Samsung LN40C530 Flickering Backlight

This problem seems to affect the following models as well

  • LN40C530 LN40C530F1F LN40C530F1FXZC
  • LN46C530 LN52C530
  • LN46C630

There are some guides online indicating cutting JP852 on the power supply board will fix this problem.  This fix disables protection circuitry and probably is not an ideal solution.

Posts on badcaps and avsforum indicate the real problem is low voltage caused by bad surface mount resistors in the power factor correction (PFC) circuit dropping the backlight voltage from a nominal 390VDC to 350VDC.  This voltage can be measured across the large filter cap on the board (use the negative leg as ground).  These are lethal and high voltages so be careful when taking this measurement.

I replaced the four series 1.5M ohm 0805 1% resistors (RP802, 805, 807,810) and the voltage came back up to 395VDC from around 360VDC.  Measuring the original resistors still indicated 1.5M ohm resistance though the discrepancy probably comes from the higher voltage applied in the circuit.

The design places multiple resistors in series because maximum voltage for an 0805 resistor is around 150V and max power is 125mW.  Calculated power dissipation is 6.7mW which well below the rating.

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