Philips 37PFL9604H/12 backlight switch off after a few minut

[oobabeloo]

Hersteller: Philips

Typenbezeichnung: 37PFL9604H/12

Inverter: DPS-298CP-A

Chassi: Q549.2LE

kurze Fehlerbeschreibung (2-3 Worte): backlight switch off after a few minutes

Meine Messgeräte::
Analog/Digital Voltmeter

Schaltbild vorhanden?:
Ja, als PDF


Hi,

My TV (Philips 37PFL9604, chassis Q549.2LE) has an annoying backlight issue since a few days.

It will power up flawlessly but, after a certain amount of time, the backlight will switch off for a second, then switch on again. It will happen a few time until the screen goes black totally.

When the backlights are on, I cannot see any dimming or artefact on the screen, thus I would not bet on a CCFL issue. The TV has been used (display on) for a bit more than 5000hours.

I used a flashlight to check that the image was still present and it was the case and I also changed the 33µf and 47µf caps connected to the backlight.

Might be worth mentionning, the ambilight will stay on when the backlight switch on and off.

Is there some other components I should have changed or some other checks I should have done ? I tried to reduce the contrast to minimum and since then, the backlight issue didn't appear back (but the display is terrible !)

I must say that I previously had another issue with the inverted/psu card (DPS-298CPA) some months ago: an annoying click could be heared after being on for a few minutes (without any backlight issue a that time). Changing the IC601 and all mosfets cured it.

Thanks for your help

 

[HermannS]

Hi,

oh dear, the powersupply from hell...
Your problem is obviously thermal. Have you tried cooling/heating parts on the power supply? Checked for broken solder joints? I would not rule out the possibility that some part that you have already replaced is broken again.

 

[oobabeloo]

I haven't tried cooling yet (I still need to buy one of this spray, any advice ?).

When running without the cover, the issue remains the same.
When lowering contrast a lot, the issue is gone.

 

[HermannS]

Just any cooling spray for electronics will do. The cover doesn't do that much.
Well, I suppose the TV has some kind of dynamic contrast switched on, which is dimming the backlight.

 

[oobabeloo]

yes, exactly : that dynamic contrast option, I switched it off but I could not see a real impact on the backlight issue.
my question will seem dumb, but how contrast and lightness are handled with a CCFL ? I mean, how the contrast can have an impact on the amount of energy the inverter needs to supply and dimming the lightness not ?

as for the spray, is it one i can use : https://www.amazon.de/Liqui-Moly-31...d=1537103064&sr=8-1&keywords=Spray+electronic ?

 

[HermannS]

That is NOT a cooling spray!

Contrast first and foremost is controlled by the panel. In low contrast images the BL is dimmed and vice versa. That's how think it works.

 

[oobabeloo]

Thanks for the explanation.
As for the cooling spray, as I'm not so confident in finding the right product myself, can you advise one please ?

 

[HermannS]

I am using this:
https://www.conrad.com/ce/en/produc...--CRC-Kontakt-Chemie----KAeLTE-75----84409-AF

 

[oobabeloo]

Thanks Hermann,
I swapped the IC601with another (again) and the backlights are on again. Either it was low quality or faulty. I added a small fan to move air a bit, hoping this will decrease the overall temperature a bit in the housing.

 

[HermannS]

That's interesting. Maybe it was a sloppy copy.

 

[oobabeloo]

what I would like to lear is how to test those IC.
I mean, I should get the datasheet first, but then, how can I check that the IC is actually of good quality or next to dead ?

 

[HermannS]

I'm afraid you can't. You would have to set up a special test circuit. Your first one did work for a while, thus you wouldn't have been able to detect any fault, anyway. It is just luck with these old devices.