Philips 42PFL7606H - won't switch on - no LED blinking - dead. TPS53126 converter chip dead - and then some...

The Philips 42PFL7606H I got from ebay for 113€ switched itself off, according to its seller, and did not boot up again.

This one was a lot more difficult than the others I had fixed before. The power supply did come up, 12V, 3.3V, Standby voltage, backlight voltage all measured ok, but after a few seconds, the machine powered down, tried one more time and then it gave up and went to protection state, resetting the "power good" line to the power supply. All voltages down, except for the stand by voltage.

So, the main board must have had a problem. This was no good news because there, diagnostics can become really hairy - and that is what happenend.

First, I spent way too much time checking the stand by voltage circuit on the power supply instead of reading the service manual properly. I paid the price for my impatience.

The first thing you want to do with a broken Philips TV is to switch it to Service Default Mode (SDM). It will produce a blink code with the stand by LED.

Switch the TV on, let it settle in protection state. Locate a large solder pad labeled SDM on the main board and connect it to ground for a couple of seconds. Then check the LED. My TV morsed me a 2-15 code. Even more bad news. The standby microprocessor could not boot.

So I started checking the voltages of the DC-DC converters on the main board and it didn't take long to find out that the only ok voltage was the 1.1V. The 1.8V was missing completely and the 3.3V and 5V where much too low.

The Philips service manuals are quite good and contain all the circuitry of the main board, which you might attempt to repair yourself. I realized that the TPS53126 dual DC/DC converter chip was not working, because it did not send the PWM signal to the output MOSFETs of the 1.8V line.

Next image: The two output chokes of the dual converter with the MOSFETs on top. The right one is the 1.8V output, which measured 0V.

Board flipped over exposes the converter chip:

I got a new chip from ebay, soldered it in and hooray, the TV booted and worked. For about 1 hour and then it was dead again. What the heck is going on?

The 1.8V was ok this time, 3.3 and 5V again as low as before.

To make a loooong story short, it took another 10 hours to identify a dead ceramic C (the one at the 6th pin from the upper right corner of the chip) and a dodgy SMD quad-resistor, which finally broke completely with a silent "click" after I douched the area with freezer spray. Until then this sucker fooled me all the time with unpredictable and intermittend reboots.

Here is the fix I applied. I soldered a normal resistor across the broken one and glued it to the board. Two convenient test points helped a lot:

The problem was as follows: the resistor is supposed to feed 12V to the enable line of the DC/DC converters. The double converter I replaced is happy with about 1.5V already, which I measured, but the 3.3V and 5V won't start. The resistor is supposed to have 22k and I measured 9k across it. So, I thought, it is probably ok. Difficult to tell when it is mounted. Then I reasoned that the double converter might pull the enable line low. But the chip was brand new and it happily produced both voltages! A bunch of transistors around it, which sense a proper 12V supply and manage some delayed power-up did not measure badly at all. Being desparate I bridged the resistor to beef up the enable line to a higher voltage and lo and behold, the tv worked!

Unsoldering the quad-resistor was no option. The possibility to break or lose it was close to 100% and I was not in the mood for finding a replacement in the vastness of the internet :-)

The TV is running fine now and I am enjoying the ambilight and 3D images.

Philips TVs seem to be quite repair friendly. The mainboard layouts in the PDF are vector graphics and zoomable. The orange circles show the corresponding areas:

The main board has plenty of test points, which are documented in the schematics, and they are exactly where you need them. However, Philips won't tell us what voltages to expect :-( You need to reason yourself with some help of datasheets.

The build quality of Philips is nowhere near a Sony. The tapes they use to dress the wiring comes loose after a few years already. It  all looks lumped together. The power supply in this model however contained only high-quality parts. It's not made by Philips.

I did an ESR check on all the electrolytics and found two dodgy 220µ 16V smoothing caps on the mainboard with .25 and .35 Ohms, respectively. That's just in the acceptable range, but same type, different values! Not good! Replaced both with decent 470µ, 0.05 Ohm ESR Panasonic FC.