Bad caps on Inverter Board?

[Remeniz]

Hi peeps.

I've got a LCD screen with a faulty backlight inverter board; everytime I try and power up the monitor the backlight comes on briefly, the PSU short-circuit protection kicks in and the whole lot turns off. This on/off cycle will carry on until you disconnect the power.

So I un-plugged the inverter board and the TV powered up fine. Plug the inverter back in and it shorts again. Now if I keep the inverter board plugged in and remove all 6 cathode connections the TV powers up fine.... then if I plug back in the tubes the TV stays on, but without the backlight.

Anyway I removed the power, plugged in 1 out of 6 of the cathode tube connections and turned it back on and the PSU, as above, cycles on/off protecting itself from a short. But when I put my digital multi-meter, on a volt range, across the small blue 18pF caps near the transformers the output from that tranny comes on and the shorting stops, TV on! I know this cause my digital meter goes crazy with high frequency AC readings. No back light though cause only one side of one tube is connected to the inverter; three tubes, 6 connections, 6 transformers and 6 caps.

So can this mean the 18pF caps are bad? I then tried this on all 6 outputs, and the same happens.

[Remeniz]

Re: Bad caps on Inverter Board?

Here's the cathode inverter board...

I did try to upload but it failed.

[retiredcaps]

Re: Bad caps on Inverter Board?

Here's the cathode inverter board...

I did try to upload but it failed.

The max resolution is 2000x2000 and the max size is around 2MB. If you exceed that, the upload will fail. There are a number of programs to resize or just take another picture with lower resolution settings on your camera.

[Remeniz]

Re: Bad caps on Inverter Board?

The max resolution is 2000x2000 and the max size is around 2MB. If you exceed that, the upload will fail. There are a number of programs to resize or just take another picture with lower resolution settings on your camera.

Thanks man.

Here's the board....

[retiredcaps]

Re: Bad caps on Inverter Board?

But when I put my digital multi-meter, on a volt range, across the small blue 18pF caps near the transformers the output from that tranny comes on and the shorting stops, TV on! I know this cause my digital meter goes crazy with high frequency AC readings.

1) It would help if you list the model number of the TV.

2) The AC voltage where you are measuring can easily get to 1500V AC. Enough to do serious damage to your multimeter and you.

3) From what I know, those blue caps rarely fail and when they do you will know it.

4) If you are getting a brief backlight flash, you have likely have 2 seconds to black.

If you hookup a dvd player, do you see the movie playing on the screen if you shine a flashlight on it?

5) If yes to #4, the purple electrolytic caps can cause 2 seconds to black.

6) Another possibility is the inverter transformers. You can measure the resistance of secondary pins (with power off and lcd unplugged). The secondary pins are the ones "feeding into" the blue caps.

7) Another possibility is that you have a bad ccfl tube.

5,6,7 and are described for computer lcds at post #13 and #14 at

https://www.badcaps.net/forum/showthread.php?t=10419

[Remeniz]

Re: Bad caps on Inverter Board?

My bad.

TV model is Toshiba NR 20VL44B. The backlight comes on very briefly, like it's trying to start up, but it's this initial start up process that is causing problems and is shorting the PSU. The PSU has a 5A limit after which it will shut down for 1 second and re-start. I just assumed that the multimeter's high internal resistance across the small cap's was fixing the problem so it needed new caps.

Also the picture is visible on the screen without the backlight.

I'll refer to the thread you linked...

Thanks.

[retiredcaps]

Re: Bad caps on Inverter Board?

TV model is Toshiba NR 20VL44B. The backlight comes on very briefly, like it's trying to start up, but it's this initial start up process that is causing problems and is shorting the PSU.

I should mention that "2 seconds" guide is meant for lcd troubleshooting. The LCD TVs are an order of magnitude more difficult to troubleshoot and you very well could be right in looking at the PSU versus the inverter section.

[Remeniz]

Re: Bad caps on Inverter Board?

I should mention that "2 seconds" guide is meant for lcd troubleshooting. The LCD TVs are an order of magnitude more difficult to troubleshoot and you very well could be right in looking at the PSU versus the inverter section.

The external PSU was cycling on/off without the inverter board.

Also I hooked up the high current output from an ATX PSU to the monitor and guess what?... It powered up, albeit without the backlight; 11.6V isn't enough for the inverter to start up the cathodes, it needs 15V.

So it seems the PSU is not liking the initial current demands; with the ATX PSU running the monitor it needed 2.5A, 4A max, to start the cathodes after which it drops to 860mA. But to have the PSU crap out with just less than an amp means it's had it.

Do I go with another switching PSU, for efficiency, or a non-switching type? If switching then i'll go with a 6A instead of 5A.

[Remeniz]

Re: Bad caps on Inverter Board?

Just a quick re-cap...

I opened up the external PSU and replaced the high voltage filter electrolytic on the primary side and it works fine now.

Thanks!