Hi, long time reader, first time poster. Great site. I'm a EE trying to keep my skills up. I've done a lot of stereo repairs but recently came across some LCDs that needed fixing. The first NEC 19" was in my neighbour's garbage and an easy cap replacement. That went to my dad who's BENQ 19" was shutting down "when it was warm". That lead me to all the BENQ repairs here, but this one has me stumped. Here's the sum up:
- BENQ FP937s worked fine then shut down when "warm"
- so i took it, left on until it shut down and then there was a "pop" after the power cable was pulled. One of those component pops, not an AC arc pop.. plus it would have had to be a 1-10A AC pop, and this thing couldn't do that..
- Now 2seconds to off when "cold". won't light backlight at all unless it "cools off" for 10-20 minutes. Super Cold can force this to do the 2s to off immediately. So a bad component somewhere has a bad connection?
- visual inspection shows all caps in tact. Power supply is making good 3.3V, 5.0V, ~16V
- diode checked all transistors: no shorts
- touched up soldering on all inverter transformers and transistors
- removed backlight panel. the CCFLs connected to the "upper" inverter transformer (farthest from the AC plug) always light up for the 2s. The "lower" CCFLs might flicker a little bit. I've swapped the CCFLs around, it is always the CCFLs connected to the lower inverter transformer.
- on the FP937s board I noticed some arcing on an upper CCFL connector - fixed. same issue
- on the FP937s I notice the lower inverter transformer has an open from the secondary pin (there's only one secondary pin?) to ground. All other xfmrs seem to have 200-400kohm. So I turned my focus to the T903 inverter board and did all the same above but the xfmrs check out fine. the PWM IC is getting 16V. Lower inverter still doesn't fire the CCFLs...
So I broke some CCFLs moving them around. That caused me to go looking for another and I came across an identical T903 for $5 that had EXACTLY the same symptoms. So now I have two inverter boards with bad lower inverters!
This has been a great learning experience on switch-mode (especially considering I am a rep for hf power supplies!), but it's starting to frustrate me. Me who is used to having it fixed by now. So I'm reaching out for some help. It's these single "Hight Voltage" xfmrs driving two CCFLs that are suspect to me. Everything I've read here has one xfmr per CCFL but I guess these xfmrs are so good they don't even have to spell right! Pics are of the T903 board.
over to you experts,
inline79
PS. A schematic would be really helpful!
- BENQ FP937s worked fine then shut down when "warm"
- so i took it, left on until it shut down and then there was a "pop" after the power cable was pulled. One of those component pops, not an AC arc pop.. plus it would have had to be a 1-10A AC pop, and this thing couldn't do that..
- Now 2seconds to off when "cold". won't light backlight at all unless it "cools off" for 10-20 minutes. Super Cold can force this to do the 2s to off immediately. So a bad component somewhere has a bad connection?
- visual inspection shows all caps in tact. Power supply is making good 3.3V, 5.0V, ~16V
- diode checked all transistors: no shorts
- touched up soldering on all inverter transformers and transistors
- removed backlight panel. the CCFLs connected to the "upper" inverter transformer (farthest from the AC plug) always light up for the 2s. The "lower" CCFLs might flicker a little bit. I've swapped the CCFLs around, it is always the CCFLs connected to the lower inverter transformer.
- on the FP937s board I noticed some arcing on an upper CCFL connector - fixed. same issue
- on the FP937s I notice the lower inverter transformer has an open from the secondary pin (there's only one secondary pin?) to ground. All other xfmrs seem to have 200-400kohm. So I turned my focus to the T903 inverter board and did all the same above but the xfmrs check out fine. the PWM IC is getting 16V. Lower inverter still doesn't fire the CCFLs...
So I broke some CCFLs moving them around. That caused me to go looking for another and I came across an identical T903 for $5 that had EXACTLY the same symptoms. So now I have two inverter boards with bad lower inverters!
This has been a great learning experience on switch-mode (especially considering I am a rep for hf power supplies!), but it's starting to frustrate me. Me who is used to having it fixed by now. So I'm reaching out for some help. It's these single "Hight Voltage" xfmrs driving two CCFLs that are suspect to me. Everything I've read here has one xfmr per CCFL but I guess these xfmrs are so good they don't even have to spell right! Pics are of the T903 board.
over to you experts,
inline79
PS. A schematic would be really helpful!
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