LCD Monitor With Flickering Dimm Backlight and Hissing Noise

[badhat]

Hi,
I've got an Acer H243H monitor with dimm and flickering backlight and a hissing noise. I opened the box to see what it's all about.

The PSU + backlight inverter board, InnoLux ILPI 129 and similar, is found in quite a couple of OEM brands and models. There is no sign of burn or overheating what I can see. Just some white residue of what I thought must be soldering flux underneath the board but only in the lower voltage department. I could easily clean it with alcohol. The source of the hissing sound, however, appeared to be the high-voltage department of the board.

Using a piece of plastic tubing as a 'stethoscope' I localized the loudest spot of the noise right between the two transformers where the low-voltage sides meet. Underneath the transformers on the soldering side of the board there's a parallel group of three 2.2 uF ceramic capacitors (according to service manual) of which one was broken. I replaced it but it made no change. I also replaced the other two but no change either.

Then I applied an 'Isolating Sillicone Oil Spray' onto and underneath the transformers. A very bad idea I have to admit. The hiss got even louder and the inverter switches itself off after a few seconds.

I'm still trying to remove that sticky fluid with benzine and alcohol. It's not easy since it still keeps leaking from the transformers to the PCB. It appears to be somehow dielectrically conductive. I even localized a 'noisy hotspot' at an unpopulated area of the PCB close to the high-voltage lines. Guess I'll have to de-solder both transformers and boil them in benzine.

Is it worth all the pain? Aliexpress lists a pair of SPW-136 transformers for about $7.50 but China is 4-6 weeks away.

I had a look at the AUO M240HW01V2 LCD's specs sheet. The CCFLs should burn at 900V / 7.5mA which is equivalent to a 120kOhms load. So I connected four dummy loads each consisting of a couple of series and parallel resistors totalling about 120kOhms plus a little glow lamp as a voltage indicator. The noise stayed about the same but the inverter doesn't switch off itself no more. The glow lamps glowed dimmer than expected and the resistors only got about hand warm. Feels like much less than the calculated 7 watts.

I couldn't measure voltages or currents, my digital multimeter is a bit deaf at 50 kHz. I'll have to build an HF rectifier probe and connect it to the lowest series resistor.

I'll try to upload some pictures and keep you updated. All hints welcome.

Best regards,
Badhat

[budm]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

http://www.shopjimmy.com/gateway-792...t-inverter.htm
Are you talking about the two high voltage transformers on the left side of the board? I would remove those two trasnformers to inspect the windings underneath. It sounds like transformer is arcing. You also need to check the resistance of the high voltage windings.

[selldoor]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

Try in a dark room - can you see any arcing

[badhat]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

http://www.shopjimmy.com/gateway-792...t-inverter.htm
Are you talking about the two high voltage transformers on the left side of the board? I would remove those two trasnformers to inspect the windings underneath. It sounds like transformer is arcing. You also need to check the resistance of the high voltage windings.

Yes, that's my board. Thanks for (the link to) this great image. My board is stamped Rev. D but I haven't found no difference yet.

I measured some 650 ohms at the HV windings of the transformers. I'll unsolder and pull them and clean everything right now. I've seen bottom views of this kind of transformers during my searches showing the open windings. I always thought high voltage transformers should be sealed somehow :-)

Pictures will follow, I promise.

Best regards,
Badhat

[badhat]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

here's some photographs







After drying the washed transformers I'll insert them inversed, I mean upside down so I might see some arcing.

[budm]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

The transformer on the left of the picture, the high voltage windings looks black, or is it shadow?
Resistance reading on the windings?

[selldoor]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

Budm - Post4
I measured some 650 ohms at the HV windings of the transformers.

Even at 641 and 659 I guess its going to be within 3%

[budm]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

Thanks, I missed that. Did you see the picture of the left transformers? Is that shadow?

[selldoor]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

I cant tell - it does look as though it could be burnt on the primary coil but as its
in a glass of benzine its hard to tell. Dont know what effect that will have on the lacquer

[badhat]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

Thanks, I missed that. Did you see the picture of the left transformers? Is that shadow?

oops, bad image, sorry. Color is new shiny copper.

[budm]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

Hmm, not sure sure at this point why you are hearing arcing sound unless the PWM is running at the wrong frequency.

[badhat]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

I've replaced the dark transformer picture above. The laquered copper wire looks fine. But I'm a bit concerned about the rosin residues.

[badhat]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

In the new image (see above) you can see the rosin crumbs. I added tin-lead solder and rosin flux to extract the transformers. Although benzine is not a good solvent for rosin I'm afraid the it could have dissolved and deposited part of the rosin at the high voltage windings. Should I use alcohol to dissolve the rosin residues and then benzine again to get rid of the alcohol? Maybe I'd first dry the transformers with a hair dryer, connect them and see what happens.

I mentioned above I could get a pair of SPW-136 transformers for about $7.50 from Aliexpress. Well, that's for the US. I live in Germany, here it's about 11 EUR - and still a few weeks away.

[badhat]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

My transformers are ok, no arcing, corona discharge or the like. The chirping / hissing noise comes directly out of the PCB. I first thought of HV creepage and sprayed some clear acrylic laquer on and under the HV area of the board. May be it's imagination but I think it helped a bit. On power-up the inverter starts with a relatively clear and stable sinus tone of a few kilohertz. After about 2 seconds it stops and after about 2 more seconds it starts again with that chirping hiss.

I disconnected all high voltage components from the board. As I wrote before, I had already turned the transformers upside down. Now I cut the 'high' terminals from the CCFL jacks and wired them directly to the transformers. I also unsoldered the parallel capacitors and the HV side of the sensing capacitors from the PCB and connected them to the new HV wires. No change, The PCB is still whitling, hissing, chirping.

Now I know it's NOT the HV. Tomorrow I'll dive into component level debugging :-)

I've got a schematic and a digital multimeter but I wonder how far I'll get with that. Any hints welcome.

Best regards,
badhat

[badhat]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

I gave up, I'm selling the good parts on Ebay.

You're right, Budm, the inverter frequency is too low, otherwise I wouldn't hear it at all. Must be something around 10 kHz or even lower. I could see a continous 'spiky' waveform on a friend's oscilloscope for a few seconds, then a few seconds pause, then the waveform again with the pulsewidth modulation kicking in. Hard to trigger since the PWM frequency isn't stable. That's what makes the chirping sound. Everything appears ok except the frequency and the output voltage. Resistors and solder joints ok, must be the MP1009 chip. Sorry, that's a bit too much effort for me. I'll only touch the inverter board again If I can't sell at least the LCD panel.

I've learned a bit on how today's TV screens and PC monitors work. Thanks a lot for your support and for your patience!

Best regards,
Badhat

[badhat]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

Now that I'm back to this forum with another case I stumbled across my old thread of January this year and I'd like to give some feedback.

What happened since? I wasn't able to sell the good parts on eBay but I found and bought a working power board for 20 EUR :-)

My Acer H243H monitor is since doing well with it.

Best regards,
Badhat

[negusm]

Re: LCD Monitor With Flickering Dimm Backlight and Hissing Noise

I would have bet that you were running into failing CCFLs....I wonder if your new board is a bit better at handling them than your old board.

I had a problem with a FHX2300 which also has the ILPI-129 board. I tore into it and found the transformers would buzz and hiss like they were possessed.

My board looked perfect, but I checked the caps with an ESR tester anyways, they were fine. I took a chance and decided that it was probably the CCFLs.

The CCFL tubes pull far more juice than they do originally when they get near to their end of life. This causes a few issues but the main thing is that it starts burning out the connections at the tips of the CCFLs....this causes that transformer buzzing and the intermittent cut-outs and flicker.

When replacing the 4 CCFLs, I found 2 burned connections. I also found that I needed to solder the replacement CCFL connections extremely well or they would burn off quickly....and cause the same buzzing in the transformers. Once I had a GOOD connection to all 4 CCFLs, the ILPI-129 is totally SILENT and the monitor is running perfectly.

Funny aside...I tried to at first source an ILPI-129 board from alibaba...but the Chinese vendor I ordered from tried ripping me off and sent me an empty envelope...but I just did a charge-back on the CC and got my money refunded. I decided to do a little more digging before trying to find another board and read more about the effects CCFLs have on these inverter boards. In a way they saved me money in not buying that board.

-Mike