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Ginko 05-02-2015 03:02 PM

HP ZR2440w backlight cutting out
I have a set of HP ZR2440w LED backlight monitors. One of them is having issues with the backlight. On certain backgrounds (like the one at I notice it flickering. The other zr2440w has no such flicker problem. The major issue with it is that the backlight cuts out for a fraction of a second intermittently. This happens a lot more frequently when it's first turned on after it has been off and cold overnight, so it seems to be temperature related. I suspected the capacitors in the power supply might be bad, so I opened up the monitor to have a look.

They all looked fine visually, but are Lelons which are on the "bad list." Unfortunately I don't have an ESR meter so I wasn't able to actually test them. I did test the voltages with my multimeter, however, and they were all within spec. I ordered low ESR panasonics, rubycons and nichicons to replace the Lelons. I cross referenced the Lelon datasheet and confirmed the originals I was replacing were all low esr type. I was not able to find a replacement for the 500V input filter capacitor, at that voltage rating and higher they are all snap in type.

I proceeded with replacing all the capacitors in the SMPS (except the input filter cap), however the problem persists still. There are two PCBs inside the monitor, one is the SMPS, and the other is the one which handles the inputs and connects to the panel. This other one is filled with poor quality caps as well, Capxon, Lelon, and Suscon. I looked up the datasheets and most if not all are low ESR type.

I thought I might be able to swap PCBs with the other zr2440w to at least pinpoint which board is the culprit, but the other zr2440w is a later revision with a completely different SMPS and input board, with different board to board connector types.

I've ordered an ESR meter since I'm going to need one in the future anyway, any thoughts/ideas on where I should proceed from here?

selldoor 05-04-2015 05:15 AM

Re: HP ZR2440w backlight cutting out
When the light goes out what does the power led do?

If the fittings are the same can you connect the flickering panel to the
known good power and main boards

rajeshkumar 05-04-2015 07:34 AM

Re: HP ZR2440w backlight cutting out
check your backlight and also check lamp may be lamp is damage

Ginko 05-04-2015 12:03 PM

Re: HP ZR2440w backlight cutting out

Originally Posted by selldoor (Post 556397)
When the light goes out what does the power led do?

If the fittings are the same can you connect the flickering panel to the
known good power and main boards

I will try to take note what the power indicator led is doing next time the backlight cuts out. None of the fittings are pin compatible except the LCD panel itself (which is LG). I am not sure if the backlight is built into the panel and therefore shares its data cable or if it is separate. When the ESR meter arrives in a couple of days, I'll disassemble the monitor again to test the capacitors, and take pictures of everything.

Ginko 05-06-2015 05:44 AM

Re: HP ZR2440w backlight cutting out
Ok, finally was able to observe the LED when the backlight dropped out. It was difficult because it happens so fast that by the time i look at the LED the backlight was back on again. The LED does stay lit during the times when it cuts out.

The ESR meter arrived yesterday and if I have time today I'll attempt to get some pictures. I did test the old lelon caps from the SMPS and the worst ESR I got was from the 10uf 50v capacitor which measured 1.78 ohms or so, but for low value capacitors I believe that's within spec.

Ginko 05-06-2015 09:49 AM

Re: HP ZR2440w backlight cutting out
5 Attachment(s)
Some progress and pictures. I disassembled the monitor again and have started testing the capacitors on the data board. C193 and C192 had a negative residual voltage of around 1.9v. These two are in parallel and are rated 470uF 16V and 100uF 16V. One is a capxon and the other a lelon. In circuit capacitance test yields 534uF. ESR/Rs tests show 130mOhm @ 100hz, 120mOhm @ 120hz, 44mOhm @ 1khz, 50mOhm @ 10khz, 63mOhm @ 100khz.

There are 25 capacitors on this board. I have to determine if each is in parallel with others first then get readings, so it will take a while to do every one.

The first two pictures are the SMPS which has been re-capped except the input filter. The next two are the data board, and the last one shows the flat flex cable which runs from the SMPS to the panel. I believe this might be for the backlight. It is not pin compatible with the SMPS board in the other monitor. I will see if it is possible to swap flat flex cables when I disassemble the other one later.

budm 05-06-2015 10:28 AM

Re: HP ZR2440w backlight cutting out
Can we see the pictures of the whole back side to see how all the boards are connected together?
The power supply board picture is cut out at the bottom, I cannot see backlights LED connector.

Ginko 05-06-2015 03:05 PM

Re: HP ZR2440w backlight cutting out
Sorry, I was in a bit of a hurry when I took the pictures. I will get some additional ones that make things clearer. The flat flex connects to the back of the SMPS in the bottom right of picture 2 right above the cutout where the flash spot is. It is labelled P851.

budm 05-06-2015 03:27 PM

Re: HP ZR2440w backlight cutting out
OK, that is the connector for the LED backlghts.

Ginko 05-06-2015 06:09 PM

Re: HP ZR2440w backlight cutting out
2 Attachment(s)
Ok, I laid the two boards on top of the shield and lined up the cables to show what goes where. The cable that goes nowhere is the data cable for the panel. The top left cable in picture 1 goes to the USB ports. The bottom left cable goes to the buttons on the front of the monitor. The one on the right as can be seen goes to the SMPS, and the bottom cable is the data cable to the panel.

I see some inductors and diodes right before the output to the flat flex. I did not measure voltage here with my multimeter before, so I will see if I can get a reading for that. I am going to disassemble the second zr2440w and see if the flat flex is pin compatible on the panel side. I just noticed the SMPS label lists the voltage rails. There is one that says 65V, I assume that is for the LED backlight since there is no 65V on the cable to the data board.

Here are the rest of the capacitor test results:

470uF, 100uF and 100uF all @ 16V in parallel. In circuit capacitance says 672uF, ESR/Rs @ 100hz, 120hz, 1khz, 10khz, 100khz: 130mOhm, 130mOhm, 55mOhm, 43mOhm, 17mOhm to 51mOhm

3x 100uF @ 16v in parallel. Cp 321uF. ESR/Rs: 60mOhm, 51mOhm, 195mOhm, 159mOhm, 45mOhm

10uF @ 50v, 2x100uF @ 16v in parallel. Cp 212uF. ESR/Rs: 53mOhm, 48mOhm, 303mOhm, 233mOhm, 67mOhm

2x47uF @ 25v in parallel. Cp 95.9uF. ESR/Rs: 1.02ohm, 89mOhm, 293mOhm, 217mOhm, 61mOhm

10uF @ 50v. Cp 10.12uF, ESR/Rs: 7.3ohm, 5.87ohm, 1.59ohm, 735mOhm, 649mOhm

47uF @ 25v. Cp 41.7uF. ESR/Rs: 2.63ohm, 2.3ohm, 753mOhm, 598mOhm, 514mOhm.

10uF @ 50v. Cp 10.04uF. ESR/Rs: 44.2ohm, 28.5ohm, 1.43ohm, 64mOhm, 55mOhm. (I think I should desolder this one and test it).

4.7uF @ 50v. Cp 4.43uF. ESR/Rs: 82.1ohm, 63.4ohm, 2.53 ohm, 1.26ohm, 1.05ohm.

220uF @ 16v. Cp 212uF. ESR/Rs: 51mOhm, 45mOhm, 221mOhm, 178mOhm, 174mOhm.

5x220uF @ 16v, 10uF @ 50v, 100uF @ 16v all in parallel. Cp 1125uF, ESR/Rs: 24mOhm, 24mOhm, 170mOhm, 106mOhm, 87mOhm

budm 05-06-2015 06:58 PM

Re: HP ZR2440w backlight cutting out
The backlights problem can be due to the boost converter power supply circuit for the LED drivers, it looks like it has 4 LED strings inside the panel. It can also be that the LED/s in the string is failing and that will cause the LED driver circuit to go into shutdown mode.

Ginko 05-06-2015 07:06 PM

Re: HP ZR2440w backlight cutting out
I will see if it is possible to swap panels to pinpoint the issue. If it's not the panel, the only other thing I can think of is checking the output voltage of said boost converter. I don't have a scope so I can't look at the waveform. If it does end up being the LEDs themselves, I presume that isn't fixable given it is inside the panel.

budm 05-06-2015 07:14 PM

Re: HP ZR2440w backlight cutting out
We have seen so many TVs with LED backlights that failed regularly here at badcaps.

Ginko 05-07-2015 11:02 AM

Re: HP ZR2440w backlight cutting out
Ok. I disassembled the other zr2440w and compared the panels. The older one is LG LM240WU8 SLA1, and the newer is LM240WU8 SLA3. Even though both cables were pin compatible, I was paranoid about frying things and wanted to check the voltages. Unfortunately it doesn't look possible to get at the SMPS PCB with probes when the backlight cable is connected to the panel because the cable is too short and the shield has to basically be on the panel like it is normally.

I looked up the datasheets on both and confirmed they are in fact pin compatible, so I performed the swap of the panels. I am now using the newer panel in the older power supply/data board and noticed's background doing the same flickering. I did see the backlight blip out for a second too. All signs point to the power supply being the problem, I'm just waiting for confirmation with another backlight cutout which I'm sure is forthcoming.

I am not sure where to proceed from here. Is the SMD chip for the backlight power supply circuit bad perhaps? I tested the 4 inductors with the LCR meter and they checked out. I also tested the 4 diodes and they seemed fine too.

budm 05-07-2015 11:21 AM

Re: HP ZR2440w backlight cutting out
Since we do not have the schematic diagram for the board, you will have to get the spec of the LED driver IC and look at the application notes and circuits to see if it is closely match your LED driver section and go from there.
It can also be that the Dimming control signal or the BL-ON signal have the glitch and cause the flickering or drop out.

Ginko 05-07-2015 11:55 AM

Re: HP ZR2440w backlight cutting out
Is it possible it might be the input filter cap I wasn't able to replace on the SMPS? I'm guessing probably not, but I'll check its esr later. If it's the dimming control signal, maybe that suspect capacitor on the data board is to blame. It would be nice if I could swap the data boards but they aren't pin compatible. I'll see if I can get the number off the LED driver IC and look up the data sheet.

Ginko 05-07-2015 01:43 PM

Re: HP ZR2440w backlight cutting out
Unfortunately I had absolutely zero luck getting the datasheet for the IC. The writing on the chip is horribly worn down and I wasn't able to pull anything up at all despite trying several combinations of possibilities. I thought maybe I could find what the IC was if I could get the datasheet on the power board itself, Tatung TPC 06034 PWB 1429. Zero luck there too.

If I had a scope I could at least determine if the problem is happening due to LED power or the data board, but I don't.

Ginko 05-10-2015 09:13 AM

Re: HP ZR2440w backlight cutting out
I'd been thinking more about this issue, and came back to the temperature correlation. That really seems to point to a capacitor problem. There is one suspect capacitor on the data board, a 10uF 50V one. This one is not in parallel with other capacitors. It measures within spec at 1khz, but when I drop the frequency down to 100hz, its capacitance jumps up to 12.35uF. That's outside the +/- 20% tolerance. In addition, the Rs measures 45ohms at 100hz. There is another identical 10uF 50V cap on the board that also isn't in parallel, and this one by comparison measures 8.5ohm @ 100hz. Its capacitance is also in spec at 100hz. I have a third desoldered 10uF 50V capacitor which I tested, it also has 8.5ohm @ 100hz.

Of course there is no way to be certain without desoldering the suspect capacitor. Unfortunately there is a lot of thermal mass in that area and my efforts to desolder it went nowhere. Desoldering wick didn't work, the combination of the small holes and large thermal mass prevented all my attempts. I would need a vacuum desoldering station to remove it, unfortunately they're all north of $150. There aren't any hackerspaces in my area either, so it seems like there's nothing I can really do for now unless something changes.

Ginko 05-17-2015 05:25 PM

Re: HP ZR2440w backlight cutting out
I was able to overcome the thermal mass problem by turning up the temperature on the soldering iron, I was reluctant to do this for fear of damaging adjacent components, but gave it a try anyway. It was still a very long and tedious process desoldering all 25 capacitors from that board. Of course, replacing those capacitors didn't fix the problem either as my luck has it. I really thought it would since the strong temperature correlation points to a capacitor problem.

I was able to solder in a test wire to the output voltage for the LED driver so I could check the voltage. It came in at 58v - 65v. The datasheet lists a maximum voltage of 56 volts for the LED backlight. Thus, it seems I finally found the issue. To confirm, I soldered a test wire into the good monitor as well and it came in at 48 volts. Oddly, they behave completely opposite when turning brightness control up and down. On the bad board, the voltage increases to 65 volts when the brightness goes down to zero. When the monitor powers off, the voltage jumps up even further to 75 volts. I'm lucky the LEDs haven't blown. On the good board, the voltage ranges from about 52 volts at full brightness down to 47 volts at zero brightness. During power off it drops down to <30v.

All signs point to the mystery LED driver chip with the worn out writing that I can't pull any datasheet on. Does anyone know if a bad film capacitor (There are 4 .1uF on the driver output), inductor, or diode might lead to these overvoltage symptoms, or is it pretty certain the chip is at fault at this point?

It looks like I'm boned since I can't find any information on that chip.

Djibrille 06-18-2016 07:57 AM

Re: HP ZR2440w backlight cutting out
I'm having a similar problem with my ZR2440w. At first I thought I was imagining but it sometimes blinks off then on really quickly. It also appears to sometimes take a lot of time to wake up (maybe this is only my imagination since I haven't timed it).
Since I had a similar problem a few years back with a 20'' Samsung that started blinking and eventually didn't turn on at all and I solved it reasonably easily by changing 3-4 caps on the power board, I thought I'd do it again with the HP. So I opened it and took out the power board but no cap seemed to be bulged or leaked. They are all Lelon brand ones. Since the 47uf 100v ones seemed to not be straight( also the board has similar sized unoccupied cap places so maybe some cost cutting was involved?) I desoldered and measured one of them and got 47.04uf, 0.14ohm esr. That looked OK to me so I assembled everything back. I haven't seen the blinking again since reasembling (a few hours ago). Maybe it only does it when cold(my room temperature is over 30C now)? Or maybe it was just a loose connection somewhere. I have pictures if anyone is interested but I haven't quite figured out yet how to attach or insert them.
I do hope there is a way to fix it since I really like this monitor, and it worked flawlessly for me for more than 4 years now.

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