24" HP monitor black/dark screen fixing - Page 1
Forum section: electronics / beginners

author: sairfan1
author_rank: Frequent Contributor
timestamp: May 23, 2023, 05:22:11 pm
content: I have a 24 inch HP monitor please refer to attached picture of its main board, suddenly its screen got dark/black and there are now signs of display, I'm not sure if there is something wrong with LEDs or LED driver or anything else like issue related to display driver.If someone have experience please help me to understand the issue. Is it possible that i can lit LEDs through a variable power supply? like I can set some voltage/amps on my HP 6543A power supply to ensure if LEDs are working, if LEDs are good then I will look for LED driver on the main board and so on.

author: u666sa
author_rank: Frequent Contributor
timestamp: May 23, 2023, 05:35:15 pm
content: You gotta check you getting backlight voltage, that's a first.If you DO. Then you check the backlight leds. If you have proper PSU you can give it voltage. If you don't, you could take apart your screen and check diods visually or give them voltage individually. (I do not recommend replacing one diod at a time, buy all diod strips and replace everything at once. If one went out, another one is on its way)Trying shining flashlight when the screen is on and there should be picture on it, in a completely dark room, you should see picture if it is back light problem.P.S. obviously, if you got the PSU out, you might as well check all voltages while it's out. But I wouldn't bother. I'd chase backlight first. If in fact monitor giving other signs that it is turning on, perhaps led is lighting up properly or there is sound when you press menu buttons.P.S.S. get exactly the same PN as on the strips, and make sure picture of strips looks exactly the same as what you pulled out, has exactly same holes, etc.

author: DimitriP
author_rank: Super Contributor
timestamp: May 23, 2023, 05:48:15 pm
content: QuoteWhat he said ^^^99% of the time (100% for my monitors:) ) replacing all the capacitors will take care of it and bring it back to life.Even if the issue is caused by something else, at least you'll have the caps out of the way.

author: u666sa
author_rank: Frequent Contributor
timestamp: May 23, 2023, 07:08:22 pm
content: Quote from: DimitriP on May 23, 2023, 05:48:15 pmCareful here.!!Pulling out a cap, and measuring its capacitance, and even installing a brand new cap might not give you results. Cap can look good, have good capacitance, but still be bad. You need to test caps on LCR meter. Often new caps you buy are trash.So sometimes you end up replacing cap with a brand new bad cap.

author: sairfan1
author_rank: Frequent Contributor
timestamp: May 24, 2023, 02:45:42 am
content: I do not have LCR meter but I have capacitors, I will change and see if that works,Does it mean there is no other way to lit LEDs without main board?

author: Jwillis
author_rank: Super Contributor
timestamp: May 24, 2023, 05:22:18 pm
content: Quote from: sairfan1 on May 24, 2023, 02:45:42 amUnless someone has made modifications to that monitor there are No back light LEDs. That Is a CCFL driver board and there should be 4 florescent lamps. These require several hundred volts to operate.[urlhttps://www.nodevice.com/service-manuals/monitor/hp/2309p/443814/download]https://www.nodevice.com/service-manuals/monitor/hp/2309p/443814Edited to fix link

author: wraper
author_rank: Unknown
timestamp: May 24, 2023, 05:35:01 pm
content: Quote from: u666sa on May 23, 2023, 05:35:15 pmThere are no LEDs, it's CCFL. And if you check voltage, there is a good chance to kill your multimeter.

author: wraper
author_rank: Unknown
timestamp: May 24, 2023, 05:37:00 pm
content: From description it's not clear if there is no backlight or if monitor does not turn on at all.

author: jmelson
author_rank: Super Contributor
timestamp: May 24, 2023, 06:04:52 pm
content: Watch out, your picture of the board looks like it has high voltage transformers (the two large black blocks) which would indicate it is NOT an LED backlight, but cold cathode fluorescent. Those generate very high voltages.Jon

author: Jwillis
author_rank: Super Contributor
timestamp: May 24, 2023, 09:01:04 pm
content: There can be as much as a 1000V to initiate CCFL operation and anywhere between 250V to 800V to sustain. So don't be poking around Willy Nilly without further knowledge of what your doing.For an HP monitor, I'm surprised the warning labels are removed from the transformers and theirs no warnings printed on the board. Not very impressed HP.

author: sairfan1
author_rank: Frequent Contributor
timestamp: May 25, 2023, 02:03:02 am
content: When I turn on the monitor its screen stays black and I can feel the backlight is not glowing.Is there some way to check CCFL lamps? still changing the capacitors/diode may work?

author: Jwillis
author_rank: Super Contributor
timestamp: May 25, 2023, 05:49:17 am
content: Quote from: sairfan1 on May 25, 2023, 02:03:02 amThe only way to test a CCFL lamp is with a high voltage inverter for that purpose. Some will say that if they are blackened at the ends they are no good, but that is wrong.Without a schematic, it is difficult to trouble shoot.I don't want to lead you down a path that may be destructive to you or your equipment. Replacement would be the safest method. The boards and lamps are available from some distributors. But it might be a gamble knowing if if they are new or salvaged.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 25, 2023, 08:39:14 am
content: As I could see from a photo - all electrolytes are fine. Except one in a pri.ary circuit which is not seen (I guess it is fine too).TS has to check power supply output voltages. Typically these are +5 VDC (it powers signal circuits, processor 'main' board etc) and 12-13 VDC (it powers backlight power supply).If both +5 and +12 VDC lines are abcent then it's a problem in a primary circuits, e.g.switching IC needs to replace.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 25, 2023, 09:08:42 am
content: As written before, your monitor does not have LED backlighting, but CCFL. CCFL replacement tubes are quite cheap, but you can also buy LED strips which are made for monitor back lighting. You probably have to dismantle the monitor to repair it. If you do so, then the main issue is to keep the stack of foils clean and free of dust. It's best to not take them apart at all and also keep your work area dust free while working on your monitor. For example choose a rainy day, then vacuum your work area and ventilate by opening some windows for some time to blow out the remaining floating dust.

author: wraper
author_rank: Unknown
timestamp: May 25, 2023, 09:31:54 am
content: Quote from: sairfan1 on May 25, 2023, 02:03:02 amSo does power indicator LED show it's turned on? If it's a backlight issue you can check resistance of secondary windings of both CCFL transformers. It should be around 1k and nearly equal for both but IMHO they are fine. If it was bad CCFL or CCFL transformer, backlight should light up for a short moment before protection turns it off as at least one lamp and transformer should be working. Capacitors not bulging does not mean they are necessarily OK.

author: u666sa
author_rank: Frequent Contributor
timestamp: May 25, 2023, 09:48:34 am
content: Quote from: sairfan1 on May 25, 2023, 02:03:02 amHere is my repair sequence. Before taking apart stuff, I turn on monitor, check if it operates. Obviously screen is dark, but you still might have menus and sound indication on menus. Shine a light into screen, see if you got picture. And there you go, you know if it has backlight problem, or it's power or other problem. You gotta do this check first, otherwise we really don't know what's happening.P.S. as far as checking CCFL voltage, I've done it using cheapo Innova multimeter since 2014.Honestly I didn't know it was dangerous or that it might kill my multimeter.But yea, don't be poking into things, make sure that it is either your backlight, i.e. your monitor turns on, but there is no backlight, or it is something else.P.S.S. Don't do CCFL to LED conversion, results usually don't look right. Identify your problem first, and if it is lamps, order new, all of them at once and replace.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 25, 2023, 11:02:07 am
content: Quote from: sairfan1 on May 25, 2023, 02:03:02 amDoes the power indicator LED show something?

author: jmelson
author_rank: Super Contributor
timestamp: May 25, 2023, 03:20:15 pm
content: I have repaired a few CCFL monitors by cleaning the inverter board. They sometimes have a plastic high voltage shield on the back side of the board to prevent arcing to the cabinet, the shield sometimes gets punched through by arcs. Additional plastic material can be inserted to block the arc path, and then removing any black dust on the back of the board. Wait a few hours for the board to dry completely and try again. I have RARELY seen monitors with totally failed CCFL lamps, usually they start going pink at startup when aged and then get back to normal when warmed up. But, most failed monitors have a problem in the CCFL inverter.Jon

author: Jwillis
author_rank: Super Contributor
timestamp: May 25, 2023, 04:58:59 pm
content: I would start with a thorough visual inspection of the components on the underside of the board. Look for cracks, disfigured componets, and/or burns to any of the components. Check for traces for breakage or evidence of arcing. You might need a magnifying glass depending on how good your eyes are.Since the main board directs the CCFL to turn on you might want to do a visual inspection of that as well.Without any visual reference. its just the blind leading the blind.Take a clear picture of the underside of the board and we might be able to direct you where to check the components responsible for driving the CCFLs Safely. You can get datasheets online in most cases for each applicable component. If we can figure out which PWM is being used, the datasheet may have a generic circuit diagram for reference.