Monitor dead, how to check transistors?

Discussion in 'General Electronics Chat' started by AE35, Dec 1, 2005.

  1. AE35

    Thread Starter New Member

    Dec 1, 2005
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    0
    Hi all!

    I'm new here!

    I just got a 21" re-badged Mitsubishi monitor, that is completely 100% dead. I
    checked the fuse, which was dead, and changed it. When I fired her up again, a
    zener diode next to the power input blew, along with the new fuse.

    http://www.avsforum.com/avs-vb/attachment....tid=47114&stc=1

    So, I've been thinking about what could be wrong. All the caps look fine, so I
    thought there may be a resistor problem!? and if it is, how do test them? can I
    test them without removing them?

    I also have a 19" Sony monitor that turns on, but no image at all. This was
    supposed to have happened during lightning. Fuse is OK.

    Thanks for your time, I really hope someone can help!

    Thanks,

    Nicholas
     
  2. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    have you check the horizontal power transistor?
     
  3. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    You can test for shorted horz. power transistor first. Most of the time this is the culprit.

    The Sony monitor needs more analysis. It is not easy to tell what's the problem is from your description. Also, Sony monitor is not the best place to start learning how to repair. They are a bit more complicated than others.
     
  4. AE35

    Thread Starter New Member

    Dec 1, 2005
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    Thanks!!

    How do I locate the H. power transistor?? and can this cause a completely dead
    monitor??

    Thanks for your time!

    Nicholas
     
  5. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    It is quite a big transistor, always cooled with a heatsink or bolted to the board's perimeter metal. Usually located in the proximity of the horz. hv transformer. You might want to check related circuits too. They might cause the horz. tr to blow up.

    Yes. Most of designs out there combined the horz, HV, and step-down switcher into the horz. transformer. It's cost efficient to do it this way. If this system doesn't work, then there'd be no power at all to most of the circuits. There are, of course exceptions such as power-saving circuit in monitor or remote control/standby circuit in TV.

    If the horz. tr is shorted, then it'd blow the fuse because it is connected between the positive supply (through the transformer to the rectified mains) and the ground. That is why the monitor is dead rather than having the power-saving or standby circuits (in case of TV) working.

    May I ask what's the zener is connected to? It seems strange to have a zener in the proximity of the mains input. Unless it is there to supply standby circuit. You might want to check the resistor that connects the zener to the supply. There has to be a reason why the zener blew.
     
  6. AE35

    Thread Starter New Member

    Dec 1, 2005
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  7. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
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    Can you post the picture in this forum? I don't want to register in another forum just to view it.

    BTW, is it a diamondtron?
     
  8. AE35

    Thread Starter New Member

    Dec 1, 2005
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    Oops sorry, I didn't know it worked like that!!

    I'll put it on my server when I get home!

    EDIT: Oh , look, there it is !! Attachment

    About what monitor, I'll have to look into that when I get home in an hour.
    It's a rebadged mits, now the front says Digital. Produced in 1995.

    Thank you very much for your help so far!

    :)
     
  9. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
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    Ah.. it looks like a stand-alone supply separate from the horz. circuit. The transistor should be the one bolted to the big slab of metal near the transformer. I'm not sure about the zener, have to see what it is connected to. Can't really tell from the picture.

    I don't think it is a diamondtron, the way to recognise it is to see the screen. If it is flat vertically but curved horizontally then it'd be a diamondtron. If not then it's not. I don't think it'd be completely flat vertically and horizontally for a monitor made in 95.
     
  10. Sebi

    Well-Known Member

    Sep 24, 2005
    59
    0
    I think You have two same zeners, parallel with big buffer-elkos. Most of case, if two elkos existiert, the supply is switchable 120 to 230V (input AC). The voltage on elkos about 150-150Volts, so i mean the zener was 160..180V (check the healthy). When the mains fuse blowed, the failure is in the primary side, so check the bridge rectifier, buffer-elkos, parallel zeners and the input voltage selector switch position. The power transistor sitting on the heatsink, the output rectifiers, and horizontal supply PNP BJT on the Al-plate.
    (The supply can work also without zeners for short time, for checking the equivalent voltages on the capacitors. Bigger difference as 10..15V indicate the capacitance diff.)
     
  11. Nicholas

    AAC Fanatic!

    Mar 24, 2005
    121
    1
    Thanks!!

    Can you tell me what elkos is??

    And by the way, the zeners are called KIV44N, but I can't find them anywhere!?

    Thanks,

    Nicholas
     
  12. Sebi

    Well-Known Member

    Sep 24, 2005
    59
    0
    Can you tell me what elkos is??

    The two big caps, about 330 or 470uF 200V

    And by the way, the zeners are called KIV44N, but I can't find them anywhere!?

    Don't worry about type, just use some 180V 1-2W type.
     
  13. AE35

    Thread Starter New Member

    Dec 1, 2005
    8
    0
    Kool!

    I checked the large caps, they seem okay.

    I'll get some new zeners, and a new slow-blow fuse. Maybe the
    problem has fixed itself:)

    Thank you very much!!!
    Nicholas
     
  14. AE35

    Thread Starter New Member

    Dec 1, 2005
    8
    0
    IT WORKS NOW!!!

    I just popped in the new zeners and a new slow-blow fuse, and the monitor works perfect! Ready to go into my arcade!

    Thank you all VERY VERY much especially Sebi and n9352527!!

    Nicholas
     
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