R&R Developpements

Fix Samsung 920N screen

A 19" LCD screen is good, a working 19" LCD screen is better!

This Samsung SyncMaster 920N LCD screen broke down overnight.
The small green LED is on, but no LCD backlight.

Trying to turn it on, the screen emits a "tic tic" sound and you can see a flash backlight of the LCD panel.
The problem seems to be cause by a failure of the power stage or inverter.

First step: disassemble the screen! There are only a few screws, but a good amount of clips very difficult to get clean.

Rear view: down the power supply board, top the logic board.

Focusing on the power supply board, we see that the lower half concerns the backlight of the LCD panel. Indeed, we see the two high voltage transformers for neon tubes and cables powering them. The green module (commonly called "inverter") between the two transformers manage their power supply, and in particular the low-voltage switching.

Before putting into question the transformers or green module, check that the power of the inverter is correct. According to the "service manual" screen found here http://www.eletronicosforum.com/monitor.htm and copy here, the voltage should be 13V.
After measuring it is well to 13V and the fuse is intact.

The HT transformers have been tested and do not seem to short circuit or open. In fact they were out of the problem, there would probably not flash at power up...
The inverter module seems concerned. The components being on the dark side, it must unsolder...

Inverter board: logic chip in the middle, above, the two MOSFETs

After heavy research on the internet, we found two interesting links :
in english: http://www.badcaps.net/forum/showthread.php?t=8453
in german : http://www.circuitsonline.net/forum/view/78517

The problem seems to come from the MOSFETs. While measuring them, one is in short circuit!...
This component, FDS4935 (datasheet) is available on Farnell (ref FDS4935BZ) at 0,99.

Once replaced and resoldered inverter module; ultimate test ... : screen lights up!
And that's one more victory against planned obsolescence, and a screen that still has good years ahead of him.
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