Samsung BN44-00264A Power Board

[cicabu72]

Hello guys, I really appreciate your efforts in this community, as I often read the discussion and find good solutions here. It is my turn this time, so a question for you on this very common BN44-00264A power board by Samsung.

My TV LE40B650 is not working anymore, no backlight and no standby led are present. I started to check some components, also reading past discussions about this board.
- fuses are ok
- CNM802 readings all 0 volts
- no backlight appears when disconnecting logic and power board
- QI820/21 not shorted, pretty high resistance on pins 1-2-3
- capacitors not swallen, at least inspecting them while onboard
- no burning signs but some rust on QP801S and 802S solder joints, I think they still make contact, maybe I can test them if you tell me what to measure (see the image).

Any additional tests that I can do to solve my issue? Thank you!

[budm]

Re: Samsung BN44-00264A Power Board

Please attache good clear straight shots pictures of the whole top and bottom side of this power supply board.
Also the pictures of the whole backside of the TV showing all the boards.

[cicabu72]

Re: Samsung BN44-00264A Power Board

Power board (front)

[cicabu72]

Re: Samsung BN44-00264A Power Board

Power board (back)

[budm]

Re: Samsung BN44-00264A Power Board

I need straight shots of the WHOLE board.

[cicabu72]

Re: Samsung BN44-00264A Power Board

[budm]

Re: Samsung BN44-00264A Power Board

You should use attachment function to attach the picture, it takes a long time to see in-line pictures.

[budm]

Re: Samsung BN44-00264A Power Board

What DCV do you have on the Main filter cap CP815/816 when measured between the two legs of the caps?
What resistance do you get on resistors RM869, RM805, RM806?

[cicabu72]

Re: Samsung BN44-00264A Power Board

- CP815 and 816 both stay at 303V
- RM869 = 0.6 ohm
- RM805 = 5 ohm
- RM806 = 5 ohm

[dick_barton]

Re: Samsung BN44-00264A Power Board

Have you checked the fuse FM802S

[cicabu72]

Re: Samsung BN44-00264A Power Board

Yes, as I stated in the first post, the two fuses are both good.

[R_J]

Re: Samsung BN44-00264A Power Board

Check if you have any voltage ACROSS CM804, this will tell you if UM801s is working or not.

[Davi.p]

Re: Samsung BN44-00264A Power Board

Ciao cicabu, questa parola l'ho gia sentita, da bambino... :-)
ma non mi ricordo cos'era...

If you want i have repaired the same board with the same exact behaviour and i have listed it on Subito.it at price of 40. I have replaced the main power oscillator IC, it is the one on heatsink with 5 pins if i remember, i don't remember the name of the IC i have to check when i'm on PC.

[budm]

Re: Samsung BN44-00264A Power Board

check if you have any voltage across cm804, this will tell you if um801s is working or not.

+1

[cicabu72]

Re: Samsung BN44-00264A Power Board

Check if you have any voltage ACROSS CM804, this will tell you if UM801s is working or not.

Between the two legs of CM804 (50V 47pF) I get 2.2 volt at the start, decreasing under 1V after 30 seconds or so. Standby led off, no chance to power it up. Good or bad sign?

If you want i have repaired the same board with the same exact behaviour and i have listed it on Subito.it at price of 40. I have replaced the main power oscillator IC, it is the one on heatsink with 5 pins if i remember, i don't remember the name of the IC i have to check when i'm on PC.

I would prefer to try to fix my board first, thank you for posting your experience.

[budm]

Re: Samsung BN44-00264A Power Board

Between the two legs of CM804 (50V 47pF) I get 2.2 volt at the start, decreasing under 1V after 30 seconds or so. Standby led off, no chance to power it up. Good or bad sign?

I would prefer to try to fix my board first, thank you for posting your experience.

You mean CM804 50V 47uF).

STRW6252 requires 15.5V for the circuit to start, see spec sheet of the IC.
When the power is first applied to the circuit, the IC internal charge pump will charge cap CM808 (22uF 50V, notes: it is typo in the schematic) to 15.5V to kick start the circuit, once it starts running the power for VCC will come from the AUX winding of the STBY Transformer to keep the circuit running.
So check the Voltage on CM808 to see how high it goes up to, if it does not reach 15.5V then you need to check the resistance between VCC pin and ground (or between the two legs of cap CM808) to see if something is loading down the VCC and not allowing it to reach 15.5V. You should also check the cap CM808 capacitance and ESR or just replace it, otherwise the problem can be bad STR6252 itself.

[cicabu72]

Re: Samsung BN44-00264A Power Board

You mean CM804 50V 47uF).

Correct, it was a typo.

So check the Voltage on CM808 to see how high it goes up to, if it does not reach 15.5V then you need to check the resistance between VCC pin and ground (or between the two legs of cap CM808) to see if something is loading down the VCC and not allowing it to reach 15.5V. You should also check the cap CM808 capacitance and ESR or just replace it, otherwise the problem can be bad STR6252 itself.

Powering the circuit, I got only 0.7V between the legs of CM808. I let the circuit discharge and I measure the resistance in between CM808 using a DMM (2MΩ setting). Resistance increase from 0.2 to 2 MΩ and voltage is about 0.25V after 20 seconds with dmm. Do you think I need to replace CM808?

[budm]

Re: Samsung BN44-00264A Power Board

Correct, it was a typo.


Powering the circuit, I got only 0.7V between the legs of CM808. I let the circuit discharge and I measure the resistance in between CM808 using a DMM (2MΩ setting). Resistance increase from 0.2 to 2 MΩ and voltage is about 0.25V after 20 seconds with dmm. Do you think I need to replace CM808?

Resistance from 200K to 2M is good, I was looking for something in 10K range if it is bad.
It sounds like the internal charge pump circuit is not working.
Do you get about 320VDC at pin 1 (D) of the STRW6252?
Check resistance on pin 7 and GND (or across cap CM802).
Check Voltage on pin 7 and GND (or across cap CM802).

[cicabu72]

Re: Samsung BN44-00264A Power Board

Resistance from 200K to 2M is good, I was looking for something in 10K range if it is bad.
It sounds like the internal charge pump circuit is not working.
Do you get about 320VDC at pin 1 (D) of the STRW6252?
Check resistance on pin 7 and GND (or across cap CM802).
Check Voltage on pin 7 and GND (or across cap CM802).

I used the heatsink as a ground reference:
- 294 VDC measured between pin 1 (D) and GND
- infinite value between 7 and GND
- 0.1V between 7 and GND when the board is powered

[Davi.p]

Re: Samsung BN44-00264A Power Board

If you want to know there are not so much components to test in the circuit, i'm gone at exclusion to find the bad Strw