Anyone have experience with ccfl/inveters in TVs? - Page 1
Forum section: electronics / repair

author: Bobblank77
author_rank: Contributor
timestamp: September 16, 2016, 05:29:54 am
content: Hey all, I bought a "Broken" TV for really cheap and am trying to fix it up. So the problem is that the ccfl backlight shuts off after ~30 mins of being on. Here's what ive found outAll the caps other components on the main power supply board look fine. No obvious bulging/burn marksIt looks the the main board provides high voltage AC to the inverter boards which also have a bunch of transformersThere is nothing obviously wrong with the inverter board that I have access to (I havent taken a look at the other one as it is below the LCD - kinda scared to move it)The signals coming from the main board to the power supply are normal (Inverter on pin is high, PWM for brightness is working fine)High voltage caps on main PS board remain charged at ~380v even when ccfls are shut offAnyone have any idea what this problem could be? I'm thinking its not the ccfl tubes themselves as the backlight definitely turns on.Also there is a ribbon cable between the inverter and power supply with markings vcc, FB1, FB2, gnd, and LB. Any idea what these mean? FB are always 0v when i measured and LB is 12vI'm not sure how to test the inverter board when its on because I believe that it is too much for my multimeter. Any ideas?Thanks everyone!

author: paulhm81
author_rank: Regular Contributor
timestamp: September 16, 2016, 06:36:20 am
content: FB should be feedback. If you have a spare ccfl you could try to test it by placing it instead of the original one by one. Usually it shuts when it senses something wrong in the feedback. It could also be one of the inverters. If it works with the spare ccfl you should look at the end of the ccfl for burned solder joint or burned isolation. If the isolation is burned it could ark to the chassis and the feedback would kick in.

author: Bobblank77
author_rank: Contributor
timestamp: September 17, 2016, 02:32:22 am
content: Thanks! I will take a look at it!

author: tautech
author_rank: Super Contributor
timestamp: September 18, 2016, 07:56:23 am
content: Got a monitor that does similar.Improved but not yet completely fixed but it's a good bet to work through replacing the PSU secondary side caps and those on the supply rail for the backlight inverter.Basic cap value checks will give some idea as to those thatneedreplacing and I set a limit of ~5% below labeled rating should be replaced.

author: Bobblank77
author_rank: Contributor
timestamp: September 25, 2016, 06:20:55 pm
content: I don't think it would be the caps. They all look fine. I did a bit more poking around and I found that if I have a fan blow across the power supply the TV stays on for much longer (14+ hours rather than 3). So it looks like the heat is causing something to fail. After the TV backlight goes out I checked the high voltage input between the PSU and the inverter and got nothing. My plan is to go backwards from there to try and figure out whats going on. Do you know if MOSFETs can temporarily fail open due to heat? Maybe a bad solder joint between the die and pins that opens the circuit due to thermal expansion?

author: tautech
author_rank: Super Contributor
timestamp: September 25, 2016, 07:30:29 pm
content: Quote from: Bobblank77 on September 25, 2016, 06:20:55 pmDon't be deceived, they can look perfect and be completely f***ed.A basic uF capacitance test with a DMM will find the culprits, any 5% from nominal value....biff.QuoteA pic of your PSU might help with some more ideas.QuoteQuite possibly solder joints.....seen these types of intermittent faults before.A PCB in a small Mig welder had a heat related fault, I was positive it was a cracked joint but could never find it, even with magnification but a hand reflow of ALL the componentry fixed it.

author: Bobblank77
author_rank: Contributor
timestamp: September 25, 2016, 09:52:22 pm
content: Okay Ill check the caps when i open it again. Ill post a pic too.

author: Bobblank77
author_rank: Contributor
timestamp: September 29, 2016, 11:01:51 pm
content: Quick question for you Tautech. Do I have to desolder the caps before taking measurements?

author: tautech
author_rank: Super Contributor
timestamp: September 29, 2016, 11:14:09 pm
content: Quote from: Bobblank77 on September 29, 2016, 11:01:51 pmBest to.Depends sometimes as to how many caps are on the same rail/track.Since Ecaps are a common point of failure there's often no time or effort wasted by removing for tests as you mostly have to replace then anyway.Sometimes when the solder is sucked away, the lead hole is large enough to give disconnection of 1 pin and that's all you need.In the old days when PCB's weren't so densely populated and mostly one sided an accepted technique was to isolate a component by cutting a track and resoldering after testing. Occasionally I still do so.Known as the razor blade technique.

author: Bobblank77
author_rank: Contributor
timestamp: October 02, 2016, 07:51:43 am
content: Here are some pictures of the board. I took measurements of the caps.Red one measured 625nF when its supposed to be 684nFGreen boxed ones are wired in parallel. Measures 183 uF when its supposed to 200 uF.Blue ones are 80pf each spot on.Should I be replacing any of these?Ill upload what ive figured out of the schematic soon

author: Bobblank77
author_rank: Contributor
timestamp: October 02, 2016, 08:05:01 am
content: Here's what i could figure out of the schematic. Sorry its not much

author: tautech
author_rank: Super Contributor
timestamp: October 02, 2016, 08:10:21 am
content: The inverter (top right) is likely sourcing it's supply from the LV (top left) and that's where I'd be focussing my efforts on those LV caps.If you DMM has sufficient frequency range and you can get to the LV rails with it powered on you might be able to measure the ripple on the AC range. You'd want to see much less than 100mV ripple on the DC rails.It's likely each rail's voltage is marked on either the PSU PCB or the main MB so try to see each rail is up to voltage.

author: Bobblank77
author_rank: Contributor
timestamp: October 02, 2016, 08:14:10 am
content: What are LV caps? Low Voltage? should I be looking at those electrolytics on the top left?

author: tautech
author_rank: Super Contributor
timestamp: October 02, 2016, 08:56:25 am
content: Quote from: Bobblank77 on October 02, 2016, 08:14:10 amYep x3

author: Bobblank77
author_rank: Contributor
timestamp: October 03, 2016, 03:22:23 am
content: Just measured all the LV caps. Everything checks out. Also no jitter on the 12V rail

author: Bobblank77
author_rank: Contributor
timestamp: October 05, 2016, 02:48:20 am
content: The gates of the mosfets that drive the HV transformer are connected to a smaller transformer (Just north of the red boxed capacitor in the picture). I measured the AC voltage across this transformer and it went from 2.8v to 0.7v to 0.1v when the backlight shut off. I guessing that whatever is controlling this is at fault for the backlight failure.