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I have this old LG Flatron w1934s that worked fine until it‘s power indicator led started flashing, and after a long while the monitor turned then on, if I heated the monitor’s back with something like a heat gun it started after a few seconds! While the starting process is going it is possible to head a switching mood from the monitor’s PCB, a sound that during operation is not present! I took the monitor apart and tested the voltages on the backlight lamps, I found one, top right in the picture, to have a lower DC voltage than the others at startup, and an equal one during operation...I also tested how log the monitor took to start depending on the zones I heated in the PCB, the two mosfet’s heatsink, if heated, is the fastest one to make it start!

My question is if it’s most likely a bad joint problem, that a resoldering could fix, or what else can I look into?pcb top

pcb bottom

capacitors and mosfets

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    \$\begingroup\$ One of the capacitors (C012? Hard to tell what the silkscreen says.) appears to be bulging. \$\endgroup\$
    – Dampmaskin
    Commented Feb 22, 2020 at 14:00
  • \$\begingroup\$ It is, tested the capacitance and seems fine, and from what I read online, heat can’t fix this issue, should I replace it anyway? \$\endgroup\$
    – EMJzero
    Commented Feb 22, 2020 at 14:11
  • \$\begingroup\$ How did you test it, by removing it? \$\endgroup\$
    – Marko Buršič
    Commented Feb 22, 2020 at 14:38
  • \$\begingroup\$ Capacitance is no the only value of importance. When capacitors fail like this, the capacitance can be fine, or even become larger than spec, but the ESR (equivalent series resistance) can shoot through the roof. \$\endgroup\$
    – Dampmaskin
    Commented Feb 22, 2020 at 19:13
  • \$\begingroup\$ Since I have to order replacements I temporarily desoldered it to take measurements! However I never knew about the ESR issue, so that may be the problem, as far as you know, that can chance with heat? (I’ll swap our the cap anyway) \$\endgroup\$
    – EMJzero
    Commented Feb 22, 2020 at 20:16

2 Answers 2

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I see at least two things:

First is the capacitor that's already been pointed out to you.

That's this one:

enter image description here

It can cause many kinds of problems. It needs to be replaced, no matter what else may be wrong.

The second thing I see is this joint on the back of the board:

enter image description here

It looks to me like the left pin of C810 has come unstuck. There's what looks like a ring around the pin. It might be just a shadow, but I think it's a gap between the pin and the pad.

I'd probably touch up all the joints just to be safe. That kind of thing will respond to heat and wiggling, so it might be your culprit.

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    \$\begingroup\$ You have a falcon eye. \$\endgroup\$
    – Marko Buršič
    Commented Feb 22, 2020 at 14:50
  • \$\begingroup\$ "works when heated" (or cooled, or rapped with a mallet) often points to failed joints like this. \$\endgroup\$
    – user16324
    Commented Feb 22, 2020 at 16:24
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    \$\begingroup\$ Thanks, I’ll attempt both solutions and then give you some news! \$\endgroup\$
    – EMJzero
    Commented Feb 22, 2020 at 18:37
  • \$\begingroup\$ Broken solder joints are a common fault with single-sided PCBs, especially on components that get hot or have mechanical stress on them (mains socket, large capacitors, power transistors etc.). \$\endgroup\$
    – Bruce Abbott
    Commented Feb 22, 2020 at 18:48
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enter image description here

Replace at least this capacitor. But you could replace all four on the other part of the heatsink.

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