Samsung LA32B350F TV No Backlight

[luqmaanb]

Hi All.

TV stopped turning on completely. I opened it up. Main fuse FM801S (250V/3A) was blown. I replaced the fuse with a 4A fuse I had laying around. The TV then turned on but no picture was visible. I noticed a very feint picture in the background and I was able to see the menu. The TV was responsive to my commands. It was clear to me that the backlight wasn't turning on. By the way, the fuse didn't blow again.

I disconnected the power board from the main board and powerered the TV back on. The backlight turned on.

On further visual inspection of the board, I saw capacitor CP810 was blown (open). This capacitor is across the drain-source of MOSFET QP802S - I'm not sure what purpose it serves.

I've done all my tests with the components still soldered to the board. All diodes checked out fine (between 450mV and 750mV). QP802S also checked out fine.

Voltage Measurements on CN802 with main board disconnected:

• PWR ON/OFF - 0.7V
• STDBY5 - 3 V - 5.1V
• GND - 0V
• 13V(S) - 11.9V
• GND - 0V
• GND - 0V
• 5.3V - 5.1V
• 5.3V - 5.1V
• GND - 0V
• GND - 0V
• 13V(M) - 11.9V
• 13V(M) - 11.9V
• INVERTER ON/OFF - 2.9V
• E-PWM - 4.5V
• DET_5 - 4.9V
• SYNC - 0V

The voltage across the big yellow capacitor was 290V. When the main board connected, the voltage on pin DET_5V was 0.22V. I suspect the problems lies there.

I know CP810 must be replaced but I suspect that's not the only component causing the issue.

Any help please?

• TV Model - LA32B350F
• Power Board - BN44-00289B

I've attached images.

[dick_barton]

Re: Samsung LA32B350F TV No Backlight

These snubber capacitors do have a tendancy to fail. Checked QP802S, RP820, RP821, RP822, RP806, RP820 also check DP802 which if damaged could have taken out UP801S.