Philips 42PFL7422D backlight troubles

[tom66]

Re: Philips 42PFL7422D backlight troubles

How did you place two leads on three pins?

[ztu165]

Re: Philips 42PFL7422D backlight troubles

No I placed the leads on the outer pins not the middle pin.

[tom66]

Re: Philips 42PFL7422D backlight troubles

That indicates a defective driver IC in my opinion -- that 24/28 pin device in the middle of the board. Also test between the large tab pin (electrically connected to the middle pin) and each of the smaller pins.

[ztu165]

Re: Philips 42PFL7422D backlight troubles

So the IC is what's causing the very slight flicker?

[tom66]

Re: Philips 42PFL7422D backlight troubles

0.04 ohms is nothing to be concerned about, the difference in the solder flux or quality of solder could easily cause that, now I would ask, are you using ohms, or Kohms?

[ztu165]

Re: Philips 42PFL7422D backlight troubles

Ohms.

[ztu165]

Re: Philips 42PFL7422D backlight troubles

Can anything else besides the backlight inverters cause the flickering? Not really flickering it's more like lines across screen moving upwards like waves or like a cascade of water. Probably regular person wouldn't be able to tell but I have same TV but with ambilightsnin the back and I can tell they are there.

[Dgtech]

Re: Philips 42PFL7422D backlight troubles

Just wanted to chime in and clear up the matter of measuring the fets. If you read my post from the link that I posted earlier, I explained that reading ohms across these fets will tell you whether or now the driver chip is good or bad. With the driver chip off of the board, and if you take the same measurement, you will read 10K ohms becasue there is a 10K resistor that is in parallel with that fet. So... that means that the fet reads open and the 10K in parallel with the fet reads 10K. But... at the same time, that driver chip is in parallel to both of these in which brings down the resistance to 8.5K As this chip goes bad, the resistance will drop. I have seen some work though they read 8.25K.

Now, since you're saying that the problem is more of a rolling, changing faint line, I'd suggest posting a picture of it.

[ztu165]

Re: Philips 42PFL7422D backlight troubles

Just wanted to chime in and clear up the matter of measuring the fets. If you read my post from the link that I posted earlier, I explained that reading ohms across these fets will tell you whether or now the driver chip is good or bad. With the driver chip off of the board, and if you take the same measurement, you will read 10K ohms becasue there is a 10K resistor that is in parallel with that fet. So... that means that the fet reads open and the 10K in parallel with the fet reads 10K. But... at the same time, that driver chip is in parallel to both of these in which brings down the resistance to 8.5K As this chip goes bad, the resistance will drop. I have seen some work though they read 8.25K.

Now, since you're saying that the problem is more of a rolling, changing faint line, I'd suggest posting a picture of it.

Thanks for coming back and clearing things up, I'd greatly appreciate your help. I am having problems getting the lines to show up on pictures and I tried video taping but you can't see them. I wish I new which were the original inverter boards that came with the TV so I can order them. It's no just one or two lines, but a whole bunch, they are thin and cascade down from the top. They look transparent and they are not visible if the image is not white or any light color.

[Dgtech]

Re: Philips 42PFL7422D backlight troubles

I'm inclined to advise you to try reseating all ribbon cables to the t-conn board. I'm trying to give you cheaper things to try than changing out the inverter boards. New boards would make this fix very expensive and almost not worth it.

[tibimakai]

Re: Philips 42PFL7422D backlight troubles

On mine ended up being the two big ceramic resistors were open.

[tibimakai]

Re: Philips 42PFL7422D backlight troubles

...on the power supply.
I have just picked up another one of these and I have re read these posts.
One FET on the slave inverter is 11.3 Ohm, all the rest are 8.5 Kohm.
Should I replace only that transistor, or the IC can be bad also?

[Dgtech]

Re: Philips 42PFL7422D backlight troubles

It most likely is the IC that has failed. Fets usually fail completely shorted. Of course, if its easier to pull the fet and check across the pads then go for it.

[tibimakai]

Re: Philips 42PFL7422D backlight troubles

It was the IC, FET it was fine. Thanks
TV is running.