Author Topic: Troubleshooting LCD monitor LED backlight circuit  (Read 16725 times)

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Offline Revenant

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Troubleshooting LCD monitor LED backlight circuit
« on: May 18, 2015, 05:20:21 pm »
I have an LCD monitor that's having backlight problems. The backlight will cut out for a split second at a time randomly, and I notice it flickering on certain backgrounds. I have a second identical monitor (dual screen setup) which doesn't have this problem. The problem is temperature correlated, when the monitor has been powered off in a cold room overnight the issue is exacerbated until it warms up. This initially led me to believe capacitors were at fault.

Steps taken to isolate/attempt to fix the problem:

    Measure 5V and 12V rails on SMPS. Within spec.

    Replace all capacitors on SMPS board except input filter with equivalent spec/ESR quality brand ones (panasonic, nichicon, etc)

    Problem persists, desoldered input filter and test, tests ok.

    Look up datasheet on LCD panel itself and find backlight connector pinout/voltage ranges. Says 46-56V.

    Attempt to look up backlight boost driver IC datasheet, but cannot find it anywhere.

    Attempted replacing all lelon/capxon/suscon electrolytic capacitors on the second PCB that has the processor on it. No dice. All but one tested ok with LCR/ESR meter once desoldered.

    Soldered test wire onto boost converter vout pin. Voltage comes in at 58-65 volts. Voltage changes inversely with brightness control. Full brightness = 58V, 0 brightness = 65V. When monitor goes into power saving, voltage jumps up to 75V. In comparison, the good monitor measures ~50V at full brightness, drops down to 48V at 0 brightness, and <30V when power saving.

    Unfortunately the good monitor was outsourced to a different manufacturer for the SMPS and processor board, so the only things interchangeable are the LCD panels themselves. I already confirmed with an LCD panel swap that the issue is not the LED strings themselves.

I have been attempting to find the IC by searching for terms like "4 channel LED string boost" and such, but application schematics all differ from the circuit on the PCB. The output side of the IC on my monitor has 4 diodes, 4 film capacitors, and 4 inductors. The schematics I see in boost driver ICs online all just show one diode and inductor, even on 4 channel ones. I guess my next step is tracing the pcb and drawing a circuit. I tried testing the diodes and inductors in circuit and they seem to test ok.

Anyone have any other ideas of things I can try? Unfortunately, I don't have an oscilloscope, so I can't look at the waveform.
 

Online nctnico

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #1 on: May 18, 2015, 05:33:12 pm »
I'd get a can of freeze spray. Start by spraying sections of the PCB and then by individual components to find the fault.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #2 on: May 18, 2015, 05:42:39 pm »
Good idea, I'll grab some canned air when I'm out this afternoon.
 

Online tautech

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #3 on: May 18, 2015, 09:12:18 pm »
Good idea, I'll grab some canned air when I'm out this afternoon.
No FREEZE spray.

Post up pics of the Backlight controller IC, quite likely we'll be able to help.
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Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #4 on: May 18, 2015, 10:34:14 pm »
Good idea, I'll grab some canned air when I'm out this afternoon.
No FREEZE spray.

Post up pics of the Backlight controller IC, quite likely we'll be able to help.
Can't get that locally so I'll have to order it online. I figured air would work since I've seen repair videos where they turn it upside down to chill a component in the process of troubleshooting. I'll get a picture of the IC.
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #5 on: May 18, 2015, 11:30:14 pm »
Here are some pictures of the IC and the traces going to the edge connector. The best code(s) I could get off it was OK(M?)9907O(0?)N on the top, and M28A3 on the bottom. The bottom code only comes up with assault rifle hits from call of duty.
« Last Edit: May 19, 2015, 08:45:56 pm by Revenant »
 

Online Rasz

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #6 on: May 19, 2015, 11:26:16 am »
blurry pictures, second diode looks fried
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Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #7 on: May 19, 2015, 08:56:14 pm »
My camera isn't the best. I upped the fstop to alleviate the focus issues and retook in better lighting. Hopefully these are better. I don't have any kind of macro lens so this is the closest pictures I can get of the IC before it can't focus on it due to being too close. The writing is really worn down so I'm afraid this is as good as it gets.

I tested all diodes in circuit and none showed a dead short in the opposing direction and the same voltage drop in the forward direction.

I think I may have goofed in measuring the voltage of the LED backlight v+ supply (sorry, I am a beginner and not familiar with boost converter circuits). I referenced to system ground and after reviewing example led backlight IC application circuit schematics I realize that it probably should have been to the current sink channels instead? When I measure to those they show up as 44v instead of 60. I can try to trace the connector pins back to the IC and post the resulting schematic if that would help any.
 

Online tautech

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #8 on: May 19, 2015, 09:00:42 pm »
Rasz is very good on this sort of stuff, but we have to identify the baclight controller IC FIRST.
The datasheet should reveal the mystery.  ;)
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Offline thm_w

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #9 on: May 19, 2015, 10:16:04 pm »
Can't get that locally so I'll have to order it online. I figured air would work since I've seen repair videos where they turn it upside down to chill a component in the process of troubleshooting. I'll get a picture of the IC.

Canned air is fine.
 

Online Rasz

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #10 on: May 20, 2015, 05:26:29 pm »
Easy way: http://www.ebay.com/itm/Universal-LED-Constant-current-board-LED-Inverter-Boost-Board-for-LED-Panel/181557989682

Look up datasheet on LCD panel itself and find backlight connector pinout/voltage ranges. Says 46-56V.

= 15s4p, this controller will work fine, thats what I use
some ukrainian dude fixing tv with it



those markings look highly custom/cryptic :(, from your defect description sounds like one of the smd caps or solder joints might be dying and intermittent, also whats with yellow around second IC on last picture? is that your flux or cooked pcb?

redo all the small smd parts with your soldering iron around that mysterious IC, hmm redo all IC legs too while you are at it. But it also might be 12V dropping from time to time and not the backlight controller at all. That yellow pcb looks suspicious.
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Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #11 on: May 20, 2015, 09:18:25 pm »
There are no capacitors around that suspect yellowed area, so it is not flux. You're right, it looks like something got cooked there. I tried to look up the datasheet for that IC but it is another cryptic one that comes up with no hits. line 1: 1920MP, line 2: 12801K. In that area is a schottky diode (sx34). In circuit diode test with multimeter shows .184 voltage drop and 180 ohms on the resistance setting. The discoloration seems to be centered around a big ferrite inductor on the other side of the board which is discolored a bit as well, but it tests to 10uH which is its rating.

Unfortunately my soldering iron isn't fine enough to get into some of those SMD parts so i'd probably make things worse attempting to resolder them. I could try with the bigger areas though. I will also try the freeze test with the canned air since thm_w indicated it should work fine.

That hong kong board looks like it might work but from the youtube video, it appears you have to connect some lines to the right spot in order for the brightness controls to work properly?
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #12 on: May 21, 2015, 10:01:42 pm »
Ok, did some more testing, no real progress. |O

I measured the brightness adjustment line with my multimeter. It shows a range of ~.1v to 3.28v or so. Duty cycle ranges from 10% at 0 brightness to OL at 100% brighness. Peak min/max function (closest I can get to a scope) shows .1v min and 3.28v max, which I guess is consistent with a pwm signal. It doesn't appear there's any issues with that signal.

Tried freezing things to see if I could induce the problematic backlight behavior, and zero results. I first targeted the suspect yellowed area IC and surrounding diodes, nothing. Then I targeted the backlight boost IC, froze it, nothing. I then tried freezing various chips on the other PCB and still could not recreate the problem. 12V rail reads as low as 11.83 at times but if the IC takes a wide range of input voltage then I doubt that would cause the problem.

I guess I can order that hong kong board since it is only $8 shipped. If that doesn't fix it looks like I'm out of options.

EDIT: The hong kong board states 0V is brightest, and 5V is darkest. "(0V-5V, the brightest 0V, the darkest time 5V) brightness adjustment"

That is opposite to and a different voltage range than this monitor (~0V darkest, 3.3 brightest). It doesn't look like that would work.
« Last Edit: May 21, 2015, 10:07:45 pm by Revenant »
 

Offline wraper

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #13 on: May 21, 2015, 10:17:50 pm »
Crappy photos with some characters invisible but I figured out that it should be OZ9902B
« Last Edit: May 21, 2015, 10:26:42 pm by wraper »
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #14 on: May 22, 2015, 12:16:29 am »
Crappy photos with some characters invisible but I figured out that it should be OZ9902B
Thank you for your efforts in trying to find this IC. Unfortunately, the characters are invisible/highly worn down on the chip itself, so it wouldn't matter which camera I used, I did the best I could with what I have available.

The 9902B is a single channel led driver, according to the LG panel datasheet, it requires 4 channels. The 6 pin cable that connects to the backlight is:

BACKLIGHT CONNECTOR PIN CONFIGURATION(CN2)

1 Channel1 Current Feedback
2 Channel2 Current Feedback
3 LED Power Supply
4 LED Power Supply
5 Channel3 Current Feedback
6 Channel4 Current Feedback

I thought it might be the 0Z9998B, but the pinouts don't match. It is a 16 pin ic with pin 14 as GND and pin 11 as vin (I think, measures 10ohms to 12V rail). On the PCB, there is an inductor, capacitor, and diode for each of the 4 channels. Maybe that is an alternative layout than the single inductor and diode I see in all the applications schematics?
 

Offline wraper

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #15 on: May 22, 2015, 08:21:47 am »
Unfortunately, the characters are invisible/highly worn down on the chip itself, so it wouldn't matter which camera I used, I did the best I could with what I have available.
Did you try to clean it with alcohol/acetone? Laser markings usually become invisible because some flux or dirt gets into them, when cleaned they usually become clearly visible again. Your IC just looks dirty IMO. By looks of it, could be also OZ9907GN. But I cannot find a datasheet for it. Anyway, it is 100% made by O2micro. Also, why didn't you say a monitor model?
« Last Edit: May 22, 2015, 08:32:02 am by wraper »
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #16 on: May 22, 2015, 11:43:28 pm »
Did you try to clean it with alcohol/acetone? Laser markings usually become invisible because some flux or dirt gets into them, when cleaned they usually become clearly visible again. Your IC just looks dirty IMO. By looks of it, could be also OZ9907GN. But I cannot find a datasheet for it. Anyway, it is 100% made by O2micro. Also, why didn't you say a monitor model?
Alcohol did indeed reveal the "worn down" (dirty) markings! I remember I had heard that tip a long time ago but had forgotten to try it. I can now read the entire chip text and as you surmised it is indeed OZ9907GN. I do find links to this chip but no datasheets anywhere, unfortunately.

I was focusing on all the other details and forgot to post the obvious - the monitor is HP zr2440w, and the panel is LG LM240WU8-SLA1.

It doesn't look like I can really make much further diagnostic progress without a datasheet or scope.
« Last Edit: May 22, 2015, 11:47:43 pm by Revenant »
 

Offline wraper

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #17 on: May 23, 2015, 09:38:34 am »
I have been attempting to find the IC by searching for terms like "4 channel LED string boost" and such, but application schematics all differ from the circuit on the PCB. The output side of the IC on my monitor has 4 diodes, 4 film capacitors, and 4 inductors.
Test those film capacitors, preferably with desoldering and checking if they are not leaky too. Maybe even just change them, especially if one of them has lower capacitance than 3 others.
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #18 on: May 26, 2015, 02:14:19 am »
Test those film capacitors, preferably with desoldering and checking if they are not leaky too. Maybe even just change them, especially if one of them has lower capacitance than 3 others.
I don't have a mega ohm meter/capacitor leakage tester so I can't test if they're leaky. All 4 are 100nF capacitors. I've desoldered 3 and they all come in at 96-98nF when tested. Will test the last one later. Maybe it's the IC itself that's bad, but even if I could get a replacement IC I don't have an smd rework station so it wouldn't be possible anyway. It's too bad that backlight replacement pcb linked earlier isn't compatible with my panel.

I'll see if I can figure anything else out with tracing, for example I'm not sure that 4ohm from the IC pin to the 12V rail is normal.
 

Online Rasz

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #19 on: May 26, 2015, 06:12:13 am »
It's too bad that backlight replacement pcb linked earlier isn't compatible with my panel.

sure it is, it might not be 100% compatible with the driver, but whats a little backwards not dimming all the way down brightness control among friends? :)
if it really bugs you that much build one transistor inverter to translate control signal

edit:
btw http://www.badcaps.net/forum/showthread.php?p=557450
is that also your post? or did DELL made a bigger oopsie
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Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #20 on: May 26, 2015, 07:29:33 am »
sure it is, it might not be 100% compatible with the driver, but whats a little backwards not dimming all the way down brightness control among friends? :)
if it really bugs you that much build one transistor inverter to translate control signal
I wasn't sure if the voltage mismatch (5v vs 3.3v) would cause problems with frying anything. I usually keep brightness at 20% (anything higher hurts my eyes), so if it is inverted and 5v instead of 3.3, 20% brightness setting translates to ~4v, so it looks like I'd need to construct a circuit that increases the voltage as well as inverting. There is already a 5v rail so maybe that wouldn't be too difficult?

btw http://www.badcaps.net/forum/showthread.php?p=557450
is that also your post? or did DELL made a bigger oopsie
I don't understand the DELL reference. Is there a forum policy against posting on other sites ("oopsie")?
 

Online Rasz

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #21 on: May 26, 2015, 09:22:03 am »
I wasn't sure if the voltage mismatch (5v vs 3.3v) would cause problems with frying anything. I usually keep brightness at 20% (anything higher hurts my eyes), so if it is inverted and 5v instead of 3.3, 20% brightness setting translates to ~4v, so it looks like I'd need to construct a circuit that increases the voltage as well as inverting. There is already a 5v rail so maybe that wouldn't be too difficult?


https://learn.sparkfun.com/tutorials/transistors under Common Emitter
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/npnce3.html#c1

I don't understand the DELL reference. Is there a forum policy against posting on other sites ("oopsie")?

either its your post, or dell sourced shitty batch of monitor internals and they are faliling everywhere
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Offline wraper

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #22 on: May 26, 2015, 09:59:31 am »
but even if I could get a replacement IC I don't have an smd rework station so it wouldn't be possible anyway. It's too bad that backlight replacement pcb linked earlier isn't compatible with my panel.
Do you mean hot air station? You don't need it and actually it is dangerous to use on phenolic PCB. Often it makes a bubble (expanding moisture) if heated with hot air. Just cut the leads of the old IC. then remove the remains of the leads with soldering iron. Then solder new IC. Possible to remove even without cutting the leads, just apply low melting point Wood's metal/Chip Quik alloy or just huge amount of solder (a bit harder) to all of the leads and heat the opposite sides fast, then quickly remove the IC before solder solidifies.
http://www.aliexpress.com/item/OZ9907GN-SOP/1881985728.html?spm=2114.32010308.4.2.txgmug
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #23 on: May 26, 2015, 10:06:56 am »

https://learn.sparkfun.com/tutorials/transistors under Common Emitter
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/npnce3.html#c1
Thanks for the references, I will review them and see if I can construct the circuit.

I don't understand the DELL reference. Is there a forum policy against posting on other sites ("oopsie")?

either its your post, or dell sourced shitty batch of monitor internals and they are faliling everywhere
Yes, I posted over there. My monitor is HP, not Dell. In my searches I've come across other posts about this particular power board used in the earlier revision zr2440w monitors, so they probably are crap quality.
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #24 on: May 26, 2015, 10:14:02 am »
but even if I could get a replacement IC I don't have an smd rework station so it wouldn't be possible anyway. it is too bad that backlight replacement pcb linked earlier isn't compatible with my panel.
Do you mean hot air station? You don't need it and actually it is dangerous to use on phenolic PCB. Often it makes a bubble (expanding moisture) if heated with hot air. Just cut the leads of the old IC. then remove the remains of the leads with soldering iron. Then solder new IC. Possible to remove even without cutting the leads, just apply low melting point Wood's metal/Chip Quik alloy or just huge amount of solder (a bit harder) to all of the leads and heat the opposite sides fast, then quickly remove the IC before solder solidifies.
http://www.aliexpress.com/item/OZ9907GN-SOP/1881985728.html?spm=2114.32010308.4.2.txgmug
Yes, I meant hot air rework. I did not know it was dangerous to use on this type of PCB, thanks for the heads up. Though upon further review I am thinking it would probably be better to replace the mystery 1920MP/12801K chip first, given the scorching discoloration on the PCB in that area.
« Last Edit: May 26, 2015, 10:23:33 am by Revenant »
 

Online tautech

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #25 on: May 26, 2015, 10:18:04 am »
Do you mean hot air station? You don't need it and actually it is dangerous to use on phenolic PCB. Often it makes a bubble (expanding moisture) if heated with hot air.
:bullshit:
Use mine all the time on my home etched PCB's, just heat gently enough to lift/wipe the IC off and you'll never damage a PCB.
I even reuse SOIC's lifted with hot air, haven't cooked one or damaged a PCB yet..........
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Offline wraper

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #26 on: May 26, 2015, 01:15:02 pm »
Do you mean hot air station? You don't need it and actually it is dangerous to use on phenolic PCB. Often it makes a bubble (expanding moisture) if heated with hot air.
:bullshit:
Use mine all the time on my home etched PCB's, just heat gently enough to lift/wipe the IC off and you'll never damage a PCB.
I even reuse SOIC's lifted with hot air, haven't cooked one or damaged a PCB yet..........
Do you etch phenolic PCBs or FR4 may I ask? :bullshit:
 

Online tautech

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #27 on: May 26, 2015, 07:47:25 pm »
Do you mean hot air station? You don't need it and actually it is dangerous to use on phenolic PCB. Often it makes a bubble (expanding moisture) if heated with hot air.
:bullshit:
Use mine all the time on my home etched PCB's, just heat gently enough to lift/wipe the IC off and you'll never damage a PCB.
I even reuse SOIC's lifted with hot air, haven't cooked one or damaged a PCB yet..........
Do you etch phenolic PCBs or FR4 may I ask? :bullshit:
phenolic PCBs
« Last Edit: May 26, 2015, 08:05:22 pm by tautech »
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Offline wraper

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #28 on: May 26, 2015, 08:55:37 pm »
phenolic PCBs
Don't know why someone would use such crappy material for their DIY PCBs  :-// . Maybe it is better than average. Made a video specially for you.
 

Online tautech

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #29 on: May 27, 2015, 07:43:40 am »
phenolic PCBs
Don't know why someone would use such crappy material for their DIY PCBs  :-// . Maybe it is better than average.
As you and I are intelligent men, I'd thought you'd grasp some of the hazards of working with FR4 in a hobbyist environment.
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Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #30 on: June 11, 2015, 07:08:38 pm »
I finally broke down and got a scope since that's something I've been planning to do for a few years anyway, and now seemed like a good time to finally pull the trigger.

I have an OZ9907GN on the way from china but it will probably be another week or two before that arrives. In the meantime, I probed the 0-3.3v vadj pin and v+led pin with respect to board ground, along with one of the 4 LED string channels.

The first 3 pictures show channel 1 as v+ led, channel 2 as vadj. The pwm signal for vadj looks messy to my untrained eye, the built in calibrator signal has a lot less noise as does a test arduino square wave generator. Maybe it's just noise from other components near the probe.

The second set of 3 pictures show the LED channel. This looks messy when brightness is set to < 100%, but as the fifth picture shows it seems fine with negligible ripple at 100% brightness. The flickering persists at 100% brightness, so I will see if any of the other 3 channels shows anything obvious on the waveform that correlates to the flicker problem. I'll also compare waveforms to the good monitor.
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #31 on: June 11, 2015, 11:15:58 pm »
Realized I wasn't in AC coupling mode :palm:

At 100% brightness, the ripple varies across all four channels. One of them is particularly bad, 2.4V peak to peak, and one is as low as 1.2V. I can imagine the varying ripple between each channel might cause interference flicker to show up, though I am not familiar with LED backlight circuits and what a normal expected ripple for them are, as well as the tolerance in ripple differences between LED channels. Next step will be to take apart the good LCD monitor again and look at the areas of interest on that with the scope to get a comparison.
« Last Edit: June 11, 2015, 11:20:37 pm by Revenant »
 

Online Rasz

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #32 on: June 12, 2015, 12:39:28 pm »
ripple

so did you, or did you not swap 4 electrolytic capacitors whose legs are visible above(below?) diodes on this pic
https://www.eevblog.com/forum/repair/troubleshooting-lcd-monitor-led-backlight-circuit/?action=dlattach;attach=152868;image
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Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #33 on: June 12, 2015, 03:58:48 pm »
so did you, or did you not swap 4 electrolytic capacitors whose legs are visible above(below?) diodes on this pic
https://www.eevblog.com/forum/repair/troubleshooting-lcd-monitor-led-backlight-circuit/?action=dlattach;attach=152868;image
The area with the 4 capacitor/diode/inductor triplets use 100nF film capacitors. I desoldered those and tested each, all came in the 96-99nF range. I wrote the ESR down somewhere and can't find the paper now but none of them tested significantly out of range of the others. It seemed like they were fine so I soldered them back in after.

EDIT: I checked the 12V rail which the OZ9907GN IC uses, the same ripple exists on there at the same frequency range, waveform looks very similar to the ripple waveform in the above posted pictures of the LED channels. Ripple Vpp is 800-920mv.
« Last Edit: June 12, 2015, 05:13:24 pm by Revenant »
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #34 on: June 13, 2015, 02:27:54 am »
I checked one of the channels on the good monitor (a lot harder to solder up test wires to the board on that one so couldn't do all 4). Ripple was a lot less on that one, about 600-800mvpp. I also checked the 12v rail and its ripple ranged from 150-300mvpp, quite a bit lower than the bad one. I have a 12v SLA battery, is there any safe way to use it in place of the existing 12v line to see if the ripple is causing the flickering? For example, one method I've seen mentioned is cutting the 12v line to the IC and substituting the battery + in its place while connecting - to ground.
 

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #35 on: June 13, 2015, 06:29:35 pm »
I decided to try and completely bypass the smps and hook up a known good ATX power supply to the 5V and 12V rails in order to see if the flickering would go away. Unfortunately, this didn't work, the monitor would briefly power up for about a second and then turn off (update- see below). I figured the reason might be the soft power on/off signal. I disconnected the ATX supply and powered it up again using its SMPS, and this time it powered up normally. I decided to probe the soft power on/off signal pin. It appears power on is +3.3V, but there is a terrible amount of ripple on this pin as can be seen from the attached picture. Could this badly rippling soft power signal be cause of everything, or is it merely a symptom of the existing ripple?

EDIT: Turns out I ran into the minimum load required ATX power on issue. Once I had a suitable load on the PS it stayed powered up. With this substitute power supply, the monitor power LED comes on, but the backlight does not. It doesn't appear the LCD is being powered either, as a flashlight to the LCD revealed no image.
« Last Edit: June 13, 2015, 11:56:09 pm by Revenant »
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #36 on: June 14, 2015, 06:19:38 pm »
It appears that the LED+ rail is not being completely boosted by the OZ9907GN. I followed the LED+ PCB trace back and it ends at a transformer with two coils. A scope probe reveals a square wave output from this transformer. There was some noise on this, I'll try and get a screenshot. It appears the other coil supplies the 5V rail. The two sides of the board are mostly electrically isolated with this transformer, capacitor (looks like a MOV but is labeled C), and an optocoupler bridging the gap. On the mains side of the PCB I found another IC, OZ531GN. As expected, zero datasheets for this and the only thing I could really find was a japanese blog with a guy who has the same monitor as I do running into issues with the backlight: http://blogs.yahoo.co.jp/little_crown36/13854669.html. Google translate I think reveals that he somehow replaced this with a different chip and got it working again? it is hard to piece together the translation.

It looks like mains comes in, passes through two inductors to a bridge rectifier, after which is a filter cap and then this IC, which connects to a heat sinked transistor, and then to one of the transformer coils.
« Last Edit: June 14, 2015, 06:55:40 pm by Revenant »
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #37 on: June 17, 2015, 04:19:35 pm »
I had some more time to try and further investigate the noise/ripple issues on the power rails. I've been able to confirm the noise/ripple bursts on the different rails (V+LED, 5V, 3.3V, 3.3v power_on signal) have the same period of ~2.25us. I traced the 5V rail back to the transformer, and was able to trigger off the 2.25us burst there too. That might just be caused from the issue downstream in the circuit, however, I'm not sure. I need to see if I can find this noise burst happening on the mains side next, but I'm limited to using differential measurements there because of the common earth ground issue.

The attached pictures show the 3.3V rail on ch1 and the output of the transformer for the 5V rail on ch2, triggered off the ripple bursts. The transformer waveform is quite glitchy as can be seen. Last picture shows DC coupled waveforms.
 

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #38 on: June 19, 2015, 12:29:58 am »
It appears the 2.25us period ripple/burst is not present on the mains side, but the differential measurement was coarse and noisy so it was hard to tell definitively. I also found out that when the backlight IC is not powered on (3.3v power on signal is low), the ripple goes away and the 5V and 12V rail are very smooth. I attempted to see if the ripple would appear if I put a load on 5V rail, when I tried a couple hundred milliamps none appeared. When I tried this with the 12V rail a small amount appeared, but not at the same period as the ripple appearing when the backlight was on. I then substituted in a ripple free 3.3V signal for the power on to see if the ripple on that was causing the issue, and ruled that out, ripple still occurs.

I found out that the IC in the scorched looking area referenced earlier in this thread is almost certainly a 5V to 12V boost converter. So, it appears the problem might be in either this boost converter or in the backlight IC itself. As is par for the course on this powerboard, I could find no datasheet for that boost converter IC. I will see if I can make and post a schematic of it, perhaps there is a pin compatible IC which can be substituted?

The replacement backlight IC arrived the other day. I am holding off on replacing it until I get a few more measurements. Not sure it will even work, might be bad/defective, so it would be ideal if I can remove the IC without cutting the pins. I'll see if I can try the solder bridge/chipquik methods perhaps.
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #39 on: June 19, 2015, 10:39:58 pm »
I was able to successfully remove the backlight IC without having to cut the pins or use chip quik. It should even be reusable if I need it for any reason. I used the enameled wire method to accomplish this. I soldered in the replacement IC, inspected each pin visually and ensured continuity/no shorts between adjacent pins. Then powered up the monitor again and................ display STILL flickers, ripple STILL present  |O

Ripple period remains ~2.25us. Decided to probe the +VLED backlight rail with the display soft powered off, the 2.25us ripple goes away, however, a 41.8us ripple burst takes its place. There is a beefy through hole diode that separates this rail from the other coil on the flyback transformer. On the other side of this diode (right at the transformer), the ripple is not present. Maybe the problem lies on the +VLED rail somewhere. There are two small capacitors directly after the diode, look like they're in the pf range, I'll test those next. There is also an unlabeled inductor (no uH value listed) with what looks to be about a 5mm core and about 8 turns of what looks like 16 ga copper wire. Not sure how I can determine the proper value for this. In circuit it tests at .8uH.

I should probably at least see if the backlight cutting out problem has been resolved or not with the IC replacement.
 

Offline Revenant

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Re: Troubleshooting LCD monitor LED backlight circuit
« Reply #40 on: July 27, 2015, 11:13:08 pm »
I have an update on this in case anyone's still interested. I ordered the ebay backlight replacement board which took a long time to arrive. It finally arrived the other day and with it I've been able to pinpoint that the flickering problem originates on the logic board and not the SMPS one. The same exact flicker behavior is exhibited with the replacement backlight circuit. In order to rule out ripple issues on the signal/power lines, I fed the replacement backlight circuit with 5V and 12V from separate isolated power rails that are free of the ripple. I pulled the dim signal to ground (brightness 100%) and enable signal to 5V. Flicker persists even with this.

At this point it looks like I'll need a replacement logic board for the monitor, and no surprise that there are none to be found anywhere. I know from searches that the LG panel in this board has been used in some Acer/Dell monitors, but am unable to find a definitive list of model numbers it has been used in. If I could, I might be able to find a non hp board to replace it with.

it is unfortunate that this has turned into such a time consuming, frustrating mess that's reached a dead end.
 


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