An LCD Computer Monitor Comes On But No Display Once Connected To a Computer Repaired
A guy who works in olive wood carving using a computerized system came to the shop and brought his LCD monitor for repair. The customer told me that it was not powering on. I powered it on and sure enough it was not coming on but when I took the back cover off, the power cable was disconnected. I connected the power cable back and the screen came on with the display “No signal”. I called the customer and told him that it was powering on but I cannot test it because it has a VGA connection and my new laptop does not have that kind of connection.
He came and took the monitor back to his shop and then a few minutes later he called back and said the screen was powering on but not detecting any signal on the display.
I took the screen cover off and cleaned all the dirt and as you can see in the photo, it is looking nice on the inside. I had this screen before in my shop but that was about five years ago because I recognized the capacitors that were replaced in the power supply section.
I started to check for bad soldering joints or any loose connections on its sockets but everything seems to be ok so I started to do what I did five years ago when this screen was on my work bench and that is looking for bad capacitors.
I got my ESR meter out and started testing of all the capacitors on board including the primary and the secondary sections of the power supply and sure enough I came across bad capacitors that were giving me high ESR reading.
I do not remember putting such a capacitor in the secondary section of the power supply especially with this kind of power ratings. It usually takes about 35 DC volts but this one rated a 100 Volts.
So I replaced this capacitor with 47UF/35 volts, soldered on board and powered on the screen. The screen came on but this was a moment of truth when I connected this screen to a laptop that a customer brought for repair (a day later) and…
At this time the computer monitor was connected to the laptop and it was the time to flip the screen and see if I have a display.
Yes, the screen had a display and I called the customer to come and pick up his screen but he could not because snow started to fall and streets were getting closed because of the storm.
A week later, the customer called if he can come and pick up the screen and finally he was able to. He was happy with the outcome and so was I.
Mission accomplished.
This article was prepared for you by Waleed Rishmawi, one of our ‘Master Authors’ and currently working in the Bethlehem area of Palestine repairing electrical and electronic equipment.
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Note: You can check his previous article on Could It Be That Easy Repair For This Battery Charger? Model: Hilty
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Anwar Shiekh
March 18, 2022 at 7:06 am
How strange that the 100V capacitor was the problem
Tayo
March 18, 2022 at 2:07 pm
It likely had a high or borderline ESR when it was installed. When I salvage Caps I always check their capacitance and ESR before they go into my repository. Then I check them again before I install them in a repair.
Waleed Rishmawi
March 19, 2022 at 4:13 pm
Tayo: higly unlikely since I do not put used capacitors when I do the replacement and even when I replace them with new ones I always check the capacitance and the ESR reading then do the replacement. have a blessed day
Waleed Rishmawi
March 19, 2022 at 4:11 pm
Anwar: strange??? I do not think so. have a blessed day
Rodolfo Gómez G.
March 23, 2022 at 1:41 pm
Los capacitores electrolíticos se degradan con el tiempo, y tal vez este componente estuviese defectuoso.
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Electrolytic capacitors degrade over time, and this component may be defective.
Tayo
March 18, 2022 at 2:09 pm
Very good job and very informative. I am surprised that the problem was on the PSU and not on main board since it basically a data/signaling issue. I'll keep this in mind if I ever come across such a case.
Waleed Rishmawi
March 19, 2022 at 4:15 pm
Tayo: thanks man. have a blessed day
Parasuraman S
March 18, 2022 at 3:00 pm
Good job there, dear friend! Your sharing these experiences enriches ours too! Many thanks!
Waleed Rishmawi
March 19, 2022 at 4:15 pm
Parasuraman: thanks my friend and you are most welcome. have a blessed day
Paul
March 18, 2022 at 6:49 pm
Have you measured the capacity and ESR of the removed capacitor?
From my experience, the 100V rating isn't the cause of the failure.
Waleed Rishmawi
March 19, 2022 at 4:17 pm
Paul: Yes I did measure the ESR meter and it was high and that is the reason it was replaced. this repair was done almost a month ago and the screen is still up and running confirming the fact it was the fault. have a blessed day
Albert van Bemmelen
March 18, 2022 at 7:25 pm
These capacitors keep causing environmental E-waste issues. Caused by the ag(e)ing electrolyte used in those caps. How different with coils that almost never deteriorate.
Waleed Rishmawi
March 19, 2022 at 4:18 pm
Albert: I totally agree with you there. have a blessed day
Babu M S
March 18, 2022 at 11:07 pm
I have learned a new thing that power supply was the reason for the said problem. Thank you very much for sharing.
Waleed Rishmawi
March 19, 2022 at 4:21 pm
Babu: yes sometime the power supply sometimes a different issue that is why we check everything and never assume things when it come to repair. have a blessed day
Imoudu Onwumah
March 19, 2022 at 6:39 am
Just as Tayo pointed out,I thought it was mainboard problem, but you prove out smps can also cause such "no signal" problem.I am learning more from you.Is this applicable to tv too? Thanks for sharing.
Waleed Rishmawi
March 19, 2022 at 4:22 pm
Imoudu: a no signal issue sometime caused by power issue so when it come to repair, we never assume, we find what was wrong. have a blessed day
Yogesh Panchal
March 19, 2022 at 8:29 pm
Good Job! Waleed,
As i can see this capacitor is on Power board & nearby cable to this capacitor is going to main Board it shows that it supply standby & 5V /12V supply to Main processing Board.
Waleed Rishmawi
March 21, 2022 at 2:02 pm
Yogesh: thanks man
yes it is and that was the fault to no display issue in this computer screen. it has been running ever since. have a blessed day
maurice
March 26, 2022 at 10:00 pm
Capacitors that are rated for an higher voltage could have higher esr than the same with same value but for lower voltage. So always check that. Normally it is better not use 100v ones instead of for example 25v because of de higher esr. This is only for capacitors that need to be low esr ones. If they are normal ones than you van use ones with a lot higher voltages.