Philips 42PFL7422D/37 backlight shuts off in 2 seconds

[brainiac88]

Hi all,

Noob to the forum. I've read dozens of threads on this unit and looking for some advice on repairing my TV. I've taken the entire TV apart, down to separating the LCD from the backlight.

System Info:
-LC420WU5
-Master Inverter: 6632L-0470A PNEL-T707A REV-1.2
-Slave Inverter: 6632L-0471A PNEL-T708A REV-1.2

Symptoms:
- Backlight goes off ~2 seconds after powering on

Observations:
- Blue LED on Master Inverter lights up when powering on, then fades to off, just as the backlight shuts off.
- The area of the backlights toward the side of the Slave Inverter appear to be dimmer than the side toward the Master Inverter. But it is hard to be definitive on this observation because it shuts off too quickly.
- Audio is OK even after backlight goes off. Test is using HDMI-2 input
- LCD image OK. [I put a high-powered light behind LCD and image is just fine]
- CCFL tubes all appear to not have any dark spots. All white, consistent across all tubes.
- Both inverter boards are pristine. No bulging caps, fried ICs, traces, or any obvious burn marks on either side of both boards.
- Power supply board does not appear to have any burn marks, bulging caps, fried traces, or other obvious damage on either the component or trace side.

Questions:
- What other tests should I perform, if any, to get a more precise diagnosis?

I do have a DMM, fine-tip and standard soldering equipment. Also have access to scopes, desoldering equipment, etc. at work.

Thanks for any advice. Hope this helps others as well.

[selldoor]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

If you have read dozens of threads you will have seen we like pictures.
Please post using add attachments jpg less than 2mb load quickly.
Your problem seems to be just the inverter/bulbs section so there are a few tests you can do. Do you have a spare ccfl tube and a working cfl 13watt bulb

[brainiac88]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

I do not have a spare working CCFL tube of the same size. Can I use a smaller CCFL from a laptop panel?

And yes, I do have working CFL bulbs, from 4W up to 23W.

Images:

[selldoor]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

I forgot philips used these I will have to look back. In the meantime have you tested the two fuses White surface mount. Also tubes in panels are CCFL and bulbs are CFL if you want to edit your post.

[brainiac88]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

Thanks for the correction.

No, I have not tested those. Will do when I get home. I assumed they would be okay because the bulbs did fire up for about 2 seconds.

[brainiac88]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

By the way *selldoor*, is the blue LED on the Master inverter supposed to be on when working properly? Or is it a fault indicator?

[brainiac88]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

Both fuses tested okay. Fractions of an ohm.

What shall I test next?

[selldoor]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

Found it !
See if your boards have been modified
https://www.badcaps.net/forum/attach...4&d=1341620604

as in this thread
https://www.badcaps.net/forum/showthread.php?t=17561

Yours may not have shorted but the mosfets may have gone.

[Dgtech]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

A few years back, I posted on a thread here how to check these inverters for a common failure.

https://www.badcaps.net/forum/showth...974#post129974

Also... If the caps on the inverters are not bulging, check them anyways with a cap meter.

[brainiac88]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

Okay here's what I've got so far:

- All 8 MOSFETs on both boards tested with ~8.5k across the two legs
- Tested DC resistance between transformer legs. See pic. All pairs (A-B, C-D, pairs on other side of transformer also) tested with zero ohms on both transformers of both boards EXCEPT for C-D on the Master board. Both transformers on the Master board between terminals C-D measured about 750k ohms.

Any tests I can do with a CFL bulb or a laptop CCFL tube?

[Ltank]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

When the backlight shuts off, is the BL-on cmmand still coming from the main board or does it shutt off killing the backlight?

[brainiac88]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

How do I test "BL-on command" ?

[selldoor]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

Unless you can find a pin on the connector between main board and power board that is labelled blon or someone replies who knows you are stuck with that pity Ltank didnt elaborate.
Going back to the resistance tests on the inverter transformers you said the pairs of pins on the other end tested at zero. Can you test them again I think each close pair may be connected and the test should be across the two pairs. Resistance should be quite high I believe 3-5 kohms. So please test A to B A to C A to D then B to C and Bto D (just copying the letters used on the other end)

[brainiac88]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

Hi guys,

Sorry, been out on vacation.

Anyhow, here's what I've got for the tests on the inverter transformers. MTs are the ones on the Master board, and STs are the ones on the Slave board. See photo...

[brainiac88]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

So because no one replied, I ordered replacement inverters off of eBay. I installed them last night and they work perfectly. Hopefully, they last.

Lesson here is that if the backlight goes off after 1-2 seconds, AND if the CCFL tubes do not have black spots or bands, the inverters are bad. So either replace them both, or try to do component-level troubleshooting.

[selldoor]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

Good fix
The other thing you could do to be helpful would have been to measure the resistances on the new ones and compare with the old ones. The list you have given is not really much use at all as the test is to look for differences of more than 3%

[brainiac88]

Re: Philips 42PFL7422D/37 backlight shuts off in 2 seconds

Right, I thought of that too. However, the new inverters were of a different design - see phot. Cost was under US$40 shipped.