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Old 04-29-2017, 04:58 PM   #23
cmg
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Join Date: Dec 2016
City & State: London
My Country: United Kingdom
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Posts: 31
Default Re: HP ZR2440w backlight cutting out

Ginko - you have a very similar problem and circuit to the one i am trying to diagnose. From your pics I can see the ZR2440W is like a cutdown version of the 2740 that I have. Probably they've used the same controller ICs.

Please try altering the brightness on the front panel control and see if it becomes stable at high brightness. I wasn't clear from the above if you tried swapping the LCD panel (i.e. including the backlights) from one of your good monitors into the bad monitor. That way you can tell if it is the LEDs or not. From what you described i guess this is not your problem. I have had one of these with one the LEDs strips open circuit and it causes the monitor to shutoff completely.

My suggestions are:
- check for dry joints in the LIVE circuit. Some look odd in the photo but i guess that is just reflections.
- read the part number on I801, see if it is the same as mine; it could be UCC25600 which is an SMPS controller. I have linked the docs for this on my thread. One good thing about that is its only $1 or $2 and you can buy it direct from TI.
- look at the signal on the isolated side of the opto-isolator I803 with a scope. I can't make out if the circuit on the driving side of that is like mine or not. You are looking to see if this is stable or not. Try adjusting the brightness from min to max while checking this. You can compare vs mine to see if the symptoms seem the same.
- look at the signal on Vadj of P803. It should be a PWM waveform at mid brightness, 5V DC at max brightness and 0V at min.
- compare readings between the good & bad PSUs. You can do both for resistance/capacitance as well as the voltage readings.

Measuring voltages on the Live side is dangerous as there are lethal voltages in that circuit and the ground is not isolated.
I reckon your problem is around I801 but you should only try measuring voltages around it if you are used to working on high voltage circuits. Aside from the safety, it if difficult since it is not isolated so you cannot use a mains-powered scope to measure the details of the circuit around I801. I have used the scope to look at the high voltage lines. My multimeter has a nice function which is that it has a bargraph. I have found that useful for measuring the SMPS circuit. As mine is flickering at a failrly slow rate (5-10Hz) it is visible on this bargraph. To take any measure measurements on this live side i have:
- left the PSU PCB bolted into the chassis but just exposed the solder-side
- soldered short wires onto the places i wanted to measure and then attached the meter or scope to these so i dont need to poke around while its on.
- stand back and ensure the monitor is unplugged from the mains immediately after. In mine there is still 320V in the PSU when the monitor is on standby.

I still havent solved mine. It is really difficult, because it manifests itself as an instability in the whole circuit, through the current-sense feedback loop. I am now thinking it is in the PFC circuit but your monitor doesn't have that - looks like it just rectifies the mains and feeds this into the SMPS circuit. Mine has another controller before this which looks like it is playing up to me. Will update my thread on this.

Best of luck and stay safe.
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