JVC LCD is too dark but still easily visible

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Andrew @andy29

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게시일: 2020년 8월 24일

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JVC LCD is too dark but still easily visible

Hi, my JVC LCD TV (LT-32DR1BJ, it’s not an LED model) has suddenly gone dark, but not really dark like the problem people usually seem to get... it's just like the brightness has been fixed at the lowest possible value or something. Turning the settings to their maximum only brightens it a tiny bit. Apparently it occasionally flicks into being bright again for a while.

I’ve attached an image but it’s hard to capture, everything is still easily visible, but just way too dark, in fact with some images it almost feels acceptable. It’s uniform over the whole picture. The menus are dark too.

Any idea about what’s going on and what I should suspect first? I’ve had a look over the boards and I don’t see any bulging or leaking capacitors, or anything else which doesn’t look right.

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jayeff @jayeff

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게시일: 2020년 8월 25일

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Hi @andy29

Don’t know the TV but if it is a LCD TV and not a LED LCD TV then it may be using CCFL tubes for the backlighting.

Couldn’t find the service manual for your model but on another JVC LCD TV in the specifications it shows that it uses CCFL tubes and also that the power board feeds “inverter boards” connected in the panel. The tubes may be mounted at the top and/or the bottom of the screen. Inverter boards are usually what are used to provide the voltage necessary to operate the CCFL (cold cathode fluorescent) tubes.

It may be that the CCFL tubes are failing or that the inverters may be the problem. Just a thought.

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Hi @jayeff , yes it quite old so it does use CCFL tubes with a big inverter board. I did suspect them. Thank you for taking the time to look for a service manual! If only some of the outputs or tubes were failing, would you see a difference in brightness across the screen or would the whole thing get uniformly dimmer? Because for this TV it's definitely not an uneven brightness/darkness.

Andrew 의 2020년 8월 25일

@andy29

Like most fluoro tubes when they age they just get duller and the colour may get more yellowish as the phosphor coating inside the tube deteriorates so this will make it seem less bright.

As the tube wears it demands more current that the inverter can't provide so again the light output from the tube is reduced.

All tubes have a certain number of hours of "run time" as specified as their "life". This number is when the light output is expected to be 50% of the original light output.

Hopefully you can still get replacement tubes if you decide to disassemble the TV to get to them that is.

jayeff 의 2020년 8월 25일

@jayeff This happened suddenly, not even any 'fizzling' or anything when it first happened, so if they do get gradually duller or yellower I wonder if it's more likely to be the inverter. The inverter does make a quiet fizzling noise but I am not sure how much is normal.

The inverter board's model number is V225-3XX and luckily second hand replacements look like they're really cheap.

If it is the tubes it feels like it might be harder to find unless they have a model number on them somewhere.

Andrew 의 2020년 8월 25일

@andy29

Inverters work very hard and get hot as well.

Check the solder side for dry joints. (use a strong light and a magnifying glass to get a good look) If the inverter uses transformers in lieu of power mosfets check the transformer terminals on the board as usually they end up having board connection problems.

I have a monitor which has ccfl tubes and had to rework all the transformer connections as a few had dry joints and I didn't trust the rest. Also some of the caps were gone as well. But my problem was no backlights not dull backlights.

Depending on how much you want to take a chance and to spend, replacing the inverter is the easier option. Maybe a supplier with a no questions asked returns policy may be an idea if it doesn't solve the problem.

There are videos online on how to replace ccfl with LEDs but I'm not sure if this is worth the effort.

Cheers

jayeff 의 2020년 8월 25일

@jayeff

I hope it will turn out to be the same kind of thing then. The boards seem to be quite cheap so that's lucky. It's good to have a starting place.

If it is the tubes and I can't get replacements, I will definitely at least have a go with LEDs, might be a fun thing to try to get it going again. I did do it once with an old 4:3 computer monitor with normal basic LED strips and it actually worked surprisingly well.

Andrew 의 2020년 8월 27일

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Hi @jayeff , yes it quite old so it does use CCFL tubes with a big inverter board. I did suspect them. Thank you for taking the time to look for a service manual! If only some of the outputs or tubes were failing, would you see a difference in brightness across the screen or would the whole thing get uniformly dimmer? Because for this TV it's definitely not an uneven brightness/darkness. — Andrew
@andy29

Like most fluoro tubes when they age they just get duller and the colour may get more yellowish as the phosphor coating inside the tube deteriorates so this will make it seem less bright.

As the tube wears it demands more current that the inverter can't provide so again the light output from the tube is reduced.

All tubes have a certain number of hours of "run time" as specified as their "life". This number is when the light output is expected to be 50% of the original light output.

Hopefully you can still get replacement tubes if you decide to disassemble the TV to get to them that is. — jayeff
@jayeff This happened suddenly, not even any 'fizzling' or anything when it first happened, so if they do get gradually duller or yellower I wonder if it's more likely to be the inverter. The inverter does make a quiet fizzling noise but I am not sure how much is normal.

The inverter board's model number is V225-3XX and luckily second hand replacements look like they're really cheap.

If it is the tubes it feels like it might be harder to find unless they have a model number on them somewhere. — Andrew
@andy29

Inverters work very hard and get hot as well.

Check the solder side for dry joints. (use a strong light and a magnifying glass to get a good look) If the inverter uses transformers in lieu of power mosfets check the transformer terminals on the board as usually they end up having board connection problems.

I have a monitor which has ccfl tubes and had to rework all the transformer connections as a few had dry joints and I didn't trust the rest. Also some of the caps were gone as well. But my problem was no backlights not dull backlights.

Depending on how much you want to take a chance and to spend, replacing the inverter is the easier option. Maybe a supplier with a no questions asked returns policy may be an idea if it doesn't solve the problem.

There are videos online on how to replace ccfl with LEDs but I'm not sure if this is worth the effort.

Cheers — jayeff
@jayeff

I hope it will turn out to be the same kind of thing then. The boards seem to be quite cheap so that's lucky. It's good to have a starting place.

If it is the tubes and I can't get replacements, I will definitely at least have a go with LEDs, might be a fun thing to try to get it going again. I did do it once with an old 4:3 computer monitor with normal basic LED strips and it actually worked surprisingly well. — Andrew

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JVC LCD is too dark but still easily visible

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