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Viewsonic VA521 dead startup resistors

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    Viewsonic VA521 dead startup resistors

    I just repaired a Viewsonic VA521 manufactured in 2005 that did not power up. To open the LCD, just pry open the front frame with a credit card, then unscrew the shield to expose the boards. The LCD has two boards, one Power/Inverter board labelled JT166W18, and one Controller board: see pics attached. First I replaced all 'Elite' caps even though they were not bulging, but the LCD still did not power up. Then I was lucky enough to find the schematics of both boards in a German web site under the title "Belinea 101536 LCD Screen": see pics attached. The primary capacitor was charging OK at 175VDC, but the PWM controller would not start.. The PWM is supplied at startup by two "startup" resistors [R804 & R805, both 950KOhms] in series directly connected to the above 175VDC: see pic attached. Turns out that one resistor [R805] was open. After replacing this resitor, the LCD worked. I read that the failure of the startup resistor[s] is a very common problem in LCD, second only to bad caps, so I hope this thread will help.
    Attached Files

    #2
    Re: Viewsonic VA521 dead startup resistors

    Did they use a carbon film resistor like that from the factory?

    A very bad practice indeed they should have used a metal film flameproof resistor.

    I was working on a TV with a shorted PSU once and mistakenly used carbon film resistors. I figured I found the short so it would be fine I was wrong.

    Whew boy did that 1/4W flame up almost like the flame from a cigarette lighter.

    I waited till I ordered the proper resistors before I eventually fixed it.

    Anything can take out startup resistors lightning strikes and power surges being the top two.
    Last edited by Krankshaft; 07-23-2009, 12:45 AM.
    Elements of the past and the future combining to make something not quite as good as either.

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      #3
      Re: Viewsonic VA521 dead startup resistors

      Nice catch!

      High value resistors are not a common problem. IMO that one must have been a factory defect.

      Since the schematic suggests (although your values differ - 560k vs. 950k) that at least those 2 are in series plus possibly another of the same value making the entire string somewhere between 1.5M to 1.9M ohms. The current presented is EXTREMELY small and the resistors simply form a voltage divider network that feeds the IC Vin at pin 3 (VCC) which per the datasheet should be 36 volts. Note that pins 3 & 7 are internally connected. With the caps C810 & C811(per schematic) rated at 35v, I would assume that the chip is running at less than 35v.

      Even at the full 400 volts of the supply cap with the resistors grounded, the max current would be 0.26 mA (400/1.5M) and the wattage would be 1/10th of a watt (0.00026*400).

      Resistors typically fail open or higher than marked value. The power dissipated by these would not be enough to fry a 1/2 watt resistor or cause it to go high in value.

      Toast
      veritas odium parit

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        #4
        Re: Viewsonic VA521 dead startup resistors

        The actual PCB differs from the schematics in one place: on the PCB there are only two equal value resistors [R804 & R805 both around 950 KOhms] in the startup series network instead of the three in the schematics. As seen on the pic, the open resistor was not burned, just probably a fatory defect. I was lucky to find the schematics: this helps a lot, and I hope that it will help other people dealing with SMPS. I also attach a higher resolution [233KB] pic of the power/inverter board.
        Attached Files

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