Philips 32PFL3505D/F7 Logo Appears and Shuts Off

After I thought I fixed this TV (see post here), I ran it for a couple of hours without a problem.  When I turned the TV off and back on, the Philips logo appeared and the TV shut off.  This was a sign the TV was going into protection mode.

An old post on a Philips TV forum indicated the PROTECT3 line from the inverter board was causing the shutdown.  I measured 0V with the backlight on and 0.8V when the TV was shutdown.  From the schematic, the line should be 3.4V during normal operation.  Looking at the schematic, there were three circuits that tested overvoltage on that line each with a transistor to pull PROTECT3 to ground.  Measuring the bases of all three pointed to Q1972 was being switched on during power up.  The base of this transistor comes from a 100VDC rail which is divided down (0.31x) and passed through a 33V (30.32-31.88V in the datasheet) zener diode D1971 (33BSB-T26).  I measured 28.7V at the cathode of the diode which is less than the specified breakdown voltage.  I lifted one end of the diode and confirmed the TV did not go into protection mode and the voltage was the same.

I ordered some BZX55/C33 33V zeners (+/-5% tolerance, 31.35 – 34.65V) on Ebay but found 1N5257B (+/-5% 500mW, DO-35) locally.  ECG5036A is another 5% substitue.  Part 568-5899-1-ND (2% tolerance 31.2 – 32.6V) at Digikey is a closer match to the original.

Update: A post on badcaps indicated the power rail should be 82V and not 100V which is caused by a bad Q1970.

7 thoughts on “Philips 32PFL3505D/F7 Logo Appears and Shuts Off

  1. regards to philips 32pfl3505d/f7 did you remove d 1971 frome the board to check for proper voltage where is the power rail thank you

    1. The schematic is on page 10-7 and you can measure the positive power rail on D1994. If you are getting around 100V, you should replace Q1970.

  2. I have 3.26v on the protect 3 from the main board the voltage drops down on the inverter side causes the stand by light to go off can you tell me what too look for or is it going to ground do you have a number to call you thanks Geoff Probert

    1. If you have approximately 3.4V on the PROTECT3 line and it does not go to zero, your problem is not the same as described here and you need to look elsewhere. If it is going to ground, you need to isolate which of the three subcircuits is triggering the error. There is a block diagram in the service manual. Transistors do fail by shorting out which can be tested with a multimeter in circuit.

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