BenQ G2420HD LCD monitor cannot stay lit up

bad electrolytics are often not visibly damaged, test them for ESR if practical. Or holding a similar one one in parallel can find a bad one by making it work ok.

If a 3.3v signal line is the cause, you could always cut & tie it high or low.

NT

If we ignore the shipping damage, it sounds like a typical back light probl em. Are you sure you have the correct CCFL tubes in the display?

There's a chance one or more of the inverter transformers was damaged when the display was run with an open tube.

What you can do is place your scope probe on the plastic part of the lamp c onnector. Where it goes exactly is unimportant as you're going to be looki ng for *differences* in the waveforms between the original tubes and the re placements. In any case, don't make any electrical connection or you'll li kely damage your scope input. But you must be sure to place the probe in th e exact same spot on the connector as the waveform will vary greatly with j ust a small physical movement.

Look at the waveforms for each of the tubes during the time it's actually l it. If one or more waveforms differs from any other, the controller IC wil l shut down the inverter. Too large a waveform usually indicates a bad, we ak, or incorrect CCFL tube. A low or distorted waveform will usually indic ate a bad transformer.

The problem with altering the enable control is that if the controller IC is shutting down the array because of a real problem, the inverter will burn up if forced to run.

Before we go further with your suggestions, please see my point No.5 again. There is no way the logic board can know about an inverter problem. It just sends and stops the ENABLE signal as described without any feedback from the inverter.

The power supply and the inverter are on one pcb while the logic and display circuits are on another board. The power supply remains fully ON as long as it's plugged in. The only connections between the two boards are via a 4-way cable:

1) +5V to the logic board 2) GND 2) ENABLE /from/ the logic board 3) DIM PWM signal /from/ the logic board

The inverter does not turn off by itself. It's turned off when the ENA signal from the logic board stops.

roblem. Are you sure you have the correct CCFL tubes in the display?

hen the display was run with an open tube.

mp connector. Where it goes exactly is unimportant as you're going to be l ooking for *differences* in the waveforms between the original tubes and th e replacements. In any case, don't make any electrical connection or you'l l likely damage your scope input. But you must be sure to place the probe i n the exact same spot on the connector as the waveform will vary greatly wi th just a small physical movement.

ly lit. If one or more waveforms differs from any other, the controller IC will shut down the inverter. Too large a waveform usually indicates a bad , weak, or incorrect CCFL tube. A low or distorted waveform will usually i ndicate a bad transformer.

yes, so overriding the ENA signal will not affect safety. What it would aff ect is the screen shutting off when pc video card stops sending signal, one would need to use the on/off switch more.

NT

I know. I'd considered that possibility but I hate band-aid solutions except as a last resort.

It seems clear to me that the problem is on the control board. I was hoping that someone would know about it and can offer a solution.

I've drawn a block diagram of the boards. Here -

Are you certain there is no feedback? I've seen systems where the load driv er, if overloaded, can truncate the PWM pulse from the low level driver cir cuitry and that circuit can sense it. Might want to put a scope on the enab le and dim lines and see if anything like that is going on.

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problem. Are you sure you have the correct CCFL tubes in the display?

when the display was run with an open tube.

lamp connector. Where it goes exactly is unimportant as you're going to be looking for *differences* in the waveforms between the original tubes and the replacements. In any case, don't make any electrical connection or you 'll likely damage your scope input. But you must be sure to place the probe in the exact same spot on the connector as the waveform will vary greatly with just a small physical movement.

ally lit. If one or more waveforms differs from any other, the controller IC will shut down the inverter. Too large a waveform usually indicates a b ad, weak, or incorrect CCFL tube. A low or distorted waveform will usually indicate a bad transformer.

affect is the screen shutting off when pc video card stops sending signal, one would need to use the on/off switch more.

It's not a fault I've ever fixed. I've no clue whether you can trace that l ine backward to see where it starts to get flakey. I'm guessing it just goe s straight into a huge chip though. And if Terry is correct that might not be the solution.

NT

Comprehensive service manual here:

It's 2 am here now and I'm getting ready for bed. I don't know if I'll have time to some more checking tomorrow. If not, it'll have to wait for Monday.

I'm certain that the two lines coming from the control board go to pins 13 and 16 of the inverter control IC OZ9933 via a few passives. These are the DIM PWM and ENABLE pins respectively. The tracks are easy to trace on the single-sided PCB and they are the only lines of connection with the control board (except for the

5V supply and GND).

I downloaded the service manual a few days ago. But while it's quite detailed about some things, it's sketchy about others or entirely omits them. Same thing for the schematic.

I already have that. See my reply to your other post.