LN-T4061F has power, sound and picture, no backlights

[Pioneerx01]

I got Samsung LN-T4061F. Powers on with splash sound, panel has image (flashlight test) but the back-lighting is not on. The power supply is BN44-00165A (IP-231135A, IP-40STD) all good caps, as far as I can see. All voltages are reading fine as well (12V and 5.4V) at least on the cables leading to main board.

There is a lead coming from the main board called inverter on/off and it reads 5.4V when on. On the other side of the power board there is lead going to inverter called VCC_12V, but it only reads 2.5V. I can't test the transformer voltage as it runs around 2100V and my multi-meter goes to 1000V only.

Would it be safe to assume that the PSU is bad? Is there anything I can test or replace? If I were to remove the VCC_12V cable from the board and pipe 12V straight to it from other 12V source on the board, would it be safe to do?

Thanks
-Peter

[budm]

Re: LN-T4061F has power, sound and picture, no backlights

Can we see the whole back side of the TV? You have power supply/inverter board combination, the High Voltage output is fed into the Balancer board, the VCC_12V is used for the comparator circuits on the Balancer board for the LD (Lamp detect, >10V = Good) error signal and the FB (Feedback for maintaining constant brightness, in mV AC range).
Do you have PFC Boosted Voltage to run the inverter circuit on that combo board?

[Pioneerx01]

Re: LN-T4061F has power, sound and picture, no backlights

Technically is is the PSU/Inverter combo, but it does have separate Inverter board as well. Besides that 2.5V DC reading on the VCC_12V I am getting nothing on the other pins.

Honestly, I am not sure what you are asking in the last question.

[budm]

Re: LN-T4061F has power, sound and picture, no backlights

That is not inverter board, it is called Balancer board to balance all the lamps current draw.
So which board did you replace, the Balancer board or the power supply/combo board?

[Pioneerx01]

Re: LN-T4061F has power, sound and picture, no backlights

I have replaced none of them. This is how I got the TV.

[budm]

Re: LN-T4061F has power, sound and picture, no backlights

Do you have the 13VDC on connector CN801 pin 19/21/22?

[Pioneerx01]

Re: LN-T4061F has power, sound and picture, no backlights

Yes, all the voltages on the CN801 are good. I am reading 13.57V on 19/21/22

[budm]

Re: LN-T4061F has power, sound and picture, no backlights

Then you need to check the UI809 12V regulator 78M12 input and output to see if 13V is present at the input pin, also check the resistance between the output pin and GND pin to see if it show low resistance, it is either that regulator is bad, the filter cap is bad, or the load (UI802S) is bad. See page 3 of the schematic for details.

[Pioneerx01]

Re: LN-T4061F has power, sound and picture, no backlights

Yep, the output in only ~2.5V with very high resistance of 8K. Input is at ~13.5V.
This is a 7812T Voltage regulator rated at 1A. All I have at home right now is L7812CV Rated at 1.5A. I should be OK swapping those out, correct?

[budm]

Re: LN-T4061F has power, sound and picture, no backlights

Higher current rating is fine.

[Pioneerx01]

Re: LN-T4061F has power, sound and picture, no backlights

Yes, that is true, I just wanted to make sure there is no other electronics aspect I am overlooking, frequency, drop in/out voltage,...

I have swapped out the regulators and the TV came to life. I wan to thank you very much for your help and patience. Looking over the problem, I should have been able to figure it out myself. It was not that hard. Oh well. Still A+ for you. Thanks -Peter

[budm]

Re: LN-T4061F has power, sound and picture, no backlights

Glad to hear it was just the bad regulator, there is flaw in that design since the input is only 13V and they want 12V output, there is not really enough head room for regulation, they should have use LDO regulator instead.