42" Philips LCD sound but no picture

[3DPiper]

I have a 42" Philips LCD tv

model # 42PFL7422D/37
chassis # Q523.1U LA
Feb 08

A friend was throwing it away.. It had the all-too-familiar "sound but no picture" problem.. I got it and brought it home.

I read many threads on here and used a flashlight to determine the LCD is working, just not backlit. I could see and navigate the menu settings, the screen just wasn't backlit.

I opened the case and examined the master/slave power inverter boards for the backlight. Each have two large caps, and three of them had bulging tops..

I went to my local electronics store and bought some caps to replace them all. I did the process, careful to keep the polarity correct and solder properly, and everything looked good. I plugged the boards back up, but no backlight.

I had someone knowledgable check my work to make sure I didn't do anything wrong, and he said the boards looked fine. We examined the rest of the boards and nothing looked burned out or bad.

Is there anything else I can look at to solve this? I know I can buy new master/slave boards on ebay for about $150, and I'm not against that, but if there is something else I should look at first I'd rather do that..

There is a green light on the mainboard, is there a way to get error codes off of that? I really want to get this going so my wife can have a TV in the bedroom..

Thanks

[Rob Northen]

Re: 42" Philips LCD sound but no picture

The new caps you installed, did you ensure that they have same specs as the old ones ?
Same ESR and same or better rippel ?
Same voltage or higher and same mf ?

[3DPiper]

Re: 42" Philips LCD sound but no picture

The caps that were on the board were

V9A PET 35V 680uF 105oC

The caps I got at the electronics store were

SHOEI 50V 680uF 105oC

They said the larger voltage wouldn't be a problem..

Could that be the problem?

[PowerAmpFreak]

Re: 42" Philips LCD sound but no picture

It's correct, the higher voltage rating won't do any harm, what is more interesting is the new caps ESR value which should be checked and verified to the old caps. You could search for a small fuse on the inverter board, a PICO fuse, or similair that could have opened if the old bad caps made the current peaks higher than normal.

[3DPiper]

Re: 42" Philips LCD sound but no picture

How do I measure the ESR value?

I still have the old caps for reference..

[cadillacman]

Re: 42" Philips LCD sound but no picture

you need an esr meter. An expensive little piece of technology. I still dont have one but i need one bad.

Id suspect some smd transistors failed, bas smd fuse, etc. Also have you checked to make sure you have peoper voltage going to the backlight inverter boards? DO they attempt to work at all?

If your voltage to the boards from the power supply is o.k., then the ebay board smight be the way to go. Just remember you could be buying somebody elses junk. If those dont work, then how would you proceed?? (ive played that game before i still have a dead tv and im pretty sure the ebay part is bad btu no way to verify without a known good board..)

[PlainBill]

Re: 42" Philips LCD sound but no picture

Several suggestions:

1. Try turning the TV on in a darkened room. See if the backlights give a brief flash when the TV is turned on.

2. The backlight control line is often labeled on the input connector to the inverter as BL_ON or ON/OFF. Check the voltage on that.

3. Check any fuses on the inverters.

4. It is easier to give meaningful advice if we have pictures of the suspect boards. Note the links in my signature, and attach them using 'Manage Attachments' (below the text entry area).

PlainBill