Samsung S24B300 LED backlight flickers like candle

[CapLeaker]

One of my Samsung monitors a S24B300 was behaving strangely for about 6 months or so now. When turning on, the back light was flickering for about 30 seconds or so, after the monitor was roughly ~ 2hrs on STBY. It wasn't a on / off flicker, more like a erratic brightness of the LED back lights, like a candle around air flow.
It drove me nuts enough today, so I decided to repair the issue.

I replaced the HDMI cable and checked the PSU and ended up with no improvement.
So after opening the sucker up, there was absolutely nothing to see with the eye. Bad capacitors? You bet! There are two electrolytic SMD capacitors C806 and C807 (50V 10uF) that are in series and feed the LED back lights (on the top left in my picture). Those caps had a high ESR value, so I changed them out to Nichicon PW series of the same value.

Happy to report, the back light flickering is gone. Another one saved.

[stj]

Re: Samsung S24B300 LED backlight flickers like candle

you should have changed the other 2.

when i do a monitor i change EVERY electrolytic.
that way i wont have to open it again!

[budm]

Re: Samsung S24B300 LED backlight flickers like candle

I guess they are using two 50V cap in series due to the LED Booster Voltage circuit (L801, Q802, D801, and drive circuit) is higher than 50V for driving the LEDs, It is basically 5uF 100V but without balancing circuit, each cap may not get equal Vdrops.

[stj]

Re: Samsung S24B300 LED backlight flickers like candle

5uf?
i didnt know series caps divided the capacitance.?

[CapLeaker]

Re: Samsung S24B300 LED backlight flickers like candle

you should have changed the other 2.

when i do a monitor i change EVERY electrolytic.
that way i wont have to open it again!

I've got a real good ESR meter and the other caps are good for quite some time yet. Until those puke, something else goes first. I always check every electrolytic cap on a board I have out.

I guess they are using two 50V cap in series due to the LED Booster Voltage circuit (L801, Q802, D801, and drive circuit) is higher than 50V for driving the LEDs, It is basically 5uF 100V but without balancing circuit, each cap may not get equal Vdrops.

Personally I don't like having electrolytic caps in series at all. However, I didn't have a 100V cap on hand. Good quality 50V caps, I always have some stock. So they went back in like the circuit was designed. The monitor is about 4 years old now. It will be good for another 4 years now.

[CapLeaker]

Re: Samsung S24B300 LED backlight flickers like candle

5uf?
i didnt know series caps divided the capacitance.?

yep they do.

[Agent24]

Re: Samsung S24B300 LED backlight flickers like candle

5uf?
i didnt know series caps divided the capacitance.?

They do. And the equation for capacitors in series is the same as that for resistors in parallel. Vice-versa also applies!

[budm]

Re: Samsung S24B300 LED backlight flickers like candle

I think STJ is joking, he has been around electronics (designing and fixing) for a long time.

[stj]

Re: Samsung S24B300 LED backlight flickers like candle

not joking, my speciality is digital design / repair.
i prefer a big line of 0's & 1's to all the analog crap!