Viewsonic VA521 dead startup resistors

Collapse
X
 
  • Time
  • Show
Clear All
new posts
  • vince88
    Senior Member
    • Feb 2008
    • 120

    #1

    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
  • Krankshaft
    Badcaps Legend
    • Jan 2007
    • 2328
    • USA

    #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.

    Comment

    • Toasty
      Badcaps Legend
      • Jul 2007
      • 4171

      #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

      Comment

      • vince88
        Senior Member
        • Feb 2008
        • 120

        #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

        Comment

        Related Topics

        Collapse

        • TinkerTron
          Schematic vs. Reality: Where Are the Resistors on My Sharp Clock Radio? Model FY-70CH
          by TinkerTron
          Hey everyone,

          I'm currently working on a mid-70s Sharp Electronic Digital Clock Radio, Model FY-70CH. I found the service manual online, printed it out, and have been studying it for a while. My main reasons for tackling this are: (1) I want to learn electronics and figured I'd start with something I had on hand, and (2) I want to fix its issues. Right now, the radio sounds awful, has about 5V DC at the speaker terminals, and the alarm time setting doesn't work. But I'm not asking how to fix these problems.

          Instead, here's what's been bugging me so far:
          The schematic...
          03-06-2025, 02:03 PM
        • double_DD
          How can I be sure that my EC chip is dead?
          by double_DD
          Hi all,

          As the title suggests, how can I be sure that my EC chip is really dead?

          I've recently gotten into this hobby of laptop repairs, and have successfully brought back to life a couple with some easy fixes, mostly shorted caps.

          Now I'm working on HP Probook 455 G8, that had some "devil" run around on 3V line, burning a bunch of stuff. Luckily I have a dead 455G7 (for spare parts) so I managed to fix it (swapped burned parts) to a point that it will charge (both on barrel jack and usb-c), but it doesn't want to turn on. There is a 3V3 on a keyboard...
          03-12-2025, 07:47 AM
        • KYBOSH
          Samsung UN50TU8000 - Dead Panel Forensic Questions
          by KYBOSH
          I came across a 50inch TU8000 that wouldnt start up properly (blinking stand-by light, backlights would cycle on and off).





          When i opened it up and removed the power to the mainboard the backlights powered up and everything looked uniform.
          When reconnected the mainboard but disconnected the flex cable to the panel i heard the TV startup with the intro sound.






          Later I checked for shorts on the panel PCB boards but could not find anything.
          When I took a look with the FLIR i noticed the 2 resistors...
          08-10-2024, 08:39 AM
        • vieuxchoub
          Psu repair: UC3845 not starting up because of NE555 defect
          by vieuxchoub
          Dear friends from Badcaps
          I have been searching for a long time to repair sucessfully a battery charger taking in 230V AC and outputting normally 14V DC. Its power mosfet has been shorted ( Infineon 06N80C3), probably because the bulk storage rectifier capacitor had swollen and had no capacitance anymore and a very high resistance.. The mosfet short has naturaly blown its 0.15 ohms resistor then its driver, the UC3845 IC which was shorted between pins 5 (ground) and 7 (vcc) plus 8 (Vref). Another small resistor surrounding was also dead. So, I have replaced the bulk storage capacitor, the...
          12-02-2023, 12:56 PM
        • disorder
          ps5 edm-033 dead after hdmi/usb ports destroyed
          by disorder
          hello guys,

          i need help to repair a edm-033 ps2
          history : as always hdmi port and usb ports destroyed after unplugged
          after change them, ps5 dead too : 1 beep, no blue light. after inspection 1.8v power was shorted, and because of dead panasonic chip. after remove it : no short anymore
          so i changed it
          now i have got 5v stby,, 3.3v stby, 3.3v power, 1.8v power and no short on them
          but the ps5 is dead again : 1 beep, near 10 seconds with power lines and then off, no blue light ...

          i don't see any projection of liquid metal on pcb.
          ...
          01-14-2025, 12:48 PM
        • Loading...
        • No more items.
        Working...