Samsung T200HD backlight issues

[gabris[LT]]

Hello, I have tried to solve problem with my monitor for quite a while now, yet unsuccessfully and hopefully someone will be able to help me.

When I got this monitor it was suffering from 2 seconds to black problem. When I have disassembled it I haven't seen any bulged caps, nothing wrong visually at all, but as I didn't have an ESR meter I decided to replace all caps on power board, after doing so I tried to switch it on, nothing changed. Then I have noticed that some solder joints on main board looked like they have suffered from oxidation, I tried to reflow the main board using some alcohol based flux, after that it started to work just fine with backlight going off extremely rarely.. Well, just for some time. When summer came in and ambient temperatures have raised I started having a lot of issues again, but at least this time it was not 2 seconds to black, or at least I thought so.. Now backlight goes off randomly after about 20-40 minutes of flawless work. The only thing I could think of at this point was cold solder joints on CFL lamps themselves, so I have dismantled the LCD panel, tried to slightly pull the CFL lamp wires to check if it was poor connection due to cold solder or burned wires, nope, wires were not burned at the ends and I was unable to pull out any of the wires, after doing so just in case I might have probably lost the contact I assembled the monitor back, and checked, no difference. Primaries and secondaries also seem so be fine (difference is well within 1%). Maybe someone has any ideas what might be the issue?

P.S. I have noticed that when backlight goes off I'm loosing voltage on Dimmer pin which goes from main board to inverter board.

Photos of the boards attached.

Tools I possess now:
Hot air soldering station.
Mega328 based LCR meter (includes capacitor ESR meter).
Regular soldering iron.
Digital and analog multimeters.

[Lumberjack777]

Re: Samsung T200HD backlight issues

Great explanation and pictures. You need to isolate the problem to see if its in the main board or the inverter board. Are you still getting power to the inverter board when the lights go out?

Since you fixed the problem by reflowing bad joints, it's also possible there's still a bad joint somewhere. As a monitor heats up there's a small amount of expansion inside that can cause a bad solder joint to fail. I use a jeweler's loupe to check for bad solder joints. Good luck.

[gabris[LT]]

Re: Samsung T200HD backlight issues

Did reflow the mainboard once more yesterday, nothing changed and having assumed that powerboard and mainboard should be fine at this point, also after doing some research I'm assuming that it should be CFL bulbs, I have checked the CFL bulb connections yesterday by pulling the wires again, now I got to 2 seconds to black again, hopefully after fixing the loose wire it will be fine, will update this post straight after I'll check.

[stj]

Re: Samsung T200HD backlight issues

connect some different lamps to the invertor and see if they stay on.

[gabris[LT]]

Re: Samsung T200HD backlight issues

really strange things happening here.. I had 2 seconds to black, after disassembling the panel ant trying out different lamp connection combinations, 2 seconds to black issue disappeared and CFL lamps started to stay lit and pulling the wires at the bulb end doesn't do anything, so yet I cannot isolate the issue, but it looks like the problem is with the contact somewhere, yet I cannot say where.

EDIT1: bending the cfl wire doesn't do anything either.

EDIT2: when another lamps are connected it just blinks for a moment when turned on.

EDIT3: after few more attempts of turning monitor on, the other lamps lit up(while pushing lamp jack slightly upwards), but after few minutes lamps went off, then I got just short blinks on power on again until I pushed the jack upwards again. (might be the issue? - socket iself?)

[stj]

Re: Samsung T200HD backlight issues

maybe - resolder the sockets.

[gabris[LT]]

Re: Samsung T200HD backlight issues

I don't have such sockets nearby, so instead I have bent the pins inside them, works without backlight going off for already one hour.

[gabris[LT]]

Re: Samsung T200HD backlight issues

Sorry I didn't want to double post, but I don't see edit button anymore.
After slightly bending the pins issues appeared again within few days, so I have soldered lamp wires straight into PCB, no more issues for already a week so far.