Samsung UN55FH6200 Intermittent Backlight

[scn101]

Working on a neighbor's LCD TV with LED back lighting. I don't have a service manual but have watched a lot of videos and read posts here. Looking for suggestions on how to proceed.

Back light (BL) may or may not turn on when TV is powered on. BL will sometimes remain on anywhere from a few minutes to up to 20 mins. BL may or may not return.

All testing is with BL set at mid level illumination

BL voltage is specified at 331V (per PCB silkscreen)
BL voltage with LEDs disconnected 395V
BL voltage with LEDs connected 260V

When BL goes off, BL voltage at Power Supply (PS) connector is 0V

BLU on/off = goes to 5V when TV is powered on and remains at 5V even when BL is off
PS on/off = 3.3V when TV is powered and is always present
PWM from main board is stable and always present even when BL turns off

To me it seems like the main board is fine. But why is the BL turning off? I suspect the PS but I'm wondering if the LED strings have an issue like drawing too much power causing the PS to shut them down? 260V loaded seems like quite a drop from the 331V spec but I'm not sure.

TV consumes about 20W when on with no BL
TV consumes about 25W with BL at min and 47W with BL at max

The PS says it can supply 289mA at 331V...which is 95W so that make me think that there's no issue with the LED string

I re-flowed many solder joints on the PS, measured some resistors values that I could get to and no bulging caps, nothing obviously wrong with it.

PS is a PD55AV1-CHS / BN44-00499A

Any test to further solidify where the issue lies? Thoughts?

Thanks!

[CapLeaker]

Re: Samsung UN55FH6200 Intermittent Backlight

back light leds are going bad. turn the backlights waaay down in the menu and see if the back light stays on longer.

[budm]

Re: Samsung UN55FH6200 Intermittent Backlight

Backlights problem, turn down backligts level as low as you can as suggested, if it stays on and the picture is OK then I would use it as is until it goes out completely.

[neilc6]

Re: Samsung UN55FH6200 Intermittent Backlight

Samsungs don't have protection shutdown in the LED driver circuit so the backlight will burn out until they are all out, there is damage to the power supply, or if one LED in the string goes open. Mostly caused by running in Dynamic picture mode which cranks the backlight level to 20/20. Lots of failures in EH, ES and H model years. Most failures in F model years I have seen are mainboard BGA failures. Not in your case since the BL-ON signal stays on.

[scn101]

Re: Samsung UN55FH6200 Intermittent Backlight

Thanks for the responses.

Can you tell me why you think the LED strings could be at fault?

Using a power meter on the AC mains, the LEDs consume about 10W at min and 30W at max BL. If I disconnect the main board to PS cable (making the PWM duty cycle look like 100%...so the PS will deliver full LED voltage) and turn the TV on, the BL voltage is 336V (close to its spec of 331V) and the LEDs consume almost 90W, which agrees with what the PS is designed to deliver; up to 95W to the LEDs. So running in steady state at 50% BL (BL set to 10 out of 20), the PS is only delivering about 1/4 of the power it is designed to deliver to the LEDs.

Since the BLU on/off is at 5V when the BL shuts itself off, I am leaning towards a faulty PS, especially if the PS doesn't contain LED voltage/current protection.

Still open to any further thoughts or comments! I really want to avoid buying a PS if it doesn't solve the issue.

[budm]

Re: Samsung UN55FH6200 Intermittent Backlight

So if you run the power supply board test when the LED are are on, the LED Voltage shows 260V (as stated on your post #1 which is quite low so it may indicate some shorted out LEDS), if you keep the meter at the LED connector when the backlights go off, does the meter jump up to 395V?
Your panel looks to have 98 LED.s so the Vf of the whole LED string is about 313V (98 LEDs x 3.2V) at full If, +about 20V Vdrops on the PWM Controlled MOSFET for maintaining constant current through the load.
BTW, what do you for ground ref. for your meter? Chassis?
https://www.shopjimmy.com/catalogsea...ung+UN55FH6200
LED strips: Notes, there are more than one version of the LED strips used in this TV model.
https://www.shopjimmy.com/samsung-lt...face-board.htm

Power supply:
https://www.shopjimmy.com/samsung-bn...-led-board.htm

[scn101]

Re: Samsung UN55FH6200 Intermittent Backlight

When the LEDs go out, the PS goes to 0V at the LED connector, which is why I'm not suspecting an open LED circuit.

I'm using chassis ground. I measured the resistance from chassis ground to the PS board's ground trace and it is next to zero ohm.

[budm]

Re: Samsung UN55FH6200 Intermittent Backlight

Check the DCV between the two legs of the Main filter cap CP806 to see if the Voltage drops down to less than 350VDC (380 ~ 400VDC is nor working Voltage) when the backlights cut out.
I was expecting to see the LED Voltage to go to 395V if the LED's go open circuit then the LED power supply will go into shutdown mode (0V output) if it has OVP circuit, but it is not what you are seeing that is why I want to verify.
BTW, what is the LED driver IC (16-PIN SMD, I cannot read the designator) P/N?

[scn101]

Re: Samsung UN55FH6200 Intermittent Backlight

CP806 is at 389V. I'm waiting for the BL to go out. It's been ornery this afternoon and staying on. Last night and this morning it was going out regularly.

I'll report back with the cap voltage when BL is off later along with the 16-pin p/n. Will need to pull the board to see it.

Thanks

[budm]

Re: Samsung UN55FH6200 Intermittent Backlight

Hope fully we can get the spec sheet of the IC to see if it has any shutdown circuit.

[nomoresonys]

Re: Samsung UN55FH6200 Intermittent Backlight

While you have the board out inspect it for any cold/cracked/loose solder joints, look closely at components on heatsinks and transformer joints as they take the most abuse.

[scn101]

Re: Samsung UN55FH6200 Intermittent Backlight

Well the BL is not turning itself off over the past few hours.

I have now fed it an antenna signal and the LED voltage with the signal is now around 290V, modulating with the PWM signal from the main board from 260V to just over 300V, which I believe is expected behavior. BL intensity is set a mid-point (10 out of 20).

The LED driver is a Microsemi LX27901 so the PS does have over and under voltage and current protection. But from all my measurements, it really does not seem like the LED string is pulling too much power to trigger the protections. From its datasheet, I could try to monitor the FAULT pin to see if it goes low when the BL goes out, signifying it sees a fault.

Also noted that with an antenna feed, the TV draws about 70W and is rated for 64W average, 130W max, so seems ok here too.

I hit the solder joints one more time to be sure. And will leave the TV on to see if I can capture CP806's voltage when (now if) the BL turns off.

[scn101]

Re: Samsung UN55FH6200 Intermittent Backlight

As one last test I disconnected the LED strip while the TV was on to simulate an open LED string. I wanted to see if the PS would recover from an open fault condition without a power cycle. It does as the BL turned on when I reconnected the LED string to the PS.

If the BL goes out again, I guess a new PS is the plan as I don't see the LED drawing too much power to trigger a true PS over-current fault unless the PS is too sensitive (which I assume to be a faulty PS).

Right now the TV is assembled and running well. BL on! I was also able to update it's firmware after trying four different USB sticks from 2001 to 2004.1 but doubt that'll have any affect to the BL issue.

[budm]

Re: Samsung UN55FH6200 Intermittent Backlight

Too bad you do not have LED tester to verify the Vf of the LED string. I am still suspecting bad LEDs since thebacklights stay on longer at low backlights settings.
I did not study the spec of the LED driver IC yet, but from what you report, the output of the LED supply shutdown and stay at 0V output when the backlights shutdown and BL-ON is still present,that means it is a latched shutdown, but when you perform open circuit test (your post #13), the power supply recovers when load is reconnected so that means the OVP is not a latched shutdown.

[neilc6]

Re: Samsung UN55FH6200 Intermittent Backlight

If you want your backlights to fail, just crank it up to level 20 again. Replacing the power supply is not the solution for bad leds. If your table lamp goes out, do you suspect the bulb has burnt out or the lamp has failed. Same with LED TVs.

[scn101]

Re: Samsung UN55FH6200 Intermittent Backlight

After running properly for many hours the backlight again decides to shut itself off intermittently. I've decided to try replacing the PS given the following observations:

- PWM signal is always present
- BLU_ON is ~5V (signaling the PS to turn on the BL)
- LED supply voltage drops to 0V (indicating the LED string is not open)
- Power consumption of the TV never hits anywhere close to its max rating (so I don't suspect an over-current condition on the LED string)
- BL "on time" doesn't seem to correlate to BL intensity setting, BL will turn off, and again back on, at random times that doesn't seem affected by the BL level setting (1-20)

I should get the PS is a few days and I'll report back. I'm not 100% convinced it is the PS but given the observations it seems the most likely culprit, and it was less than $45.

[CapLeaker]

Re: Samsung UN55FH6200 Intermittent Backlight

Time will tell...

[scn101]

Re: Samsung UN55FH6200 Intermittent Backlight

New power supply has been running now for four days with the TV being on many times for several hours at a time without any BL issues. The old PS would turn the BL off after 5-10 minutes of TV on time just before the new PS was installed. So in this case the PS was the issue. Now solved. Hopefully this thread with the voltage measurements taken, etc., helps someone else with a similar issue.

[nomoresonys]

Re: Samsung UN55FH6200 Intermittent Backlight

Good to hear, thanks for the followup, wonder if you don't just have a bad cap or 2 on the original powerboard??