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LCD TV stand by voltages. 5 volts and?

Discussion in 'Electronic Repair' started by Dave, Jun 21, 2008.

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  1. Dave

    Dave Guest

    I'm wanting to determin if my power supply voltages are correct on my
    dead LCD TV. On my power supply board the output voltages are printed,
    but since the set doesn't turn on, the only voltage I see present is 5
    volts on the stand power supply pin.
    I realize the power supply isn't completely turned on until the
    microprocessor on my video board sends it the correct turn on pulse,
    but isn't there more I can do aside from assuming the video board is
    likely defective. I checked ESR on all capacitors, looked for shorted
    diodes and transistors, burnt resistors, etc. What is a high failure
    item on the video board which might cause this? The set doesn't come
    on for even a second.
    This is a Westinghouse model SK32H240S

    Thanks in advance.
  2. Arfa Daily

    Arfa Daily Guest

    On many, although not all, LCD TV sets that I've had the 'pleasure' to work
    on (!) the standby supply produces +5v and +3v3. Normally, when you have one
    that won't come out of standby (do you actually even get a standby light
    showing?) the first thing to do is to locate the standby switching line to
    see if that is toggling. If it is, you then need to look at each individual
    output rail from the power supply to see if they start to try to come up.
    Often, they will, but fall over again so quickly that you don't see any
    activity at all in terms of the set coming to life. The rails are normally
    something like +3v3 switched, +5v switched, +12v, +24v high current (for the
    backlight inverter) and +33v.

    If the power supply is trying to start, then the next thing to do is to
    unplug the backlight inverter board. It is common for these to fail, and if
    they impose a heavy load on the 24v rail, the power supply will shut back
    down before it ever really gets going. Another common failure for a heavily
    loaded rail, is a failed audio IC.

    In general, LCD TV power supplies are *very* heavily self protecting, and
    they will either shut back down very quickly, or not even come up at all (as
    far as you can see) if they detect the slightest thing wrong with any of
    their rails - thats over-voltage, under-voltage, or excess load.

    It can be very tricky to get to the bottom of what's causing an apparently
    dead LCD TV. I have also had system control micros fail, and also corrupted
    EEPROMs causing startup problems.

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