Monitor turns on occasionally!

[Srivatsa R Upadhya]

I have a cheap quality monitor. The problem is it turns on randomly. Once it turns on it works fine. I can turn it on and off with the buttons behind the monitor, but if I turn off the power to the monitor I cannot get it to turn back on(It sometimes turns on after several tries to turn it on).

I checked the power supply voltage(12v), it is good. There is a step down DC/DC converter(FR9888) on the display controller board which is also alright(5v output).

What should I check next?

[LateLesley]

Check the 12V secondary capacitors on the power supply, It's known if they are bad, they can cause problems powering on, due to ripple on the powerlines.

https://www.ifixit.com/Guide/Repairing+an+LCD+Display+with+failed+capacitors/4131

[m3vuv]

I had a pioneer amp tuner combo that did pretty mutch the same a few weeks ago,did the norm replacing caps etc,made not a scrap of difference,checking the standby/power switch should of been my first port of call but it was a pig to get to,in the end i bit the bullet and removed the tactile switch,that tested iffy,replaced it and fixed it 100%,first tactile dome switch ive ever seen go bad,just goes to show.

[samnmax]

I second the recommendation of checking the On/Off button. Those can go bad and start conducting (~kOhms) even when not pressed.

https://www.eevblog.com/forum/repair/samsung-p2770hd-monitor-brightness-setting-goes-to-max-after-10-15-minutes/msg3503784/#msg3503784

[Srivatsa R Upadhya]

I checked the capacitor voltage keeping my multimeter in ac voltage mode for ripple, there was nothing. I also tried a different power supply. It still does not want to work . The same problem !!!

[james_s]

I've seen this happen when somebody washed the screen of a TV with glass cleaner and it dribbled down into the buttons. If you have high humidity this could also cause a similar problem. Can you unplug the power button from the main board easily? That would be a good way to isolate it, or you could try measuring the resistance across the button and see if there is any conductivity when it is not being pressed.

[MathWizard]

Where does the power to the on/off switch come from? Is it a separate DC/DC converter and transformer for STBY power ? I'm thinking more of an isolated 'forward converter' type SMPS.

Are there any transformers in the monitor ?? Either way where does the power to the on/off switch come from ? Maybe it's button bouncing on the power switch, and or i mean it's cap/supply voltage

[Srivatsa R Upadhya]

I checked the voltages, the capacitors, the power switch. They all seem to be fine. Those are the ones that usually cause the problem and I think I can rule them out! I have some pictures of the boards. In the board having the buttons, I have retained only the power and the auto adjust buttons and removed the rest.

[kripton2035]

I would check what's happening WHEN IT DOES NOT TURN ON.
measure voltages, ripples, but only when it doesn't power on. then only you can find what's wrong.

[Srivatsa R Upadhya]

Yes,  I did the measurements when it didn't turn on. Most of the time it will not turn on!

[m3vuv]

just a thought,i lived in france about 20yrs ago and had my old uk crt tv (240v) france at the time was on 220v,most of the time it worked ok but in times of peak power demands on the grid the power would sag to about 190v and the tv would not turn on at those times!.

[james_s]

Wow, 190V is quite a sag.

If it has intermittent faults I would suspect electrolytic capacitors or solder joints. You could try warming up the board with something like a hair dryer and see if that makes it start working. If it does then try cooling certain spots of it using a can of air duster held upside down.

[ambrosia heart]

" but if I turn off the power to the monitor I cannot get it to turn back on(It sometimes turns on after several tries to turn it on)."

How many people here do you think they are really know the working principle of a SMPS, just as simple as yours?

Very easy to fix, not related to caps but several SMD components on PCB. Turn the pcb over and find them out.
I know they are located on the back of pcb from your photo.

See pm to find out how to fix and remember don't quote what you read on here (my pm) , just FYI.

Remember to use google translate to see what I wrote in thread 12 and 22. I gave the brother a complete explanation of your case.

Enlarge thread 22 photos to trace the ic Vcc from positive pole of the large cap.

Tell me the result if you want.