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LCD monitor won't turn on

Discussion in 'Electronic Repair' started by james, Oct 6, 2008.

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  1. james

    james Guest

    I have an old broken viewsonic VG150 LCD monitor. Basically, it cannot be
    turned on even though the adapter is supplying a healthy 12V DC (measured
    with load).

    I opened the case and measured a pin labeled +12V and found only 2V when the
    power switch is held down, and goes to 0V when released.

    The front panel power switch itself is functional -- I short out the two
    contact points with a wire and got the same 2V as above.

    This could mean the circuit that controls the main power (probably a power
    transistor) is bad, or there is a short circuit dragging down the voltage.
    Since there is no smoke when I held down the switch, I'm hopeful it's the
    first case.

    The power supply (external adapter) is 12V 4A. The wires leading to the
    front panel power switch is very thin, and the switch itself is a push
    buttom with momentary contact. So I figure the power switch is not what's
    directly turning on the power; it must be controlling a semiconductor that
    turns on the full 12V 4A to the entire unit. However, I did not find any
    power semi-conductor that looks like it can switch on/off 4A of current.
    Here's a photo of the circuit board:
    (put mouse on the photo and click "original" to see full resolution)

    Any idea which chip/transistor controls the 12V 4A to the rest of the

    To further debug the problem, I need a circuit diagram. Any idea where/how
    to get a circuit diagram of a viewsonic monitor? I looked on viewsonic's web
    site and the phase "cicruit diagram" or "repair manual" returned zero hit.

    Without circuit diagram, the only thing I can think of is to solder a wire
    directly from the input 12V DC to one of the +12V lead. If this works, it
    means the unit cannot be turn on/off via the front panel switch. If it
    doesn't work, it may generate some smoke...
  2. lurch

    lurch Guest

    Don't know if it helps but Google the number stamped near the top of the
    board (3150-0122). Got several hits and at least one source for a
    replacement board ($80).
  3. Adrian C

    Adrian C Guest

    IC18 to the left of the power socket? I can't read the type number other
    than it's made by International Rectifier..
  4. Franc Zabkar

    Franc Zabkar Guest

    It took my browser forever to render your X3 image (in fact I gave up
    waiting for it), but I finally worked out how to link directly to the

    The fact that the something is happening when you press the on/off
    button suggests that the uP is alive, and that the +5V(?) supply is
    OK. I suspect that one of the AmTRAN chips (AM30 or AM100B) controls
    the power to the backlights. It appears that Q15, the PNP (?) smt
    transistor near the lamp connector, switches the +12V power to an
    off-board inverter. I'd monitor the +12V supply on its Emitter (input)
    and Collector (output) pins. The Base should be getting an on/off
    signal from the uP via a resistor. Perhaps the inverter has a short
    circuit which has taken out Q15 and/or a fusible resistor.

    BTW, the Mitsubishi M52743BSP chip is a "I2C BUS CONTROLLED 3-CHANNEL
    VIDEO PREAMPLIFIER", so I don't think it handles power control:

    FWIW, a Nokia LCD 510L monitor appears to use the same chips, so maybe
    your Viewsonic is a rebadged unit.


    - Franc Zabkar
  5. Andy Cuffe

    Andy Cuffe Guest

    The first thing I would check are all those electrolytic caps. About
    half the LCD monitors I see just need caps.
    Andy Cuffe

  6. james

    james Guest

    I have an old broken viewsonic VG150 LCD monitor. Basically, it cannot be
    Forget what I said about 2V. I measured the wrong pin.

    Surprisingly viewsonic gave me the repair manual, but I have to sign a NDA.
    So I can't reveal too much here.

    There is an IC that implement the power switch toggle effect. That is where
    the problem lies. The front panel power switch is connected to the clock
    input of this flip flop via some RC network but for some reason it doesn't
    want to flip. If I manually connect the +12V to the set or reset pin of this
    flip flop, I can turn the monitor on / off at will. So I can add a toggle
    switch to do this; it would just look a little ugly.

    I hope other LCD manufacturers are as nice as viewsonic in providing circuit
    diagram. Has anyone ever obtained repair manual for other brand of LCD
  7. Franc Zabkar

    Franc Zabkar Guest

    I thought you may have been measuring the +12V at the lamp/inverter
    connector. It may have helped if you were more specific.

    BTW, does Q15 switch power to the inverter?
    So you are talking about IC30, the MC14013B dual D-type flip-flop?

    AFAICT, the circuit looks like this:

    | |
    | |-- R -- Vcc |
    | | |
    | ____o___ |
    Vcc | | S | |
    | | | | |
    R |---| D Q*|-- R94 --+
    | | | +_|_ C11
    +----+---|>Clk Q |--> ___ 10uF 16V
    | _|_ | | on/off |
    / SW ___ |___R____| |
    | | C o _|_
    _|_ _|_ | =
    = = |--- R --- Vcc

    R94 and C11 effectively debounce the switch. If the chip is OK, then
    I'd suspect that C11 is open. Try connecting a 10uF cap in parallel
    with C11.
    Mitsubishi Electric Australia will sell me a manual for mine, without
    an NDA. LG supply manuals for their LCD TVs, without an NDA. The FCC
    database has many circuits for LG monitors and other manufacturers,
    all of which you can download ... without an NDA.

    - Franc Zabkar
  8. james

    james Guest

    You're a walking IC encyclopedia? :)
    Close enough. I tried assuming each of the component is bad and did a patch
    test (e.g. short the cap to Vcc or to gnd) but still couldn't make it work.
    I hooked up a scope on the Clk and it shows a pulse when the power switch is
    pressed. If I use a frayed wire to short the switch (this creates many
    pulses) then it sometimes work after many attempts. Increasing or decreasing
    the voltage by 1.5V doesn't help (use an ohmeter Rx1 range to short the

    I don't know if it's a good idea to try to replace this chip. It's surface
    mount and the chip is small (about 1cm) and the leads are even smaller.

    I'll play with this until I get tired of it. If I can not figure out the
    definite cause then I'll just glue on an additional switch.
    Thanks for the info. I didn't know FCC has circuits in their database.
  9. Franc Zabkar

    Franc Zabkar Guest

    4000 series CMOS logic has been around for decades. Anyone from the
    old school would recognise that chip. BTW, I think that the R and S
    inputs should be active high, not active low as I've drawn them.
    If you short the cap to ground, then the D input is always 0, which
    means that Q will always be a 0. If you tie the cap to Vcc, then Q
    will always be a 1 ... if the chip's clock input is working.
    Practice on a scrap board.
    Being CMOS, and the fact that the chip is connected to the real world
    (ie the switch), it may be that the clock input has been damaged by
    ESD. (???)
    Neither did I until recently. Most circuits appear to have been added
    after 2000. Not only that, but there are user manuals, block diagrams,
    operational descriptions, and internal and external photos. They're
    not always good quality, but they might be better than nothing.

    One major downside is that the "searchability" of the database is very
    poor. For example, I'd like to be able to locate all LG LCD monitors
    for which circuits have been submitted. Or I'd like to find all ADSL
    modems which use a Globespan Virata chipset. Maybe the FCC should let
    Google manage their database.

    - Franc Zabkar
  10. Franc Zabkar

    Franc Zabkar Guest

    This is it:

    Grab the FCC ID off the label on your device and type the characters
    into the Grantee Code and Product Code boxes.

    Note that most products will not have circuit diagrams because their
    manufacturers often choose to hide behind confidentiality provisions.
    I still managed to find about 100 circuits for LG LCD monitors and
    TVs, though.

    - Franc Zabkar
  11. Guest

    Could you please provide me Viewsonic VG150 repair diagram or manual?
    I also have a broken VG150.
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