# PSU in LCD1550V NEC monitor troubleshooting



## Brandenramer (Jan 30, 2010)

I have 4 lcd monitors NEC LCD1550v is the model number, one works fine and three do not power on they instead have a quickly flashing power LED and no other symptoms. I know from exchanging the boards that all 4 work fine save for three of the PSU's this is my problem- these PSU's are not really Physically damages in any visible way and are not replaceable so repair is seeming grim. I have gone over them with the multimeter a great number of times looking for a difference between the one working board and the other three. All I have really found is that the non working boards seem to have slightly lower readings all the way around as far as resistors and capacitors but nothing jumping out as bad. There are some component i can not seem to get a reading on either board and seem to be the likely culprit how ever i can not test to be sure and i am not sure how to test the transformers. I am really new to this but im a quick learner, also very dedicated and i have accrued many broken lcd monitors and will not stop until they are mostly all fixed. Solving this one would be a tremendous help as i am struggling financially and will not sell the working one until the other three are fixed. Posted are two pictures one of the monitors and the other of the board, circled in yellow are some of the components i can not test. Any pointers or help of any kind is super appreciated, Thank you


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## sagey (Mar 1, 2010)

Did you ever find out what was wrong with this supply
I have 4 bad ones.4-dontkno
Thanks:


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## Brandenramer (Jan 30, 2010)

no but i have a good board for reference like i said im finding that the bad boards resistors are all reading either lower or higher than the good board but im not sure that they are actually bad. its starting to seem that i will never know the prob. do you get the flashing power led aswell?


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## octaneman (May 13, 2009)

Hi Brandenmramer 


The parts you have circled in your picture are called non polarized capacitors, to test capacitors connect your leeds and set your meter on the high scale. The meter will jump to zero and then it will slowly go to infinity. This means that the capacitor is being charged by the meter, once it reaches infinity the capacitor is fully charged.


A bad capacitor will show 0 readings the meter will not change value or If the capacitor is open you will get no reading at all. The most common problem LCD screens have is that the inverter board gives out. The inverter board is usually located at the top of the screen and it has several processors back to back. These are the drivers for the screen, to test them you will need an oscilloscope. If you don't have one use your meter to test for the voltage that goes to the drivers. To do this there should be a small plug or a ribbon cable (or both) that goes from the video card to the drivers. 


post back your findings.


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## sagey (Mar 1, 2010)

Yes I do get the flashing power led & I also have a good supply & don't see the difference of the good board & bad. 
There must be a component that quits when voltage is applyed.
I will put more effort into this.
Thanks


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## Done_Fishin (Oct 10, 2006)

Contrary to Octanemans comments, non polarised capacitors do not charge. Only Polarised capacitors charge.
The only way to see if a non-polarised capacitor is faulty is to measure it with a capacitor tester. This will measure the capacitors value. Note that a short circuit capacitor will show a low resistance. a good capacitor and an open circuit capacitor will both show high (infinite) resistance!
One of the items shown is an overvoltage protection device for the input AC. If this was faulty it would blow the fuse. It's the black item by the bridge rectifier. 

The blue items just below are ceramic capacitors, probably feeding to earth or chassis. 
Although it does happen, it's rare to find these items failing.

I personally would start off by replacing all polarised capacitors on the left hand side of the board that you show above. also there is an optocoupler labelled U800 just above the heatsink and by the side of it a small transistor like device that is most likely a TL431A. This gives a reference voltage for the voltage being controlled, and the control signal is fed back via that optocoupler.

there is an IC to the top centre labelled KA3842. this is the switch mode power supply controller. there are two electrolytic caps next to it. 
They could be the cause of your fault. they are important components in the way that the controller operates. you may want to consider replacing the IC as well , although normally replacement of the capacitors is usually enough, should that be the circuit that isn't working.
There are also another two caps by R809 R810. I cannot actually figure out what the might be responsible for from the picture but it's possible that if they aren't working properly, the set wouldn't turn on.

I would also check all diodes and transistors around the transformers. Although it's difficult to tell using a multimeter whether a diode is faulty or not. I have seen many tired diodes on Transformers that have measured Ok but just wouldn't cope with the frequency and reverse bias voltages. Replacement after checking with a component tester and noting the slow reaction to recovery, cured the fault. but how would you check that without equipment. only by replacement with something as good as or better than the original.

NOTE . This is dangerous stuff you are getting involved with here. Make sure that you work with the mains lead removed. try to power the set on without the lead connected to try drain any unwanted voltages from the capacitors.
capacitors should always be replaced with the same voltage rating and value as the Original. In switch mode supplies always use 105 degree C capacitors even if the original was 85 degree C. It will last a lot longer.


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## Brandenramer (Jan 30, 2010)

well i noticed that atleast one of the bad boards does not even flash the power led so im convinced they are different parts responsible. What is the process of elimination on this, shouldnt i be able to test the voltage out to the signal and inverter board to narrow it down to identify which cicruit to start with?


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## Done_Fishin (Oct 10, 2006)

if the switch mode supply is malfunctioning you are faced with a loop situation. any malfunction, anywhere in the loop causes a shutdown. 
Since the LED is flashing it's a good chance that your power supply is trying to start and then resetting. 
It's one of the most difficult faults to solve. 
Hence the reason why I told you everything in my earlier post.


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## sagey (Mar 1, 2010)

Update-I still havn't figured this out but I did replace u801 & did not change.
When this monitor is turned on I don't get a power light but when I unplug the vga cable the power light starts blinking.
Any ideas.
:4-dontkno


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## Done_Fishin (Oct 10, 2006)

I have no idea what U801 is apart from the fact that you changed it!

power light blinking could just be because the VGA cable is disconnected or it could be that the power supply is resetting itself, trying to start.

if you have several of these monitors and they have more than one board, being the same in all models, you may be able to number them according to the set from which they came, and then swap boards, trying to see if any "set' of boards and display might work.

Mark the sets as 1,2,3, etc and the boards accordingly. keep notes as to what each combination does as you check them out


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## atif32 (Feb 11, 2012)

dear i have also nec 1550v monitoe lcd my lcd os totaly dead
no led is lighting power section is working fine .producing 5 volt to control board and to the on switch ,but when i press button nothig happen


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## Done_Fishin (Oct 10, 2006)

if the power section is dead start by checking that you have standby voltage, the power switch makes contact, check for short circuits on the secondary, check electrolytic capacitors, transistors & diodes in power supply area, check the fuses .. and if yo have a second GOOD known working power supply anywhere, swap it out to see whether the set powers on ..


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## atif32 (Feb 11, 2012)

dear sir i have no other good power supply 

i yhave checked transister change capastor fuse but on the power switch 5 volt present when i press the switch the voltage cam down bit but not start power i thnk some thing is short but i cheked all thnik that i can do any troublshooting guide is this is the problem of control board or power supply


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## Done_Fishin (Oct 10, 2006)

when you say 5 volt drops a bit .. what reading do you get and with what type of voltage meter ?

Impossible to say where the problem is. Most times the problems are power supply related but I have had problems on monitors with display panels.

The major problem with switched mode power supplies is that they tend to refuse to work when an over-voltage or over-current condition exists and it's extremely difficult to tell which is causing the problem or if it is due to a failure in the start up circuit for the smps controller


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## atif32 (Feb 11, 2012)

so dear sir what can i do

wsap the board or 
i have check all the part one by one
any troublshooting sechamatic


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## Done_Fishin (Oct 10, 2006)

I have no information at all about this monitor. I usually work by experience based upon knowledge of a particular circuit or model and depending upon the way that the model behaves when I am checking it out. If you have checked all the basics including possible solder joint problems, then you'll need a second working unit in order to pinpoint what parts are functional and what parts need attention.


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## rooky72 (Jul 27, 2012)

Brandenramer said:


> I have 4 lcd monitors NEC LCD1550v is the model number, one works fine and three do not power on they instead have a quickly flashing power LED and no other symptoms. I know from exchanging the boards that all 4 work fine save for three of the PSU's this is my problem- these PSU's are not really Physically damages in any visible way and are not replaceable so repair is seeming grim. I have gone over them with the multimeter a great number of times looking for a difference between the one working board and the other three. All I have really found is that the non working boards seem to have slightly lower readings all the way around as far as resistors and capacitors but nothing jumping out as bad. There are some component i can not seem to get a reading on either board and seem to be the likely culprit how ever i can not test to be sure and i am not sure how to test the transformers. I am really new to this but im a quick learner, also very dedicated and i have accrued many broken lcd monitors and will not stop until they are mostly all fixed. Solving this one would be a tremendous help as i am struggling financially and will not sell the working one until the other three are fixed. Posted are two pictures one of the monitors and the other of the board, circled in yellow are some of the components i can not test. Any pointers or help of any kind is super appreciated, Thank you


mayby by repalcing C847 polarised capacitor it will fix the problem, in my case (See pic)
worked


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