Samsung LNT4032H BN044-00134A No Back Lights

[culcuhain]

Good morning!
I am working on a Samsung LNT4032H TV with the power board BN044-00134A Power/Inverter Board.

The will turn on, have audio and video is displayed on the screen when shown with a flashlight. The backlights do not come on.

The voltages to the main board were within Spec. ALL the caps, including the 220uf 450v have been replaced with Panosonic or Nichicon (I don't use cheap parts) QI801 and QI802 (FQPF11N50CF) where both toast. QP801 (SPW12N50C3) was toast as well (tested those on my parts tester). All have been replaced. Resistors all have continuity.

Here are some noted voltages while I check a few components last night (Black plug of DMM was grounded to chassis)
DP801 55v/325v
RP804 -55v
BI801 325v/325v
FI801 325v
DI801 325v/325
DI814 -40v

That is all that I had time to check before it got too late.
Any ideas on my next steps?

[budm]

Re: Samsung LNT4032H BN044-00134A No Back Lights

Are you testing those Voltages on the HOT PRIMARY side of the circuit? If you are, then you are using the wrong GND ref. for your meter, that is why those readings are showing Negative number and wrong levels.
When making the measurement in the HOT PRIMARY side, you HAVE TO USE the negative leg of the main filter cap as the GND ref for your meter, when working on the COLD SECONDARY side you can use the Chassis GND as the GND REF. for your meter.
We also need good clear pictures of the top and the bottom side of YOUR board and the pictures of the whole back side of the TV that shows all the boards.

[culcuhain]

Re: Samsung LNT4032H BN044-00134A No Back Lights

Yes. I was testing the hot side of the circuit. I will solder a wire to the bottom of the main cap to make it easier to access when I test it tonight. I will post some pics up after I get home. Thank you for the quick response!

[budm]

Re: Samsung LNT4032H BN044-00134A No Back Lights

The diagram attached my not be 100% accurate.
The IC driving the QI801/802 may also be damaged when those MOSFETs shorted out.
Pictures provided by shopjimmy.

[ReeceyBurger123]

Re: Samsung LNT4032H BN044-00134A No Back Lights

Broken solder joint on the inverter transformer is common on these boards.

[culcuhain]

Re: Samsung LNT4032H BN044-00134A No Back Lights

Thank you. That is the correct PSU/Inverter board and Schematic. I have ordered a few of those IC chips and should have them Saturday. I will check the inverter transformer solder spots and just reflow them. I will report back sometime over this weekend. Again, thank you!!

[culcuhain]

Re: Samsung LNT4032H BN044-00134A No Back Lights

Ok. The parts came in yesterday and while the kiddo napped I went to work. i tested the Mosfets, Diodes, Bridge Recifier and resitors that I could. I reflowed the solder on the Inverter too... Looks like a bad FAN7382 chip from when QI801 and QI802 (FQPF11N50CF) shorted out. She has been up and running for about an hour with no issues... My friend heard what I was doing and dropped off his tv for repair... That is another thread....

Thank you all for all the help! My patio will now have a tv!!!

[budm]

Re: Samsung LNT4032H BN044-00134A No Back Lights

Thanks for the follow-up on the repair.

[culcuhain]

Re: Samsung LNT4032H BN044-00134A No Back Lights

and no more backlights. Going to tear into her this weekend.

[culcuhain]

Re: Samsung LNT4032H BN044-00134A No Back Lights

Okay this is pretty weird. Reflow the solder on the transformer for the inverter. That had no effect. I checked all the diodes in the inverter section in line. They all tested okay. I pulled the MOSFETs and diodes can my test read them all good. I went ahead and replace the IC that I replaced before. When I plug it all in and tested it. And now I have back lights. Either the IC chip failed or I had some bad soldering on the board. I have the TV running in the garage right now. I'm going to run her 24 hours and see if all is well. I will also check temps about the power board and inverter board.