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.
scn101
New Member
Last Activity: 02-18-2023, 07:52 PM
Joined: 02-06-2016
Location: Dallas, TX
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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...Leave a comment:
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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...Leave a comment:
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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...Leave a comment:
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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.
ThanksLeave a comment:
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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.Leave a comment:
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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...Leave a comment:
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Samsung UN55FH6200 Intermittent Backlight
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... -
Re: New Members - please post your introductions here
Hello all,
Graduated as an EE many, many years ago. I truly enjoy fixing things and saving them from the landfill. White goods, cars, TVs, anything broken often presents me with a new opportunity to learn.
There's some super smart people on this forum and many great knowledge building posts to read.Leave a comment:
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