I'm working on a ViewSonic n2750w with this weird image distortion. The best way I can describe it is the image has artifacts, but these artifacts fall into a very uniform grid pattern. But the interesting thing is, if you pull up the TVs on screen display, the OSD is crystal clear with no distortion. I've attached two pictures showing the TV with and without the OSD. I've tried almost every input on this TV; coax, S-Video, composite, VGA, and DVI. The distortion occurs on every input I tried.
I opened the TV to have a look. I included is a picture of the back of the TV. In the center is the video processing board. To the right is the power board, and lower right is an audio board. This TV does have CCFLs, the inverter board is not visible in this picture, it's located underneath the piece of metal holding all these circuit boards.
I know the power and the backlights aren't the problem, so I'm ruling out the power and inverter boards. Also, since the OSD has no distortion issues, I'm ruling out the panel itself as well as the LVDS connector. So I believe the problem to be somewhere on the video board.
The service manual for this TV included a helpful block diagram on the video board circuitry (also attached). The various inputs (in purple) are not displaying correctly. The OSD comes from the MCU and that's working (marked in green). Also, the LVDS to the panel is fine (blue). So I believe the problem is in the scaler/de-interlacer area of the circuit (red).
The video board has something like 50 caps on it. I tested the ESR of every cap. Attached is a table of every cap on the video board. The ones in green are good. The ones in red are too high, and the ones in orange are right at where most charts consider the cap to be bad. I did my best when identifying the cap number; sometimes it's hard to tell if the printing on the circuit board is for the electrolytic or a nearby surface mount cap.
I've ordered replacement caps for all the ones I marked orange and red, although they haven't arrived yet. But my question is, has anyone run into a video problem like this before? Any thoughts if changing a few caps will solve this problem? Some other ideas I had include reflowing the solder on some of the ICs on the video board, also checking and, if necessary, replacing the oscillators on the video board.
I opened the TV to have a look. I included is a picture of the back of the TV. In the center is the video processing board. To the right is the power board, and lower right is an audio board. This TV does have CCFLs, the inverter board is not visible in this picture, it's located underneath the piece of metal holding all these circuit boards.
I know the power and the backlights aren't the problem, so I'm ruling out the power and inverter boards. Also, since the OSD has no distortion issues, I'm ruling out the panel itself as well as the LVDS connector. So I believe the problem to be somewhere on the video board.
The service manual for this TV included a helpful block diagram on the video board circuitry (also attached). The various inputs (in purple) are not displaying correctly. The OSD comes from the MCU and that's working (marked in green). Also, the LVDS to the panel is fine (blue). So I believe the problem is in the scaler/de-interlacer area of the circuit (red).
The video board has something like 50 caps on it. I tested the ESR of every cap. Attached is a table of every cap on the video board. The ones in green are good. The ones in red are too high, and the ones in orange are right at where most charts consider the cap to be bad. I did my best when identifying the cap number; sometimes it's hard to tell if the printing on the circuit board is for the electrolytic or a nearby surface mount cap.
I've ordered replacement caps for all the ones I marked orange and red, although they haven't arrived yet. But my question is, has anyone run into a video problem like this before? Any thoughts if changing a few caps will solve this problem? Some other ideas I had include reflowing the solder on some of the ICs on the video board, also checking and, if necessary, replacing the oscillators on the video board.
Attached Files
I tested the ESR of the caps without removing them which, as I understand, is fine except for caps in parallel.
I looked at the piece of paper where I jotted down the numbers and I can totally see how I thought some scribbles were numbers. Since I made that mistake, I went back and took all new set of readings. Everything was the same as what I reported above. For the 12.XXX readings, just drop the last digit.
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