Dell U2212HMc monitor power board repair or ad hoc replacement - Page 1
Forum section: electronics / repair

author: ashlin4010
author_rank: Newbie
timestamp: December 12, 2017, 12:25:42 am
content: I have a Dell U2212HMc monitor my goal is to repair it to a mostly functional state.The problem is the power supply has failed and because I'm not the greatest at the electronicsand I suspect multiple parts have failed I have decided to try and replace the functions of the board.The power board (715G4542-P03-000-0H1H) has three main functions. It supplies 5 volts to the controller board, some audio stuff to deal with an external sound bar (irrelevant) and the LCD panel driver.The LCD panel driver is the only part of the board I'm interested in. Thankfully it is nicely isolated so it's easy to see what traces are going into that area. Unfortunately I've been unable to identify what two of the traces do I believe one of the traces is for a lower voltage for the driver chip and the other is a higher voltage for the LCD panel. The ones I have identified are ground and some dim controls. I've tried to find schematics but I've been unable to find some.So I have came to this forum in a last ditch effort to try and fix or replace this board I will attach some other stuff I have found.https://imgur.com/a/vBf7S\http://www.avnet-embedded.eu/fileadmin/user_upload/Files/Displays_NEW/Colour_TFT/LG_Display/LG_Display_LM215WF3_SLC1.pdfSo what I would like from you people is you help, advice maybe some schematics, anything really I've done as much as I can on my own. you need any information from me definitely ask you can get me on discord at ashlin4010#9723Thank you

author: Kilo Tango
author_rank: Regular Contributor
timestamp: December 12, 2017, 10:32:17 pm
content: Hi, welcome to the forum.It would be handy to have as much diagnostic info as possible, eg you say the power supply is at fault, how did you come to that conclusion ?.I know from experience that the backlight system can be fussy and if the backlight bulbs are worn out, then the backlight chip just shuts down and so no backlightWith the power supply, first let it sit disconnected from the mains for 5 mins to allow any capacitors to discharge. Not having a circuit is a pain, however if you can identify the power supply controller chip you can look up the data sheet for it which give you an idea of how it works. A check for faulty bulging caps is a good idea, they tend to fail after a long time in service, they dry out. Also look for anything that looks broken, burnt, any fuses blown ?.Next being very careful about live voltages on the power supply ( the big storage caps can be up at 375V DC ) check power rails are there and not loaded up, eg 5V is within 10% ish. The most common rails used are 5V, 3V3, and 12V so look for them, Basically look for the output connector, there should be several power rails fed there.Do this and then report back.Cheers Ken

author: ashlin4010
author_rank: Newbie
timestamp: December 12, 2017, 11:30:41 pm
content: The original fault occurred when the back of the LCD panel which is metal came into contact with the negative leg of a 450V 180 (Mu)F capacitor. So I'm not certain of the LCD panel has survived but I hope. As for the power supply the fuse has blown and a thermistor in series with it had also gone. So because I did not have a spare fuse or thermistor laying around I completely bypass them which I know is probably not the best idea but I got this thing out the trash anyways so I thought it was worth a shot and of course they didn't work it trips the breaker every time I try to turn it on. So after look at the board very closely I believe I did find a connection between one of my unidentified traces and a 5V rail. As for the other it goes through a fe YG868c15 componenthttps://ae01.alicdn.com/kf/HTB1spWAKFXXXXXVXXXXq6xXFXXXJ/50PCS-New-original-YG868C15R-TO-220F-Schottky-rectifier-150V30A-YG868C15.jpg_640x640.jpgunfortunately I don't know enough to know what was component is or does and can not find much about it.thank you

author: Kilo Tango
author_rank: Regular Contributor
timestamp: December 13, 2017, 11:02:48 am
content: Well that shows a mistake on my part, I should not have said measure the power rails without first getting a complete picture of what had gone on before. Bypassing fuses is never a good idea, a fuse is a protection device and it protects your house wiring and you from faults. Maybe you might do it temporarily if you had repaired the unit and wanted a quick test, but not hoping a fault has magically gone away.In view of what you have said, I would avoid trying to fix the power supply; it's likely blown up several parts and they can be dangerous to work on without a lot of experience, plus you need a properly equipped lab and someone to pull you off if you get an electric shock. They reckon 50V is enough to kill you, well there is 375V on the storage cap so extremely dangerous. In fact if you look at a typical switched mode controller circuit there are many places that will normally have dangerous voltages on them and in case of a fault the whole thing could be live.The best thing might be to look for another 2212 with a smashed screen and use yours for parts to repair it.This isn't a chicken way out, its called "I enjoy life to much without taking unnecessary risks". Google "what voltage kills you" and have a read.Cheers Ken

author: ashlin4010
author_rank: Newbie
timestamp: December 14, 2017, 09:43:20 am
content: oh I should have stated this more clearly I'm not actually interested in repairing the board as such I'm only interested in this one very small section. The plan was to identify what the traces are doing that are going into this very specific part of the board which I have done with the exception of one or two then remove the part of the board I'm interested in and connect an separate external supply. Which I am confident I can do. I just hope that this very important part of the board still works but I believe it well because the LCD panel Control Board survive. As for your concerns about my safety I am aware of the dangers of high voltages and capacitors and am taking care to not get my self killed. All I need is to know what these traces are. And as for replacing the power board that's not really possible cause I live in New Zealand I have to be super lucky to find one of these monitors.

author: Kilo Tango
author_rank: Regular Contributor
timestamp: December 14, 2017, 02:45:37 pm
content: OK, how about getting some photos of the inside of the monitor ?.As an example here are some pictures from one of my U2410's. You can see the "L " shaped separate power supply section and the various interconnect cables. You can work out what they are for, eg the connector on the right is mainly the power feed to the interface board and those signals should be mainly power, although there might be a start up/shut down signal from the logic to the power supply. The 2 multi legged ICs flat on the board are most likely linear regulators, by checking the part numbers you can find out what voltage they are, and trace their output lines to the connector. They sometimes use multiple wires to give more current handling, so although there are 14 wires, there may only be 3 or 4 power rails.The component you gave a picture of, looks like a big dual diode, maybe to supply the 5V from the switch mode transformer ?. I would expect to see a large capacitor next to it.Ken

author: ashlin4010
author_rank: Newbie
timestamp: December 14, 2017, 10:02:28 pm
content: ok so that's how everything is connected of course they're not just sitting on the back of the monitor like that the board set in a metal frame. You might noticed the wires going from the controller board to the power board have some black thing attached to it that is a Molex connector I added so I could test the Control Board still works(I think it does).https://imgur.com/a/Xl4bZ

author: Kilo Tango
author_rank: Regular Contributor
timestamp: December 15, 2017, 11:48:46 am
content: Good pictures.On the diagram where you labelled components, I am bit surprised by the T4AL/250V fuse over on the LHS, all the mains input stuff looks to be on the lower RHS. and the 33uF 100V cap suggests there is some high volts here. However it might just be a fuse for the LED backlight supply, curious but ... YG868C15 feeds the 3 off 1000uF caps, I would guess that's the LED backlight supply.I would try feeding 5V into the control board, don't bother with the backlight initially. You should see the power LED light on the front panel go green. I would then feed in a video signal, and if the monitor doesn't go into standby that means its working. If you shine a torch on the display you should see a faint picture.a good site on testing and driving LED backlights, technibble.https://www.technibble.com/forums/threads/guide-how-to-test-and-use-an-lcds-led-backlights.38262/I would guess the backlight supply to be 12V, and you could possibly run the backlight controller part of the power supply board on its own. Interesting thought, the fuse that's gone appears to be for the backlight, maybe that's the main fault with the unit ?. OK shorting out the main storage cap to the metalwork is not a good idea, but it shouldn't damage the rest of the circuit. If the backlight part doesn't work and keeps taking too much current, I would check that 3 legged device on the bottom of the board, maybe its dead.Ken

author: Kilo Tango
author_rank: Regular Contributor
timestamp: December 15, 2017, 11:54:33 am
content: Ah just found out LED backlight strings are often 35V that's why there is a 33uF 100V cap.It might be the backlight has gone short and taken out the fuse. Someone else on the forum said they tended to go.this is getting interesting !.Ken

author: ashlin4010
author_rank: Newbie
timestamp: December 16, 2017, 04:04:02 am
content: Good news and bad news. I wired up all the traces I knew and for the one trace I didn't know I connected it to 5 volt because it was obvious it wasn't ground. So I powered it up and it worked...for about 10 seconds.What happend. I powered it up and the back light came on but began to flicker not very fast I was thinking maybe it's doing that because it has no PWM input and was just confused. But a couple seconds later two resistors went up and smoke and it stop working. After that I switched off the power and and check to see if any of the components got hot and two did the LCD driver chip and a P8008BD(MOSFEThttp://www.unikc.com.cn/UploadFile/products/2015713152034-P8008BD-REV1.2-20140611.pdf).So I have there theories as to what went wrong.5 volts was too much,Should have not been a voltage at all should have been something totally different orThe world hates me.Anyway I have a pictureshttps://imgur.com/a/41FVFSo what went wrong and what's next? besides fixing the resistors obviously.Thank you

author: Kilo Tango
author_rank: Regular Contributor
timestamp: December 16, 2017, 09:40:12 pm
content: Hmm never mind. I would have thought the backlight controller would have used 12V as a supply, its unlikely to use capacitors rated at 25V if the rail is only 5V. Can you tell us the part number of the backlight controller chip, the data sheet will tell us the supply rail.I would also be inclined to try and check the backlights via the ribbon cable for shorts or low resistance; as I said earlier, the fuse blowing suggests there is something wrong with the backlights.It might be that one string is faulty, the circuit is trying to light both, one works but the other is shorted, hence the flickering, over current and bits blowing up ( which wouldn't happen if the fuse was still working .......)However the good news is the LCD and video side of things is probably working OK, so even if you had to bypass the backlight controller you could still get it to work.Ken

author: ashlin4010
author_rank: Newbie
timestamp: December 16, 2017, 11:16:49 pm
content: The backlight driver is OZ9998AGN I've been unable to find the data sheet for that exact chip this is what it looks likehttps://sc01.alicdn.com/kf/HTB16DOJMpXXXXa4XVXXq6xXFXXX1/OZ9998GN-OZ9998AGN-SOP24-LCD-backlight-chip-imports.jpg.The part of the board that actually drives a backlight has a fuse in total there are three fuses on the power board the fuse that blew was between the mains voltage and the Transformer. I'm do you think the P8008BD or the backlight driver survive? Because if they haven't there's nothing really else I can do with the exception of replacing every component that's broken which could be very expensive considering I got this monitor out of the trash. Anyway I have replaced the resistors which ones with the same value written on them they're also a little bigger in physical size but I can't be sure they're the same value (because I can't test it because the old ones exploded) but I hope so.So now I need to test the circuit do you think there'd be any harm in running the backlight circuit without the backlight attached? I haven't yet tested the black light for short circuit but you probably right because that's where the big capacitor made contact with.Also I don't know how to test the back light.On page 12 of the dater sheet is a pinout of the black light but I do not understand what "Current feedback" is.http://www.avnet-embedded.eu/fileadmin/user_upload/Files/Displays_NEW/Colour_TFT/LG_Display/LG_Display_LM215WF3_SLC1.pdfhttps://imgur.com/a/GPceCthank you

author: Kilo Tango
author_rank: Regular Contributor
timestamp: December 17, 2017, 09:37:11 pm
content: on page 7 it gives the LED backlight spec. It says 110mA at 40V. The connection diagram is given on P12. I think that the positive should go to VLED and the negative to the FB1,2,3,4 channel current feedback terminals.I would first check the resistance of the strings, in particular looking for a short, then try feeding in a voltage.You could run the controller without the backlight, the controller is trying to feed 110mA into the string so without a string it will ramp the volts up to maximum.Ken

author: ashlin4010
author_rank: Newbie
timestamp: December 18, 2017, 01:57:08 am
content: all right so I disassembled the panel and the LEDs Look to be ok but I don't have a 40 volt supply so I tested it with 12 volts but nothing might be just not enough voltage as for measuring the resistance I was getting around about 15 to 20 Megohms each and found shorts.Unfortunately the P8008BD mosfet after the destroyed resistors is dead so I am deciding whether or not to replace it maybe I can scavenge one from somewhere but I haven't managed to find one yet.