A broken TV

 I was requested to fix a broken TV. According to the owner of the TV, the

back light turns off within 10 minutes after the power on. As I checked it, it

worked perfectly for more than an hour. In order to duplicate the problem, I

have kept the TV on for 3 hours and there appeared the symptom as was

below.

The flush light of the camera helped me to judge that the LCD itself was OK.  Only the back light

was stopped working.

Then I applied a shock using my hand. Hitting the TV, it worked nicely. I

quickly understood that there would be a poor connection somewhere in

the TV. Hitting the back light board easily duplicated the problem letting

to know me that the problem was in that board.

As I measure the voltage of the regulator on the back light board, it dropped

when the shock was applied. I measured the resistance between the fuse F1

and the regulator Q1. F1 and Q1 are connected by 4 resisters R5, R6, R42

and R49 inseries. The total rsistance  should be 480 Ohm (120Ohm X 4) but

it varied synchronizing with a shock. Sometimes it was about 500 Ohm

and sometimes more than 1KOhm. As I see the board carefully, I found a

soldering crack at the point where a red arrow was indicating. Re-solderling

R6 fixed the problem but I suspected that the problem would be caused again

sometime in the near future. I must say the designer of the board is not skilled

enough. For the application like this, he or she has to pay more attention

on the heat and the expansions of the parts. A resister's expansion and the

board's expansion are totally different. A heat cycle test can catch this type

of problem easily too. Judging from the situation, the manufacturer of the TV

was not doing it enough, I can assume easily. In order to not have the problem

again, I re-designed the circuit using another resister which is not for the

surface mount.