Philips 52PFL5704D/F7 main board short

[Sevenqi]

Philips 52" LCD model 52PFL5704D/F7, plug the ac cord, relay click, click, click.., no led.
Open the cover (wow, the screws need torx t10 screwdriver, I had to buy one to open this tv). Checked the 3.3 standby, 0v, disconnected the connector to the main board, the standby comes back to 3.3v, strange, checked the standby pin to the main board, it's short to the ground! this explained why the relay clicked.
Now my next step is, how I can isolate the short ? capacity, diode, rectifier ?

[rievax_60]

Re: Philips 52PFL5704D/F7 main board short

Its usually a shorted SMD ceramic capacitor. I try to find the problem by applying less than 1v via a one ohm resistor to the shorted rail.
The trick is to follow the rail in the direction that shows decreasing voltage measurements.

[Sevenqi]

Re: Philips 52PFL5704D/F7 main board short

HI Rievax, thanks for your reply. Still not fully understand your way to find the short, the whole rail should be 0 ohm, I may need to cut somewhere ? could you please draw a simple schematic. Thanks

[Th3_uN1Qu3]

Re: Philips 52PFL5704D/F7 main board short

There is no such thing as "0 ohm". PCB traces have resistance you know. What he said is this: Take a low voltage source such as an AA battery, and a 1 ohm resistor. Wire the negative of the battery to ground of the PSU, and the positive to the 1 ohm resistor. The other lead you wire to the red lead of your multimeter. Black lead goes to ground as usual. Now probe each part on the shorted rail and look for the lowest voltage indication on your multimeter. When you read very close to zero, you have found the shorted part.

[Sevenqi]

Re: Philips 52PFL5704D/F7 main board short

Please correct me, not sure if I'm right to draw the connections like the picture. The A terminal is the Red probe of the meter, it should probe to different postions on the shorted trail, the lowest voltage on the meter should be the short part position?

[rievax_60]

Re: Philips 52PFL5704D/F7 main board short

Please correct me, not sure if I'm right to draw the connections like the picture. The A terminal is the Red probe of the meter, it should probe to different positions on the shorted trail, the lowest voltage on the meter should be the short part position?

Not quite. The battery and resistor need to be permanently connected to some point on the rail, preferably near the power connector. It can be difficult to find a place that is safe and easy to solder to. The battery negative connects to the PCB ground.
I didnt think of using a 1.5v cell, next best thing if you dont have an adjustable power supply. Maybe use a D cell and a bit higher value resistor to increase the time.
The voltage that develops on the rail needs to be within of the millivolts range on your DMM.
Connect the black probe to something that is ground like the tuner can or input connector shell.
The red probe needs to be very sharp to pierce the lacquer coating the tracks. I use homemade sharpened steel probes, old CD player motor shafts.

There is a slightly alternative method of making the measurements. Best to explain this later. Let us know when you are set up.

[Sevenqi]

Re: Philips 52PFL5704D/F7 main board short

I had no luck to figure the shorted parts until today, I used a high risk method to fix the problem:

1. Use a computer ATX power supply to short the green and black wire ( power on switch).
2. Cut the two wires that output 5V.
3. Connect the 5V to the mainboard 5V standby pins, ground to ground, +5V to +5V, NEVER make mistake !

I didn't hear any sound, any burnt smell nor any burnt parts. But the standby input pins were not shorted any more !
I then installed back the main board to the TV, guess what, it works !

[jbfr_54]

Re: Philips 52PFL5704D/F7 main board short

Well you probably burnt a shorted cap ! Good try, this is a common method to use when no luck in finding the shorted component more smoothly. Lucky if it is a cap, the board is back on life, bad news if it is an IC, the board was dead anyway, it'll remain dead !