Re: Samsung 940N 2 Seconds fade to black
Hi guys
Long time lurker here, I don't even remember if I ever posted or not...
Anyway, I have two monitors, 940N and 943N, with this type of problem. They both turn on, then lose the backlight after about a second.
First I cracked open the 940N, and, predictably, there were bulging C(r)apXons on both the MCU and inverter supply rails. Replaced every cap with corresponding Rubies, ZLJ series (CapXons were GL and KM (I think) series). The problem is now downgraded to "backlight off". Measured the inverter transformers, primaries have negligible resistance, secondaries are at about 1.2 kOhm, they differ on the third digit, so it's all neat. CCFLs are good, tested with known good 650V source. Baffled.
Then I took the 943N, thinking I may have more luck with that one. At first the backlight didn't come on at all, but I could see the panel producing a picture without error. Elite caps inside, all good. Took out a couple, ESR is fine, capacity is fine, put them back in (against better judgement, but I figure they aren't the problem, just like the CrapXons weren't in the 940). So I measure the inverter transformer and find that one of the secondaries is open-circuit. So I take a spare one (with ~1.13 and ~1.15 kOhm on the secondaries, a couple of Ohms on the primary), solder it in and... Now I've upgraded the problem to "blank after a second". Baffled.
Then I took to the multimeter. It turns out that the 13V rail (on the 940N) and the 14V rail (on the 943N) are too high! On the 940N this rail reads ~17V, and on the 943N it's around 16.5-16.8V. The voltages are pretty stable, meaning no fluctuations greater than ~2%. I measure the same voltage across the whole path, from the Schottky rectifier to the control board connector.
So, where do I start?
Hi guys
Long time lurker here, I don't even remember if I ever posted or not...
Anyway, I have two monitors, 940N and 943N, with this type of problem. They both turn on, then lose the backlight after about a second.
First I cracked open the 940N, and, predictably, there were bulging C(r)apXons on both the MCU and inverter supply rails. Replaced every cap with corresponding Rubies, ZLJ series (CapXons were GL and KM (I think) series). The problem is now downgraded to "backlight off". Measured the inverter transformers, primaries have negligible resistance, secondaries are at about 1.2 kOhm, they differ on the third digit, so it's all neat. CCFLs are good, tested with known good 650V source. Baffled.
Then I took the 943N, thinking I may have more luck with that one. At first the backlight didn't come on at all, but I could see the panel producing a picture without error. Elite caps inside, all good. Took out a couple, ESR is fine, capacity is fine, put them back in (against better judgement, but I figure they aren't the problem, just like the CrapXons weren't in the 940). So I measure the inverter transformer and find that one of the secondaries is open-circuit. So I take a spare one (with ~1.13 and ~1.15 kOhm on the secondaries, a couple of Ohms on the primary), solder it in and... Now I've upgraded the problem to "blank after a second". Baffled.
Then I took to the multimeter. It turns out that the 13V rail (on the 940N) and the 14V rail (on the 943N) are too high! On the 940N this rail reads ~17V, and on the 943N it's around 16.5-16.8V. The voltages are pretty stable, meaning no fluctuations greater than ~2%. I measure the same voltage across the whole path, from the Schottky rectifier to the control board connector.
So, where do I start?
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