Phillips 42PFL3704D/F7 - Power Up Issue

[jcm21884]

Model: 42PFL3704D/F7
Serial: YA1A********

This TV is my daughter's TV until it completely stopped working.

Initial problem: no display. If left a while and/or opened connections checked would sometimes come back on and work for “weeks.”

Then started blowing fuse F1601 as soon as power cord plugged in.

By removing R1424, R1604, and C1507, would not blow fuse. Isolated the problem to Q1602 (2SK3561) and replaced (TK8A50DA). TV has problem described below. I have also replaced Q1601 with a TK8A50DA (because it was part of the same switching circuit and didn't know if they had to be matched), but still have below problem.

Problem: When the power cord is plugged in, the TV powers up – Relay RL1201 engages and front Power Light comes on - no display. After 7 seconds, the relay drops and the Power Light goes out. The screen never shows any signs of coming on.

I have checked the following voltages at the Inverter Board. The first number is the power on voltage and the second is after 7 seconds and the relay drops:

HOT Side:
+345V: 345V – 0V
HOT +17V: 16.5V – slowly to 0V
POWER-CONTROL: .68V – 0V
HOT +22V: 19.95V – 0V

COLD Side:
AL +3.3V: 3.3V – 3.3V
AL +5.0V: 5.3V – 5.3V
PANEL +24: 0V – 0V
T1701 Terminals 10-11: 0V – 0V
T1701 Terminals 14-15: 0V – 0V

I need some help in determining where I need to test next. I am attaching schmatics.

Any assistance would be greatly appreciated.

Jeff

[munirutech]

Re: Phillips 42PFL3704D/F7 - Power Up Issue

lets see the pictures of the power board front and back,make sure its clear

[jcm21884]

Re: Phillips 42PFL3704D/F7 - Power Up Issue

See attached photos

[jcm21884]

Re: Phillips 42PFL3704D/F7 - Power Up Issue

Looking for some help in determining which area of the circuit is responsible for the shutting down of the power. If the power button is pushed after the initial cycle, the relay R1201 re-engaging, the voltages come back up as described in the original post and after 7 seconds, the relay and voltages drop as described above.

[jcm21884]

Re: Phillips 42PFL3704D/F7 - Power Up Issue

Problem Isolation:

IC 1401 – Texas Instruments UCC28060 -- http://www.ti.com/product/ucc28060

Pin 9 PWMCNTL: From data sheet: "PWM enable logic output: This open-drain output goes low when HVSEN is within the HVSEN good region and the ZCDA and ZCDB inputs are switching correctly if operating in two-phase mode (see PHB Pin). Otherwise, PWMCNTL is high impedance."

I found when plugging in the power cord, the Pin 9 voltage to be 5.75 and after 7 seconds dropped slowly to zero.

I used a 10 ohm resistor to ground at the same time I pressed the power on button and the TV came on.

I then unsoldered one side of R1439 effectively removing the PWMCNTL to the rest of the circuit. Now the TV operates normally.

From the description above it appears that this IC is bad because it is not turning on and pulling Pin 9 down. I believe this is an important part of the circuit protection and cannot leave this disconnected.

Before trying to remove a surface mount IC, I wanted more experienced opinions as to whether this IC bad or there is another issue.