Relay in CRT TV's power board clicks on then off

Discussion in 'General Electronics Chat' started by tpny, Oct 16, 2014.

  1. tpny

    Thread Starter Member

    May 6, 2012
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    When I plug in the wall plug of my TV I hear the relay clicks on then off. TV never comes on. What is making the relay go off immediately? Thanks
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,789
    1,103
    Your TV probably has an over-current fault which causes the power-supply to shut down.
     
  3. ian field

    Distinguished Member

    Oct 27, 2012
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    782
    The relay could be a "red herring" - some CRT TVs have a relay to disconnect the degauss PTC thermistor after a preset time delay.

    My first thought is the small electrolytic cap in the PSU error sampling circuit, which often doubles as the Vcc reservoir for the control chip - keep trying to get it to start before finding and fixing the cause, and it'll eventually go bang.
     
  4. tpny

    Thread Starter Member

    May 6, 2012
    216
    0
    relay coil voltage is 12v dc at both + and -, which means the voltage difference is 0V thus relay stays off. When cord is plugged in initially I imagine the voltage difference is 12V thus relay clicks on, but then immediately voltage difference goes to 0V.. Any ideas?
     
  5. ian field

    Distinguished Member

    Oct 27, 2012
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    Start by inspecting *ALL* the electrolytic capacitors for bulged tops - some evaporate electrolyte when hot and develop an internal vacuum when they cool, these show up as having sunken tops. Some corrode round one of the leads and leak electrolyte on the board - going over the soldering will heat the spilled electrolyte and produce a nasty smell when you heat that joint.

    The small electrolytics rarely show any of the above signs, but if they've been getting hot; they usually deteriorate the plastic sleeve - otherwise, get an ESR tester or make an educated guess and test by replacement.

    Obviously an overload downstream can cause immediate shutdown, there's a few prime suspects in the horizontal output stage - a short cut used by some engineers is to isolate the horizontal stage from B+, and load B+ with a mains light bulb. That doesn't always help much - some sets sense proper operation, and if that doesn't happen; the front panel micro shuts everything down!

    There's no way to sugar coat that bit - immediate shutdown could be a fault *ANYWHERE*.
     
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