an
overload of the flyback derived supplies. The horizontal output,
switching regulator and pincushion amp are all running cool.
Here's what I have tried:
1. Checked 130V B+ line; no measurable ripple and the line stays at
129V regardless of what the raster is doing.
2. Checked the hold down input to see if tv is going into x-ray prot;
stays under .09Vac/.02Vdc regardless of what the raster is doing.
Any ideas? I checked the faq and did a google groups search for
similar problems, but came up empty (usually the horizontal seems to
work fine or not at all).
My next course of action is to check the electrolytics in the
pincushion correction circuit and if that doesn't turn up anything,
try cold spray around the horizontal output and pincushion circuits to
see if I can identify the culprit.
Thanks, Rob