Samsung ln40c630k1fxza no backlight

[curranb]

I'm looking for help with my Samsung ln40c630k1fxza. The set powers up, has sound but no picture. However, shining a flashlight on the screen reveals that there is a picture just no backlight. The last few times the set had been used the screen flickered for a few seconds at startup until a normal picture appeared. Now it will not light.
After reading several threads on this issue in this forum and others I took a chance and replaced the power supply capacitors that have been known to fail in these sets. Unfortunately there was no change.
I've downloaded the schematic for the power supply (bn44-0034b) from this site. Below are the readings at the connector that runs to the main board:
Pin Desc. Reading
1. B13v. 13v
2. 5v_DET. 0v
3. B13v. 13v
4. BLU_ON 2v
5. B13v. 13v
6. E-PWM. 5v
7. GND. 0v
8. H_SYNC. 0v
9-10. GND. 0V
11. B5v. 5v
12. GND. 0v
13-14. B5v. 5v
15. B13v. 13v
16. A5v. 5v
17. B13v. 13v
18. PS_ON. 1.5v

I also took measurements at the connector to the inverter board

1. Vcc B12v. 13.24v
2. FB2. 0v
3. FB1. 0v
4-5. GND. 0v
6. LD. 12.38v
7. LD. 12.38v

All readings were with the tv powered up. Any ideas or next steps?
Thanks.

[budm]

Re: Samsung ln40c630k1fxza no backlight

"bn44-0034b" Recheck the P.N, it is not correct number; we also need to see the picture of it too. It may be just bad dc blocker/coupling cap.

[curranb]

Re: Samsung ln40c630k1fxza no backlight

Thank you for the reply. Power Supply Model is I40F1_ADY. Code is BN44-00340B.
Please see the attached photos. The caps I've already replaced are the 9 in the lower left corner of the 1st and 3rd pics and the upper right of the 2nd pic.

[budm]

Re: Samsung ln40c630k1fxza no backlight

I am talking about DC blocker/coupling cap CI802 in the inverter section.
Fuse FI801S is good?
If you turn the TV on in the dark room, do you see the backlights even flash on for seconds?
The BLU-ON is a little on the low side.
Do you have about 390VDC measured right between the two legs of one of the two yellow main filter caps when TV is turned on?

[curranb]

Re: Samsung ln40c630k1fxza no backlight

I will check the capacitor and fuse when I get home tonight. I had checked the voltage of the two large yellow capacitors in standby and on as described in a previous thread and found both readings were good. I will also double check.
I've started the tv with room darkened several times and haven't seen any flashes - the screen is always pitch black.

[curranb]

Re: Samsung ln40c630k1fxza no backlight

OK. I removed the fuse and cap you mentioned and both were good. Any other ideas? I really appreciate any thoughts you may have.

[curranb]

Re: Samsung ln40c630k1fxza no backlight

And I rechecked the yellow caps and was getting 385v at each.

[budm]

Re: Samsung ln40c630k1fxza no backlight

OK, so the boosted Voltage is OK, one thing we can try is to bypass the PFC Under Voltage protection, see the picture attached.
You can short out the photo transistor of the Opto PCS804S to see if that will allow the Inverter circuit to come on or not, we know that the PFC Voltage is normal so it is OK to bypass the protection just to be sure that we do not have faulty protection circuit due to bad Opto or resistors that drifted their values. Make sure to look at the diagram really well to make sure you do that to the correct Opto on the board.

[curranb]

Re: Samsung ln40c630k1fxza no backlight

Great. I"ll give that a try tonight. Thanks.

[budm]

Re: Samsung ln40c630k1fxza no backlight

OPTO LTV817M spec.

[curranb]

Re: Samsung ln40c630k1fxza no backlight

OK. I shorted the pins you mentioned together - still nothing. Not sure if this helps but I took voltages at the 4 pins of the photo coupler and they are as follows (using the pin numbers found on the photo coupler data sheet):
1 -54.7 V
2. -55.8V
3. -6.9 mV
4. -7.2 MV
Note that all voltages are negative with regard to ground.

[R_J]

Re: Samsung ln40c630k1fxza no backlight

to measure the voltage correctly, pin 1 & 2 are reference to HOT ground (ie black lead of meter on negative of cp801/803) Pin 3 & 4 you can use the chassis as ground
If you disconnect the main board from the power board (cn802) and plug in the power, do the back lights come on? (not sure if this works on this model but worth a try)

[curranb]

Re: Samsung ln40c630k1fxza no backlight

Thank you for your help RJ. Using the - of CP801/803 as hot ground
Pin 1 = 3.0 V
Pin 2 = 1.9V
Using chassis ground
Pin 3 = -6.9 mV
Pin 4 = -7.2 mV
I've tried disconnecting the main board from the power supply but it doesn't light.

[budm]

Re: Samsung ln40c630k1fxza no backlight

Thank you for your help RJ. Using the - of CP801/803 as hot ground
Pin 1 = 3.0 V
Pin 2 = 1.9V
Using chassis ground
Pin 3 = -6.9 mV
Pin 4 = -7.2 mV
I've tried disconnecting the main board from the power supply but it doesn't light.

That cannot be right, pin 3 and pin 4 per spec sheet of the opto are the Photo Transistor side which are in the cold side of the circuit, and it is fed by 5.2 power supply through QI815 (KRA104S) which should be on to supply about 5V to the EN pin of the backlights PWM IC.
So to have mV at pin 3 and pin 4 it can either be that QI815 (KRA104S)is bad or it is not turned on by QI814 (2N2222, NPN).
Start checking DCV on those two Transistors when BL-ON is present.
Notes: those pin number assignment in the diagram may not match the spec sheet of the Transistors.
You do have 5.3V in that section of the circuit, right?
See schematic for more details.
BTW, I see that the power supply has pull-up resistors RS861 for the PS-ON and RI868 for the BL-ON which means you can self-test the power supply board by itself without having the cable connected between the power supply board and the main board.

[curranb]

Re: Samsung ln40c630k1fxza no backlight

I double checked the voltages at pins 3 and 4 and they are definitely what I previously posted. In the attached picture pcs804s is the nearest photo coupler in the row of four and pins 3 and 4 are on the right. Out of curiousity I measured the right hand pins on the other 3 photocouplers (cold side) and the readings were between 1 and 5 volts. The transistors you mentioned are on the backside of the board and very small - if they're in need of replacement I think that would be beyond my soldering skills! If you believe that readings are indicative of a power supply board problem and not an issue with another component (i.e. main board, etc) I'm thinking the best solution might be for me to just replace the PSB. What do you think?

[budm]

Re: Samsung ln40c630k1fxza no backlight

That is correct, the right side is the cold side of the circuit and they are pin 3 and pin 4 so I expect to see about 5V on those pins when BL-ON is present. So it is either the 5.3V power supply is not working or bad Transistor. At this point based on your Voltage readings, the power supply board does have problem in that section.

[curranb]

Re: Samsung ln40c630k1fxza no backlight

Do you have any recommendations of where to purchase a new or refurbished PSB?

[budm]

Re: Samsung ln40c630k1fxza no backlight

I use shopjimmy.

[curranb]

Re: Samsung ln40c630k1fxza no backlight

Thank you very much for all your help. I really appreciate it.