Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schematic

[ngth82]

Good morning all!

I have a Samsung UN40H6203AFXZA that has a constantly blinking power light. The backlight doesn't turn on. The power board looks like it has heated up enough to discolor the four vertical axial lead resistors in the center of the board. (See pics) After an initial check, I think the power board is bad.

The power board outputs 0V to the main board when the backlight power connector is connected (connector CNL802). The two large caps on the hot side only show ~5V across it when everything is connected.

However, when the backlight power is disconnected, i get 13V @ CNM803 to the main board. The two hot side caps charge to ~150V

I haven't checked if the LED string is shorted yet.

Does anyone have a service manual or schematic for the UN40H6203AFXZA TV, or specifically the BN44-00769A power board?

I found a replacement board for ~$30 shipped, but i figure its worth a shot to take an initial stab at fixing it.


Thanks in advance!
-Thomas

[budm]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

Something is not making sense to have 5V across the main filter cap, for it to drop from 160VDC to 5V the load must be really HEAVY or the main filter cap just cannot maintain the Voltage.
This power supply board is an always on power supply with some of the output are switched on when PS-ON is present.
You should check the ACV between the two inner pins of the bridge rectifier to see what you have when the LED connector is not in place and when it is in place.
I would inspect the solder joints of all the parts which are soldered to the through hole rivets, those joints are know to have cold solder joints.
The board is BN44-00769A.
Pictures provided by shopjimmy.

[ngth82]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

Hey Budm -

As usual, thank you for the replies. I'll take a look after work. I should be home by 7pm PST. I didn't have much time to look at it yesterday but should have some time today and the remainder of the week.

I'll check the bridge rectifier when I get home w/ and w/o the LED connector plugged in.

I didn't get a chance to remove the board yesterday, but i'll definitely check the solder joints on this.

-Thomas

[budm]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

The ACV reading between the two legs of the bridge rectifier will give you the clue when you have/have not LED connector in place.

[ngth82]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

Hi Budm:

I ended up working from home today. We have some freezing weather coming to Washington tonight.

I measured the AC pins of the bridge rectifier (two middle pins -- pin 2, pin 3). Here's what i found:

w/LED string disconnected:

1. AC pins of bridge rectifier reads: 120VAC (as expected)
2. DC pins of bridge rectifier reads: 160VDC (as expected)
3. LED connector voltage reads: 200VDC (???). Voltage on the power board says its supposed to be 160.
4. 13VDC provided to Main board
5. PWM_BLU pin is 1.6VDC
6. PS_ON pin is 3.3VDC

w/LED string connected:

1. AC pins of bridge rectifier reads: 120VAC (as expected)
2. DC pins of bridge rectifier reads: 160VDC (as expected)
3. LED connector voltage reads: 0VDC (???). Voltage on the power board says its supposed to be 160.
4. 3-5VDC provided to Main board.. appears to oscillate (see below)
5. PWM_BLU pin is 0VDC
6. PS_ON pin is 0VDC

So one thing i didn't notice before is the sound the TV makes -- almost like an analog clock ticking. Seems like the power board is over currenting and retrying. This is confirmed by looking at the 13VDC signal with my oscilloscope. There is a 4V DC bias, and tries to start up every 1s. The supply ramps up to 6VDC before shutting down...

Notes:

1. The PS_ON rail at 3.3VDC would seem right to me (assuming 3.3VDC logic levels).
2. The PWM_BLU is the PWM signal for the "backlight unit" -- at a 33% positive duty cycle by default

Any ideas?

1. Could the LED string just be bad? (it did not measure as a short across the + and - terminals when disconnected)
   

Thanks!
-Thomas

[budm]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

"The two large caps on the hot side only show ~5V across it when everything is connected." So just to be sure your post 1 is wrong then?
Now you are showing 160vdc with everything connected.

[ngth82]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

I would have to assume that my first post was incorrect since i checked and double checked the DC output from the rectifier. From what i recall from other TV boards, the rectifier output is directly connected to the 400V caps.

I would sincerely hope the TV doesn't have multiple failure modes... :/

I'll go back and look again after my work day is over from the home office.

[ngth82]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

Hi Budm -

I can confirm that the caps are charged to 160VDC and the rectifier bridge output is also 160VDC after looking at it a third time.

I looked through a couple of other forum posts on badcaps for other Samsung TVs. Seems to me that disconnecting the power to the main board should cause the TV backlights to turn on. In this case, they remain off.

I think the problem exists with either the power board or the LED backlights. Is there a good way to isolate the problem with out gambling on purchasing items?

I have a benchtop power supply that can provide up to 80VDC... can't do 160VDC. So... other than disassembling the screen and checking LEDs individually, do you or anyone else have any other suggestions?

Thank you,
Thomas

[budm]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

OK, this TV use one big string LED.
1) Put the LED connector back in place.
2) Leave off the cable between the main board and the power supply board.
3) Keep the black probe of the DCV meter to the chassis.
4) Keep the red probe of the meter on the either pin of the D+ pin of the LED connector.
5) Plug the TV power cord into the AC outlet and see if the DCV will jump up close to 200VDC or not, then wait a few seconds to see if it will drop down to much lower Voltage.

[ngth82]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

Hi Budm:

OK. Here's the latest results per your test suggestion:

The LED connector is back in place; the power connector to the main board is disconnected. Digital ground is connected to the chassis as the reference point.

The auto-range function on my DMM shows it's fluctuating up to about 2.1VDC every ~1 second or so (Picture 1).

Probing it with my oscilloscope shows that the LED voltage rises up to 8VDC for approximately 175ms before shutting down (Picture 2).

The measured positive duty cycle confirms the power supply retry is approximately once a second. This process constantly repeats.

-------
In the same setup, disconnecting the LED string will bring the voltage up to 177VDC once the TV is plugged into the AC outlet. This voltage remains at 177VDC with no LED load and no main board connected. (Picture 3).

===

Bad LED string?

[budm]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

OK, with the LED connector still in place, check the resistance between chassis GND and the D+ to see if it shows very low resistance.
At least the boost converter is working when load (LEDs) is disconnected..

[ngth82]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

Lol... yep. The D+ to the chassis GND shows 7.1 ohms. In contrast, when the load is disconnected, the resistance is several mega Ohms. Seems like then the + voltage side of the string is shorted to the chassis at some point in the string. Do yo concur with that?

-Thomas

[budm]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

Yep, you found the problem. I have seen the wires going into the panel got nicked by the chassis. Now the bad part is if it is inside the panel then you will have to take the panel apart to inspect the wires, etc.
You can try moving that bundle of wires around to see what happen.

[ngth82]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

As always, thanks for the support there budm. That's great. I'll start to remove the boards this evening and see if i can get the reflector panel removed without too much trouble.

I'll post back when i get somewhere (or if i run into issues).

Testing individual LED strings shouldn't be too bad

-Thomas

[ngth82]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

I disassembled the panel. There are three LED strings in all - the top right LED on the upper most string ("string 1") has blown and shorted out. Isolating that upper string shows a 7.1 ohm impedance between "+" and the metal plate on the back of the string (note that the metal plate on the back of the string doesn't seem to be the "-" side of a "good" string either - good as in the other two strings that visually look okay). I was able to remove the blown LED and cleaned up the charred material on the end. This cleared the 7.1ohm low impedance path

The short has been cleared, but when i power up all three strings with the TV supply, the LEDs do not illuminate. I see 177VDC at the LED connector. I traced the LED connector circuit... looks like these are really wired as one single string (hence the silk screen markings D+, S1-, S2+, S2-, S3+, and D-).

More tomorrow i guess.

[budm]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

The three strips are connected in series to form one long string so if one LED goes open circuit then the whole string will not light up. If you look at the traces of the LED connector pins you will see how they are joined together to put the three strips in series connection.

[ngth82]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

Hi Budm -

I was able to get the three strings individually tested. 6 LEDs from the top string are dead (the same one with the charred LED/PCB/trace), 6 additional LEDs from the middle string are burnt out as well. The bottom string appears to be okay. See pics.

I wired up a string of discrete LEDs (the red ones) for testing and have since removed them after fixing traces on the top string.

Time to put the TV back together and see if there is anything else broken (or if I broke anything). If everything appears to work, i'll spring for a replacement LED string set.

Thanks!
-Thomas

[ngth82]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

Tv has been reassembled. It works for now. I'll look for a replacement set of LED strings... hopefully the cost can be justified.

[vinceroger69]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

Thanks for the update and glad you found the fault and fixed it, that was a lot of failed bad leds in one go with the damage as you say if you can find some cheap strips i would change them too.

[ngth82]

Re: Samsung UN40H6203AFXZA power light blinks; no video; to BL need power board schem

Also FYI for anyone that is salvaging LED strings with charring:

The top strip (the one with the blown LED) required Kapton tape on the back of the strip to prevent it from shorting to the chassis. Everything worked fine when it was out of the chassis, but the lights would blink (and the power supply would over current) when it was in the chassis. The charred FR-4 material is semi-conductive.

I found a new set for about $100 bucks. I'll contact the TV owner and see if they want it repaired; or just expect the TV backlight to eventually fail and we're back at square one.