Trying to fix my monitor

[Cyborg666]

Hi there!

I'm quite new to electronics, and Dave's video inspired me to try to fix my old LCD monitor.

I've tried a few things but now I'm stuck, so I figured I'll ask here
It has an external power supply, and it seems to function. I measure the voltage it says to give out, and no caps looks damaged in it.
When I plug it in to the monitor the backlight (fluorescent tubes) only lights up for about 1/3 of a second accompanied with a fizzing sound. Then it goes black.
I can't see any physical damage to the tubes or any blown caps.

Any ideas what might be wrong, or how I should go about finding out?

[digsys]

Does it work off an external DC Supply? I notice a DC socket. Do you have a power supply with V and I adjustments?
IF so, unplug the LCD / Back-light etc and see what the Power section draws - then plug in each "module" and recheck.
Somehow, you need to isolate where the problem is - which MAY be a shorted component. Start with that.

[Cyborg666]

Yes, it uses an external DC supply. Unfortunately I don't have a power supply, only a multimeter. Should I try to hook it up to measure the current?

[Cyborg666]

Oh and btw, I've unplugged the LCD screen and the button panel, with no difference.
When I unplug the last board you see to the right of the photo, I don't get anything happening with the tubes...

[dfmischler]

The most important thing to do is to decide if the failure is in the external power supply or in the monitor itself.  Can your multimeter monitor the external power supply voltage while you try to turn on the monitor?  Does it dip substantially when you do that?  If so, it might be bad... Monitoring the current might work even better, if you can see the maximum current draw (but inrush current might not be what you are after).

The board to the left in your picture seems to be the inverters for the backlights.  I suggest that you take any numbers off of that board and search ebay for a replacement board.  You could also search the web looking for a schematic, but I personally would doubt my ability to find the problem without other test equipment (e.g. an oscilloscope).

[digsys]

Yes, you definitely need to see the current. maybe you'll see a huge current surge, then a current limiter kick in, and see it drop.
There's 4 HV switchers in the pic. Look for a "feed" component and maybe disable each one in turn - connect the rest of the circuit.
Maybe one of those is shorted.

[peter.mitchell]

Probably a short somewhere in the inverter transformer windings. measure the primary and secondaries of the inverter transformers and compare them, one may be very low resistance compared to the others.

[Radio Tech]

I am an old TV tech that worked for Curtis mathis years ago. Never got into the LCD area.  I let the other tech's handle the new fangled stuff.  Then a few months ago I saw Daves video on fixing these buggers.
Thought to myself thats neat!. But I always run into murphy!
Like yesterday a friend brought in a Samsung 943n. Turned it on and it shows my desktop for about 1 second. then goes black while the power light stays lit.  Ripped all the caps off and replaced to find same issue.
So figure it is probably the inverter. So will save it for parts.

Good luck on fixing yours Cyborg.

[jaycee]

The ends of some of those tubes would suggest that the tubes have reached their end of life. Backlight circuits often have a circuit that will shut down the inverter if one or more of the tubes fails to strike.