Samsung UN55J620 - BN44-00774A

[jason123]

Hi folks..

I got this Samsung UN55J620D (similar to the 55J6200,...)

Backlight dead - TV turns on, picture comes up (with flashlight)

So I have the backlight set on order. However, I was bored and I started repairing the strips. LEDs are standard 3 V, and I have tons of these.

Got them all done (many were bad) - put them in the TV.. testing checks out with the tester all the way from 5 mA to 220 mA..

With just power supply connected to LED strips.. I turn the TV On, and they start flashing almost 1Hz - then the weak unreplaced LEDs would blow, and I would replace .. so on. Now they're just flashing, nothing is failing.

so I unplugged the LED strip and I see pin 1 of the LED string reach ~320 V.. they slowly drop to about 275V, then back up to 320 V then slowly down.. so on.

I tested the large diodes on the power supply - seem to be fore the 13 V, and I get steady 12.9 V. The smaller diodes all tested (desolder one lead and check resistance both direction - one direction is about 6 M Ohm, and other direction is open (High Impedance)

I checked D9101 Voltage and its similar to M1+ (pin 1 of LED string)
R9161 reads about 0.4 Ohm, R9162 is 1.9 Ohm.

All the LEDs still check out (I will install the new set when I get it.. just want to make sure I don't have a PS problem as well)

Thanks in advance.

[nomoresonys]

Re: Samsung UN55J620 - BN44-00774A

If you unhook the cable from powerboard to mainboard and plug the power cord back into powerboard leaving mainboard unhoooked, are the backlights on and steady?

[jason123]

Re: Samsung UN55J620 - BN44-00774A

No, they do exactly the same thing. Flashing at 1 Hz or slightly faster.

[R_J]

Re: Samsung UN55J620 - BN44-00774A

It looks like the led string is going open, tripping the ovp and shutting down the led driver circuit.

[jason123]

Re: Samsung UN55J620 - BN44-00774A

Is it normal for the voltage to fluctuate even when the leds are disconnected?

[R_J]

Re: Samsung UN55J620 - BN44-00774A

Yes, There is the base voltage (across CM841), and the boosted voltage (across C9131), Q9101 is the mosfet along with T9101c that does the boosting, When the voltage goes too high due to an open circuit, the ic detects this and shuts down the boost circuit.