27" Samsung monitor

[RyanG]

I was given a monitor that worked for about 5 minutes. The monitor turns on but no picture comes up. Whatever computer I hook it up to detects it, and I can change its settings if I am using it as a second display. But, nothing ever shows up on the monitor. I tried plugging it in with no video signal and with VGA, DVI, and DP. The power supply is putting out the correct voltages and every test point on the board comes up correct except 1. In the picture that I attached it is the point labelled T_P_5V just above and to the left of the 2 LD1117 voltage regulators. The 6 pin IC to the left of the test point is labelled DFYA2X, and I have no idea what that is. Any help would be appreciated otherwise I'm throwing it away. The bottom of the board only has test points on it.

[bitseeker]

So, what voltage do you get at TP 5V?

The pads for C421/C422 look like they've been reworked. What kind of voltage is across there?

Also, does the backlight come on? If not, can you see an image if you shine a flashlight into the screen when it's connected to a video signal?

[RyanG]

I get 0v at the TP and on the C421/C422. I had soldered a wire onto the C422 spot to check the voltage as I was messing with some stuff. That's why it looks that way. I haven't had any backlight since it worked for the few minutes that it did. I tried the flashlight thing too and I saw nothing. I hooked my garbage scope up to the TP and it gave me a reading of about 4.7v for about 0.1s every ~6.5 seconds. The 5v coming in from the power supply isn't very smooth either with the worst of it coinciding with when I get the voltage spikes at the TP.

[RyanG]

I now tried hooking up the monitor with just the front panel connector and power out to the LEDs. The screen lit up and blinked a blank screen repeatedly. I tried plugging in the flat cable to the back of the panel and the flashing of the screen stopped immediately with the LEDs off.

[RyanG]

I hooked up an external 5v supply to the board and I got video but the backlight flashed on and off the entire time. I'm thinking it's the power supply.

[Neganur]

Could it be a fw error? A lot of Samsung monitors suffer from fw corruption and they look as if they're broken. You'll need a VGA cable and with a little luck you'll find the software on Samsung's website.

[bitseeker]

I'm not familiar with that specific monitor, so Neganur's suggestion is worth investigating.

Generally speaking, dead monitors are due to the power supply. If that one has a switching supply, it behaves as though it isn't starting up or going into safety/overload shutdown. A bad starter capacitor, dead optocoupler (no feedback, can't regulate, so shuts down), shorted rectifier, or burned out load resistor are common faults.

[RyanG]

At the moment it is working fine. I resoldered all the power connector pins onto the board I posted before and everything seems alright.

[bitseeker]

That's great news, Ryan. Congrats and thanks for the update.

[Rasz]

At the moment it is working fine. I resoldered all the power connector pins onto the board I posted before and everything seems alright.

it might start ok after resoldering because heat makes dead capacitors temporarily heal
what is your power supply situation? correct voltage with no load is no indication

[RyanG]

It has been working fine for 2 days now. I desoldered and tested the electolytic caps. The voltage was actually low without a load when I was checking the supply.

[RyanG]

It finally went out again, so back to my guessing.

[Rasz]

It finally went out again, so back to my guessing.

does sound like heat healing

>I desoldered and tested the electolytic caps.

capacity or resistance?

[RyanG]

I tested them all in circuit for resistance and then desoldered them all to check them again

[Rasz]

well, Im at a loss then.
symptoms you describe sound like a classic textbook capacitors healed (for couple of days) by desoldering.
There is an easy way to check this theory - aim a hair drier on max heat at the supply until its very warm/hot and try powering it on again. Its not necessary the big caps at the output, might also by one of the smaller caps powering buck converter.
 as a matter of fact I have 26' LCD on my desk doing almost exactly same thing right now (backlight for <1sec and then going blank audio only), I heated it with a hotair on monday and it lasted till Saturday.

easy way is just replacing all electrolytics in bulk if you value your time above couple of bucks, you can always pull them off if it doesnt work

[james_s]

I have to agree there, just replace the capacitors and go from there. Bad electrolytic capacitors are such a common problem with this sort of thing and they're difficult to really properly test in some cases so if in doubt, change them out. The heat from desoldering will temporarily lower the ESR and make the capacitors test better than they actually are.