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Old 03-09-2018, 12:02 PM   #1
Pimpom
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Default BenQ G2420HD cannot stay lit up

I have a BenQ G2420HD monitor that was damaged during transportation. Two of the four CCFL tubes were broken and I've solved that problem but it still has a fault. These are the symptoms:

1. When switched on, it lights up and presents a normal display FOR ONE SECOND, then goes dark for 3 seconds, lights up again for one second and then goes dark permanently. Power cycling makes it repeat the same behavior over and over.

2. It detects the video and sync inputs. The pilot light stays green as long as there's a signal input (even when the screen goes dark by itself as described) and turns amber when there's no signal.

3. The LCD panel syncs and displays correctly even when the screen goes dark by itself. I verified this with an improvised backlight. It is only the backlight that is misbehaving.

4. The light/dark cycle is caused by the 3.3V ENA signal from the control board to the inverter coming on and off. It's too regular to be a thermal issue.

5. All power supply voltages (17V, 5V & 3.3V) are OK. The control board controls the inverter with one-way ENA and DIM signals. There is no feedback path from the inverter to the control board.

6. The fault seems to be in control board. It doesn't have obvious cracks, burn marks or bulging capacitors.

Any help will be greatly appreciated. I drew a block diagram of the main parts:
Attached Images
File Type: png BenQ G2420HD.png (14.6 KB, 9 views)

Last edited by Pimpom; 03-09-2018 at 12:05 PM.. Reason: Minor change of wording
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Old 03-11-2018, 06:04 AM   #2
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Default Re: BenQ G2420HD cannot stay lit up

You might want to check this thread out which might give you some ideas on what to check (the section on the inverter control chip):

https://www.badcaps.net/forum/showth...ght=benq+g2255

Also, you will probably need the service manual for your monitor, so here it is attached.

Let us know your progress.
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File Type: pdf benq g2420hd service manual.pdf (2.12 MB, 5 views)
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Old 03-11-2018, 12:03 PM   #3
Pimpom
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Default Re: BenQ G2420HD cannot stay lit up

Thanks for your interest. I thought no one was going to reply.

I already downloaded the service manual a few days ago but while it's quite detailed about some aspects, it's sketchy about others.

That thread of yours is quite long and will take some time to go through.

It's getting close to midnight here now and I have a heavy day tomorrow. So it may be some time before I can pursue this further and have something to report.
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Old 03-11-2018, 12:47 PM   #4
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Default Re: BenQ G2420HD cannot stay lit up

No problem, take your time. You should see how much time it took us to figure the thread I mentioned, there is now a part 2 of that thread where we are trying to diagnose what kills the main chip on the logic board.
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Old 03-12-2018, 04:37 PM   #5
Dumah Brazorf
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Default Re: BenQ G2420HD cannot stay lit up

Maybe it was better to change all the ccfls.
I think it's not the logic board to cut the enable signal but the inverter chip that pulls it to ground due to a problem (overvoltage/overcurrent) of the inverter or the lamps.
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Old 03-13-2018, 04:53 AM   #6
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Default Re: BenQ G2420HD cannot stay lit up

Quote:
Originally Posted by Dumah Brazorf View Post
Maybe it was better to change all the ccfls.
I agree, but I can't find spare CCFLs. In fact, I have not replaced the broken CCFL at all. When I said that I solved the problem, it was a simplification. I have ordered LED replacements from China but they'll take at least a month to arrive.

I tested the monitor with only two CCFLs installed. Only one of the four CCFL outputs has a load sense feedback to the inverter control and I made sure that one CCFL was in that socket.

As it is the Enable signal from logic board that keeps turning the inverter off, it doesn't seem to be related to the fact that only two CCFLs are fitted.

Quote:
I think it's not the logic board to cut the enable signal but the inverter chip that pulls it to ground due to a problem (overvoltage/overcurrent) of the inverter or the lamps.
That would make sense except for a couple of things:
First, I haven't been able to find a detailed datasheet for the inverter chip OZ9933, but the ENA and PWM pins are almost certainly input-only.

Secondly, I've traced the ENA and PWM control lines from the logic board as shown in the diagram here. There's a 1k resistor (R805) in series with the ENA input to the inverter and there's a 4.7k pull-up resistor (R114) on the logic board.

Even if the inverter chip somehow shorted the ENA input pin to ground, R805 and R114 would form a voltage divider and there'd still be at least 0.5V at the board input. But I measured only a few mV there. This indicates that the line is pulled down to ~0V from the logic board.
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Old 03-13-2018, 04:39 PM   #7
Dumah Brazorf
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Default Re: BenQ G2420HD cannot stay lit up

So there's no problem. The image is still present so you can get the enable signal from elsewhere.
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Old 03-13-2018, 11:23 PM   #8
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Default Re: BenQ G2420HD cannot stay lit up

Simply disconnecting the RTD2482RD pin 102 would do that as R114 will then permanently pull up the ENA line. But then the monitor will stay lit up even when the rest of the circuit goes into standby.

I suppose I can live with that if all else fails. I could also design and add a separate sync detector and use that to turn the backlight on and off. Thing is, I don't like leaving a problem unsolved. If this were a known problem that sometimes happens with this display controller chip (the RTD2482RD, not the inverter controller), I'd feel easier about using a bolted-on solution.
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