Philips Lc4.31EA Strange Backlight Characteristics

[JonathanAnon]

Got this to fix for a friend. When I turned on the TV the backlight came on for a split second then shut down. I shone a torch on the screen and was able to see the static and channel change numbers, so I knew it was inverter/backlight problem.

Tested the PSU / caps / rails / B+ and all check out good.. The transformers, fuses and other bits on the inverter board check out good too... I was just gonna bin it but the curiosity got the better of me, cos I thought the backlight looked odd when it did come on... I opened up the LCD Panel and found the backlights and observed the following.



Half of all the bulbs turn on for a split second, hold the above lighting, and then immediately shut off... What the hell is happening????

[tom66]

Re: Philips Lc4.31EA Strange Backlight Characteristics

The alternate inverter, slave most likely, is not working. Budm says they are used in push-pull config. I do not know much about the master/slave inverters (never had to fix one, it's on the wish list), but if that is the case, then only one side will get the ac excitation it needs, so only about half a bulb will light, individual bulb characteristics make it vary slightly of course.

[JonathanAnon]

Re: Philips Lc4.31EA Strange Backlight Characteristics

I hear what you're saying tom, but I would have thought that if that was the case then for the positive period of the A/C cycle the light bulbs would be fully lit, and then fully UN-lit for the negative period of the AC cycle ..?

[tom66]

Re: Philips Lc4.31EA Strange Backlight Characteristics

That is why I think it's more likely that both sides are driven independently (not necessarily in phase) and both need to be energised to get the bulb to work completely -- but as I said, I have no experience with these. I have seen them in TVs, but never done a repair on one.

[budm]

Re: Philips Lc4.31EA Strange Backlight Characteristics

Just think of it as a full bridge drive circuit, only half of the bridge is working so the output will not be at full voltage on each half of the cycle. Lets say that the power supply is 10v, in bridge mode, the load will see positive swing of 10v and negative swing of 10v, total will be 20v Peak-to-peak.

[JonathanAnon]

Re: Philips Lc4.31EA Strange Backlight Characteristics

I might swap the dual NPN PNP MOSFET chips and see if the other half of the bulb lights up..

The two chips seem to measure differently, think an internal diode may be blown in one. I'm gonna pull them off the board and check them fully later.

[JonathanAnon]

Re: Philips Lc4.31EA Strange Backlight Characteristics

Just for anybody else following the thread... I got a new inverter board from the UK for this one for 16.99 STR, and it sorted out the problem..