Old Dell 15" - 1 second lights up, then goes black

[tom66]

I have been given for repair a 15" LCD monitor which has no backlight. When you turn it on you get nothing but a very faint image. So most likely, the inverter or the CCFLs are dead.

I found a blown fuse on the inverter board. I removed the tiny 1206 SMD fuse and replaced it - temporarily - with a solder bridge. Now the monitor lights up for about 1 second then shuts down. So I know the CCFLs are probably good right? Question is, why is the inverter shutting down after 1 second? Could a bad CCFL be shutting it down? The inverter shutting down also turns off the LCD display. (Before, with the blown fuse, you could at least faintly see the display.)

When lit up, the display is the normal, expected brightness and is a good, pure white. No yellow or pink.

I tried to test the CCFLs as so. I unplugged one and then noted the result:
Both plugged in: switches off in <1 second
Top only: switches off in <1 second
Bottom only: nothing (never comes on)

Seems somewhat inconclusive.

I found the inverter part for just $10 + $4 shipping so I will probably buy it if further troubleshooting proves tricky, but I don't want to waste money on an inverter when the CCFLs could be bad.

Any idea what the problem might be?

Thanks,
Tom

[alexanna]

Re: Old Dell 15" - 1 second lights up, then goes black

When lit up, the display is the normal, expected brightness and is a good, pure white. No yellow or pink.

I tried to test the CCFLs as so. I unplugged one and then noted the result:
Both plugged in: switches off in <1 second
Top only: switches off in <1 second
Bottom only: nothing (never comes on)

Any idea what the problem might be?

Thanks,
Tom

The Transistor Controlling the bottom CCFL is shorted

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

The Transistor Controlling the bottom CCFL is shorted

That might explain the spark I got when I first powered it on, but no further sparks. I thought it might be HV touching metal but it was a good distance from the casing at the time.

Both CCFLs work though?

[PlainBill]

Re: Old Dell 15" - 1 second lights up, then goes black

The bane of everyone trying to help is those who fail to give enough information. 'Old Dell 15' is almost as meaningless as saying 'old monitor'. A number of companies supplied OEM monitors to Dell. An exact model number is very helpful - mandatory, I would say. Good pictures of the power supply / inverter are also very useful.

IF you have a monitor supplied by Benq (the last character of the model number is a b), you have a very common problem. The solder joints on the transformers fail, the transistors short, which blows the fuse. Now when someone foolishly bypasses the fuse, even more damage is done.

The results of your tests are exactly what I would expect if you had destroyed the driver and control transistors for the bottom CCFL.

PlainBill

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

The bane of everyone trying to help is those who fail to give enough information. 'Old Dell 15' is almost as meaningless as saying 'old monitor'. A number of companies supplied OEM monitors to Dell. An exact model number is very helpful - mandatory, I would say. Good pictures of the power supply / inverter are also very useful.

IF you have a monitor supplied by Benq (the last character of the model number is a b), you have a very common problem. The solder joints on the transformers fail, the transistors short, which blows the fuse. Now when someone foolishly bypasses the fuse, even more damage is done.

The results of your tests are exactly what I would expect if you had destroyed the driver and control transistors for the bottom CCFL.

PlainBill

All I know is the monitor was made by Phillips and the inverter part # is Delta 18B004.

There was no magic smoke released, only a spark. Transistor definitely dead?

The fact is, the inverter works, but only for a few seconds. If the transistors were dead it wouldn't work at all. Right?

Anyway, it might be worth buying a replacement inverter.

[PlainBill]

Re: Old Dell 15" - 1 second lights up, then goes black

All I know is the monitor was made by Phillips and the inverter part # is Delta 18B004.

There was no magic smoke released, only a spark. Transistor definitely dead?

The fact is, the inverter works, but only for a few seconds. If the transistors were dead it wouldn't work at all. Right?

Anyway, it might be worth buying a replacement inverter.

Again, Dell bought their monitors from several different manufacturers. Benq supplied monitors have two separate inverter driver sections with a single inverter control IC. A failure of one driver section will usually blow the fuse. If the fuse doesn't blow (or some fool bypasses it) the situation you describe can occur.

Fixing an inverter is no more difficult than fixing an SMPS, and is often much less expensive than buying a new inverter. However, repair requires some knowledge of electronics, knowledge of the operation of the inverter, and SOMEONE has to be willing to pick up a camera and attach pictures.

At $10, purchasing a replacement may be a good idea, but expecting someone to confirm that a new inverter will actually repair a monitor of unknown design is ridiculous. I'd give the odds as a 40% chance that the real problem is a CCFL, 40% that the problem is the inverter, and 20% that it is (or was caused by) bad capacitors.

PlainBill

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

Again, Dell bought their monitors from several different manufacturers. Benq supplied monitors have two separate inverter driver sections with a single inverter control IC. A failure of one driver section will usually blow the fuse. If the fuse doesn't blow (or some fool bypasses it) the situation you describe can occur.

Fixing an inverter is no more difficult than fixing an SMPS, and is often much less expensive than buying a new inverter. However, repair requires some knowledge of electronics, knowledge of the operation of the inverter, and SOMEONE has to be willing to pick up a camera and attach pictures.

At $10, purchasing a replacement may be a good idea, but expecting someone to confirm that a new inverter will actually repair a monitor of unknown design is ridiculous. I'd give the odds as a 40% chance that the real problem is a CCFL, 40% that the problem is the inverter, and 20% that it is (or was caused by) bad capacitors.

PlainBill

I design electronics as a hobby. I have a fair idea how a circuit works.

Yes, it is foolish to bypass a fuse. But, the board is powered with 18V from a short circuit protected power supply. Worst case is, the inverter blows up. It was dead already. But I tested it on my bench power supply before hand. The board was drawing less than 0.5A with no lamps attached, which is good. I suspect the fuse failed because it was running near limits. I^2*t limits come into play over the lifetime of a device. This was a monitor used in an office (old business stock) so it's quite likely it was on for 8+ hours a day. It's almost 6 years old. Who knows, one day that fuse might have reached its limits and poof... dead.

There are no bad caps on the board. The controller is filled with Teapos, but surprisingly they are in almost good as new condition. There is one YXA Rubycon on the inverter, also in good condition. No other caps of note.

The power supply is a brick which attaches to the monitor and provides 18V at up to a number of unspecified amps. However it checks out too, with low ripple. Under no-load conditions it is a bit unstable (1Vp-p) but that is not uncommon.

All I know is the Dell serial number and part number. I have found occasional references to it but they haven't helped narrow any problem down.

For your enjoyment, pictures are attached, but I doubt they will help any more than my description.

[PlainBill]

Re: Old Dell 15" - 1 second lights up, then goes black

The pictures of the inverter confirm it. It's our old friend the Royer oscillator. The transformers are driven independently, but controlled from a single controller. If either CCFL or inverter section is bad, you will get 'two seconds to black'.

Earlier you did a test where you tried using only one CCFL at a time; the top one lit, the bottom one didn't. Let's describe it this way. You have two plugs on cables going to top and bottom CCFLs. You have two sockets for top and bottom plugs. They are the outputs of the top and bottom inverter sections.

Connect top plug to bottom socket and test. If the CCFL comes on, the bottom inverter section is good. The problem is the bottom CCFL or the CCFL wiring. If BOTH CCFLs will light on the top socket, but neither will light on the bottom socket, the bottom inverter section is bad. If the top CCFL will light on either socket, but the bottom CCFL won't light on either, the bottom CCFL (or it's wiring) is bad.

PlainBill

[alexanna]

Re: Old Dell 15" - 1 second lights up, then goes black

The inverter shutting down also turns off the LCD display. (Before, with the blown fuse, you could at least faintly see the display.)

Top only: switches off in <1 second
Bottom only: nothing (never comes on)

Any idea what the problem might be?

Thanks,
Tom

Were missing something, with the inverter fuse bridged the whole monitor is shutting down [loosing the very dim display] There is something shorted in the inverter section.
I would suggest powering on the monitor without the inverter plugged in, see if you can see the dim display

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

Were missing something, with the inverter fuse bridged the whole monitor is shutting down [loosing the very dim display] There is something shorted in the inverter section.
I would suggest powering on the monitor without the inverter plugged in, see if you can see the dim display

The power indicator light stays on and the supply voltage stays at 17.87V when it switches off...

Without inverter, the display is visible, if you look carefully.

The pictures of the inverter confirm it. It's our old friend the Royer oscillator. The transformers are driven independently, but controlled from a single controller. If either CCFL or inverter section is bad, you will get 'two seconds to black'.

Earlier you did a test where you tried using only one CCFL at a time; the top one lit, the bottom one didn't. Let's describe it this way. You have two plugs on cables going to top and bottom CCFLs. You have two sockets for top and bottom plugs. They are the outputs of the top and bottom inverter sections.

Connect top plug to bottom socket and test. If the CCFL comes on, the bottom inverter section is good. The problem is the bottom CCFL or the CCFL wiring. If BOTH CCFLs will light on the top socket, but neither will light on the bottom socket, the bottom inverter section is bad. If the top CCFL will light on either socket, but the bottom CCFL won't light on either, the bottom CCFL (or it's wiring) is bad.

PlainBill

Both plugged in: 2secs to black
Bottom not plugged in: 2secs to black
Top not plugged in: nothing

I then flipped the inverter board around. Top output -> Bottom CCFL. Bottom output -> Top CCFL. The result was a small, brief buzzing sound (<200ms) from the inverter and nothing else - no light.

Then I connected just the bottom CCFL to the top output. Result: VISIBLE ARCING on the PCB pins of the transformer and on the component side of the board as well as a loud buzzing/clicking sound (like a cricket.)

Then I connected the top CCFL to the bottom output. Result: nothing. No light.

Any idea?

[PlainBill]

Re: Old Dell 15" - 1 second lights up, then goes black

The power indicator light stays on and the supply voltage stays at 17.87V when it switches off...

Without inverter, the display is visible, if you look carefully.



Both plugged in: 2secs to black
Bottom not plugged in: 2secs to black
Top not plugged in: nothing

I then flipped the inverter board around. Top output -> Bottom CCFL. Bottom output -> Top CCFL. The result was a small, brief buzzing sound (<200ms) from the inverter and nothing else - no light.

Then I connected just the bottom CCFL to the top output. Result: VISIBLE ARCING on the PCB pins of the transformer and on the component side of the board as well as a loud buzzing/clicking sound (like a cricket.)

Then I connected the top CCFL to the bottom output. Result: nothing. No light.

Any idea?

At a minimum you have a broken CCFL or wire (probably the high voltage wire). This may have damaged something in the bottom section of the inverter.

If I had the monitor on my bench at this point I would be saving the CCFLs and the power adapter. The rest would go to a recycling center. A working 15" LCD monitor goes for anywhere from 'Name your price' to $30 locally. I don't think many get sold at $30 or $25.

PlainBill

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

At a minimum you have a broken CCFL or wire (probably the high voltage wire). This may have damaged something in the bottom section of the inverter.

If I had the monitor on my bench at this point I would be saving the CCFLs and the power adapter. The rest would go to a recycling center. A working 15" LCD monitor goes for anywhere from 'Name your price' to $30 locally. I don't think many get sold at $30 or $25.

PlainBill

As a learning experience, I will try to repair this. So what would be the next steps.

I already have two 14" LCDs (just found the other one in the roof), and an 18". This one would just be a trophy for me.

[PlainBill]

Re: Old Dell 15" - 1 second lights up, then goes black

As a learning experience, I will try to repair this. So what would be the next steps.

I already have two 14" LCDs (just found the other one in the roof), and an 18". This one would just be a trophy for me.

A trophy monitor? Well, it's less expensive than a trophy wife.

Disassemble the LCD panel until you reach the CCFLs. This has been a royal PITA every time I have had to do it. Look for signs of damaged wires, arcing, or broken CCFLs. Try not to get any dust into the layers.

PlaiinBill

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

Okay, they weren't too difficult to remove once I figured out how to do it. They are on little sliding rails and come out in these modules.

So, when I was removing one of them, the pink wire on one of them came loose very easily. Could this have been causing the problem?

What source for CCFLs would you recommend?

Do you think a new inverter would be a good idea too?

BTW, the LCD panel part number is LM151X2 and it is made by LG-Philips LCD (Korea).

[PlainBill]

Re: Old Dell 15" - 1 second lights up, then goes black

Okay, they weren't too difficult to remove once I figured out how to do it. They are on little sliding rails and come out in these modules.

So, when I was removing one of them, the pink wire on one of them came loose very easily. Could this have been causing the problem?

What source for CCFLs would you recommend?

Do you think a new inverter would be a good idea too?

BTW, the LCD panel part number is LM151X2 and it is made by LG-Philips LCD (Korea).

Yeah, a loose high voltage lead would explain everything. You were lucky; every one I had to deal with required a complete disassembly of the LCD panel.

Several users have reported excellent results with CCFL Warehouse.

As far as replacing the inverter, it's your call. I guess what I would do is repair the broken lead and seal any damage with silicone adhesive, letting it cure for a day or so. Then test both CCFLs on the existing inverter. Using the swap technique you can determine if there is any problem with the existing CCFLs and inverter. If everything works, put it back together and cross one more item off your bucket list. If there is any problem, I'd replace both CCFLs and the inverter.

PlainBill

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

Yeah, a loose high voltage lead would explain everything. You were lucky; every one I had to deal with required a complete disassembly of the LCD panel.

Several users have reported excellent results with CCFL Warehouse.

As far as replacing the inverter, it's your call. I guess what I would do is repair the broken lead and seal any damage with silicone adhesive, letting it cure for a day or so. Then test both CCFLs on the existing inverter. Using the swap technique you can determine if there is any problem with the existing CCFLs and inverter. If everything works, put it back together and cross one more item off your bucket list. If there is any problem, I'd replace both CCFLs and the inverter.

PlainBill

Hmm, I tried to push the lead back in, but I got a sound like glass being crunched. Sounds almost like the tube is broken or was broken by me putting the lead back in. I guess, since the tube is broken, I will need to dispose of it properly; is there a way to remove it from the support it sits in and change it out for a new one?

[PlainBill]

Re: Old Dell 15" - 1 second lights up, then goes black

Hmm, I tried to push the lead back in, but I got a sound like glass being crunched. Sounds almost like the tube is broken or was broken by me putting the lead back in. I guess, since the tube is broken, I will need to dispose of it properly; is there a way to remove it from the support it sits in and change it out for a new one?

Yes. The best way is 'carefully'. I'd suggest spending some time on the CCFLwarehouse site, looking at all the information they have on supplies and procedures. I've never reached that point. I have heard that one way is to simply remove the old tube and the silicone mounting blocks, solder the leads to the new CCFL, then use silicone adhesive to hold the tube in the channel.

PlainBill

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

I'm almost tempted to do an LED conversion as the CCFLs are $10 each and the inverter is $14. Not really worth it. I can buy two strips of high power white LEDs (total around £9) and run them straight off the 18V supply (possibly using a relay or FET to switch them on, so they don't run when the monitor is in standby.) To be considered, I'll let everyone know how it goes .

[tom66]

Re: Old Dell 15" - 1 second lights up, then goes black

As an idea on how to size my LED strip, can anyone give me a source for the average light output from a single CCFL in a 15" monitor at full brightness?

[PlainBill]

Re: Old Dell 15" - 1 second lights up, then goes black

As an idea on how to size my LED strip, can anyone give me a source for the average light output from a single CCFL in a 15" monitor at full brightness?

The original CCFL is about 310 mm long. I checked a spec sheet, and the rated brightness is 46,000 Cd/m^2

PlainBill