Trying to fix a monitor power supply

Discussion in 'Technical Repair' started by littlerocker, Jan 31, 2019.

  1. littlerocker

    Thread Starter New Member

    Jan 29, 2019
    2
    2
    Hi guys, I have a problem with a blown power supply from a Samsung monitor. Specifically the power supply is a bn44-00750A. Basically bought a broken monitor mainly for myself to try and fix it and also learn in the process. Myself I am more of IC design engineer, but I have worked with pcb’s and components in my student life and I feel confident doing the repairs, but I am not sure exactly what to look for as I have no experience with power supplies (but hey, never too late to learn).

    And after doing some tests myself I found that:

    1. First things first, do we have expected 13V output voltage? Nope :(

    2. Ok then let’s check the components: Fuse is gone

    3. Bridge rectifier is gone-> the bridge simply outputs the 220 ac voltage. plus all pins are shorted.

    4. There was a 0.10ohm resistor that was clearly burned further down the path from the rectifier

    5. I am pretty sure that some of smd transistors on the hot side are also burned. (in red circle what looks like a transistor, all pins are shorted, but the ones next to it seems fine)

    6. The main capacitor is not shorted, but I cannot test it properly without taking it out, since it is glued like hell to the pcb.

    7. I have checked all diodes and they are ok.

    In the pictures I have circled in red what I think it is still broken, and in blue where the resistor, bridge rectifier and the transistor were. I have taken progressively zoomed in pictures to have a clear view of the board. I focused mostly on the hot side of the board.

    Now before going into the details, I would like to know if there is a possibility that the cold side was also affected and if (I hope not) the main board of the monitor could also be damaged. I personally think not since I do not read any voltage at the output.

    Also how can I find a good replacements for some components (for example the small smd transistors). On the internet I couldn’t find any schematic and specs of this board.

    I am looking if there is anybody who could give some pointers as to where exactly to look for more damage and if it is worth trying to fix. Thanks!
    20190127_103151.jpg 20190131_094709.jpg Picture 2.jpg Picture1.jpg 20190131_094609.jpg
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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  3. wayneh

    Expert

    Sep 9, 2010
    16,102
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    Of course it is possible it was part of the event that brought the monitor down. I think it is impossible to estimate the odds but, like you, I would probably hope for the best and see if the power supply can be brought back on line.
    That makes repair very difficult. SMD transistors are notoriously hard to identify especially if they have been damaged so any markings are gone. I'd keep looking for the service manual or schematic. It can be hard and take a while. Sometimes you can find a schematic of a related model.
    I suspect our best monitor repair expert around here is @R!f@@ but he is busy and doesn't show up often. Maybe you'll be lucky.

    As to whether it is worth fixing, I think that is a solid no if it is a CRT monitor. If you succeed, you have something you'd have to pay to have hauled away. They have negative value these days. It's probably not worth it for an LCD either – it depends how nice it is. Or was. But you mentioned wanting to fix it yourself, and that's a real thing. Only you can assess whether the challenge is worth the payoff. I've been in your shoes and it's easy to keep plodding forward long past the "worth it" stage.
     
  4. littlerocker

    Thread Starter New Member

    Jan 29, 2019
    2
    2
    Hi! Thanks a lot for the answer. I did already order some parts to change to see if it is of any luck. I ordered the bridge rectifier, the fuse, the blown resistor and one of the power transistors that was also blown.

    As for the monitor, I basically got it for free from a friend and new it is still worth (here in the Netherlands) 500 euros. And, except the learning part, I think it is worth trying to fix. Of course if the damage goes further to the main board of the monitor, or the screen itself (which I highly doubt) then maybe I just have to throw it away.

    The smd transistors I took them off the board and tested them separately, and they are not shorted, I have realized that the pins were connected via a small resistors (33 ohms) which was enough for my multi meter to consider as a short.

    Once I get the part and solder them in, I'll update. Also with details of what I've done, maybe it will be helpful for other people looking for the same answers :).
     
    subshadow1 and wayneh like this.
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