Anyone knowledgable about TFT/LCD inverters?

Hi all,

My 18" Philips 181AS TFT monitor isn't working so well. The inverter wasn't working, found the SMD fuse open and replaced. All lamps light up now, but the brightness seems too low - the whites aren't as bright and fresh as they should be. Along with this, the inverter transformers are squealing at a high-pitch.

It has 4 lamp outputs, with each pair at either end being run by a dual-output PWM controller. Each lamp has it's own circuitry after that, mirroring the other 3. I can supply the datasheet for this controller if necessary, but have no schematic for the inverter (TDK TAD383).

The start-up voltage for the lamps must be reaching the right levels as they fire up alright, so I don't know what could be wrong to have them run improperly after that. Yesterday I removed the lamps to view them whilst running the monitor. At one point I found just tapping the board ever so gently caused the lamp circuit on that side to shut down. Seems that side is sensitive anyway, as before that I found just touching a screwdriver tip against one core of a particular transformer will shut the circuit down on that half of the board (core sparks a bit first, as they all do). Not so with the other transformers though. Anyway, after moving the lamps about some more, tapping and banging the PCB wouldn't shut it down, so maybe a lamp connector wasn't in properly (although I'm sure they all were).

It's got me very confused. I wondered if a SMD device was cracked or internal breakage was present in a component, but banging the board and flexing it won't make that half of the inverter shut down again.

I'm at a loss as to what to do next. Any ideas?

Thanks,
James

I would suggest checking any electrolytic capacitors in the inverter for high ESR - it's an EXTREMELY common failure in switchmode PSU's.

It's also possible that your lamps are failing?, they generally have a fairly short lifespan? - and often aren't replaceable?.

Not sure if you want to go this way, but this site has backlite inverter boards for many LCD monitors, as well as lamp tubes and other LCD repair parts.

https://www.lcdpart.com/

I did not see a specific listing for Phillips 181AS but they had a Phillips listing xx181 that may be your part. Price was USD$69.00

I used them for parts on my Viewsonic backlite board. The first one I received failed after 3 weeks use, but they 90 day warranty the part. So far, no problems with replacement.

Dialtone

Thanks for both responses.

I changed the 3 capacitors on the PCB. 2 small ones which are part of the primary PWM stage were a bit low, but all it has done is change the frequency of the squealing a bit.

The website given is an excellent resource, thank you for that. Postage is cheap to the UK too, which is a great bonus.

Looking around the net, it seems a dull display is mainly due to the CCFL tubes being old and getting weaker.

I can now get the new tubes thanks to that website, but first I want to be fairly sure it's them... Would it seem right that old CCFL tubes in need of replacement could cause the inverter to squeal like it is? I'm not too sure what happens when the lamps start to fail, but I assume the current doesn't pass through them so easily, so could that cause the transformers to oscillate at the wrong frequencies and hence cause what I can hear?

Thanks again,
James