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

[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

[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

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

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

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 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?

[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]

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