HP w20 LCD Display Inverter, running out of options.

[PlainBill]

Re: HP w20 LCD Display Inverter, running out of options.

What if it's not overvoltage that causes shutdown? Could it be undervoltage or undercurrent?

I was looking post #69 mine and noticed that AC drops over time. That's also what happened when I adjusted brightness down from 95% and it caused shutdown when brightness reached about 50% level.

Here's the way this was designed to work, and my current theory as to the nature of the failure. The brightness is controlled by pulse width modulation - PWM. CCFLs work much like standard fluorescent lights - current flow through the lamp generated ultraviolet light which causes phosphors coating the inside walls of the tube to glow. They don't respond well to reducing the voltage; rather the duration of the 'on' time is varied. At 100 Khz, they are turned on every 5 microseconds. If they are on 1 microsecond out of every 5 they will be much dimmer than if they are on 4 microseconds out of 5. The current sensing circuitry is really responding to the peak current, not the average current, just like the voltage sensing circuitry is responding to the peak voltage.

Now, repeating what I wrote yesterday: The ... the voltage at the right end of R32 goes through R51, Q17 (actually a dual diode where the negative transistions are clipped), and on to the Isen input of the inverter controller. That signal ALSO goes to R46 and Q22, which I believe is a true transistor. When the spikes get high enough to turn on Q22, it reduces the Vsens threshold and causes the inverter controller to shut down.

You bring up a very good point. Q22 appears to be SMD code RKM; Q23 certainly is. That is a RK7002 MOSFET. Because of the way it is wired, it certainly changes the voltage on the OVP input of the controller. Because of the tests you did with jumpers we know that reducing the voltage on the Vsens input will prevent shutdown.

What I don't know - yet - if Q22 is triggering on increased or reduced current. And I'm having an absolute block when it comes to figuring out how to change the trigger current WITHOUT changing the current at the Isens input. One part of figuring that out is to really understand the differences between P-channel and N-channel mosfets, and between depletion and enhancement mode.

PlainBill

[CapBlown]

Re: HP w20 LCD Display Inverter, running out of options.

Thanks PlainBill.

Thinking about that OVP input I took a closer look at it. Makes me wonder if I've found a bad capasitor. At least it looks like there are fractures in it so I think I better change it. How could I identify it's capasitance as I have no schematics.

[CapBlown]

Re: HP w20 LCD Display Inverter, running out of options.

I was watching some old pictures and it looks like that cap was looking allright when I got this monitor. It must have gone like that during testings or when resoldering it.

[PlainBill]

Re: HP w20 LCD Display Inverter, running out of options.

It's going to be difficult to determine the value. The first step would be to determine which pins on the IC it is hooked to. There are several timing caps that set delay times. If this is one of those, it will be fairly easy to select a 'common' value.

PlainBill

[CapBlown]

Re: HP w20 LCD Display Inverter, running out of options.

It's going to be difficult to determine the value. The first step would be to determine which pins on the IC it is hooked to. There are several timing caps that set delay times. If this is one of those, it will be fairly easy to select a 'common' value.

PlainBill

It's connected to pin 7, OVP, parallel with that 100kΩ

[PlainBill]

Re: HP w20 LCD Display Inverter, running out of options.

It's connected to pin 7, OVP, parallel with that 100kΩ

Try something in the range of 1 - 10 nanoFarads. If you have a junk box with through hole ceramic caps, experiment. It shouldn't be critical.

PlainBill

[CapBlown]

Re: HP w20 LCD Display Inverter, running out of options.

Thanks again PlainBill.

I think that eventually I will have a junk box . As a beginner I need to go shopping again. This time I'll get some extra components though, I never seem to have what it takes to do anything.

[CapBlown]

Re: HP w20 LCD Display Inverter, running out of options.

I got replacement cap for fragmented one. I purchased MMC with capacitance measurement capability and found out it was 98 nF so I replaced it with 100nF

I measured also IC pin 13 (CT). I don't know if it means anything but if I measure it with multimeter, backlights immediately go off and AC voltage goes like this:
1,8 V~ -> 9.9 V~
Seems pretty sensitive circuitry... even though in this situation monitor can be switched back on immediately.

You bring up a very good point. Q22 appears to be SMD code RKM; Q23 certainly is. That is a RK7002 MOSFET. Because of the way it is wired, it certainly changes the voltage on the OVP input of the controller. Because of the tests you did with jumpers we know that reducing the voltage on the Vsens input will prevent shutdown.
PlainBill

I was wondering would it be worth trying to get rid off Q23. If that is the part which screws VSENS input signals, removing it like it's been done on REV 3 board might help?

[PlainBill]

Re: HP w20 LCD Display Inverter, running out of options.

I got replacement cap for fragmented one. I purchased MMC with capacitance measurement capability and found out it was 98 nF so I replaced it with 100nF

I measured also IC pin 13 (CT). I don't know if it means anything but if I measure it with multimeter, backlights immediately go off and AC voltage goes like this:
1,8 V~ -> 9.9 V~
Seems pretty sensitive circuitry... even though in this situation monitor can be switched back on immediately.



I was wondering would it be worth trying to get rid off Q23. If that is the part which screws VSENS input signals, removing it like it's been done on REV 3 board might help?

Try it.

PlainBill

[CapBlown]

Re: HP w20 LCD Display Inverter, running out of options.

Thank you for your advices and encouragement PlainBill. I've learned a lot within last few months.

Removing Q23 seems to have solved this backlight shutdown issue for now. I just don't feel too good about it. Looking REV 3 board without that component I think I can live with it.

I managed to destroy Q23 while removing it, so there is no going back anymore.

It's great that this monitor is working again, but I have to admit that I am disappointed that I could not understand or locate the real reason causing this problem. Though I can always comfort myself thinking that there must have been a problem with the original design, why else would they have removed that Q23 from the next revision.

[retiredcaps]

Re: HP w20 LCD Display Inverter, running out of options.

Thank you for your advices and encouragement PlainBill. I've learned a lot within last few months.

Good job in getting this monitor working again.

I think this thread and the "Acer AL2423W Recapped" by Per Hansson demonstrate the patience and persistance in troubleshooting the "tough dogs".

I, for one, will have to re-read both these threads a few more times in the future for everything to sink in.

Like smason wrote in another thread, I enjoy the detailed explanations and have lots more to learn.

[PlainBill]

Re: HP w20 LCD Display Inverter, running out of options.

Thank you for your advices and encouragement PlainBill. I've learned a lot within last few months.

Removing Q23 seems to have solved this backlight shutdown issue for now. I just don't feel too good about it. Looking REV 3 board without that component I think I can live with it.

I managed to destroy Q23 while removing it, so there is no going back anymore.

It's great that this monitor is working again, but I have to admit that I am disappointed that I could not understand or locate the real reason causing this problem. Though I can always comfort myself thinking that there must have been a problem with the original design, why else would they have removed that Q23 from the next revision.

I'm glad you have it working, too. Like you, I'd rather have understood exactly the reason for the problem. In general, there are two reasons for a new revision. In many cases it's to accommodate revised performance of one of the components. In the case of an inverter, to allow use of a different panel with different CCFLs. Sometimes it's to correct a design defect. I assumed the first reason; it may be that the design tolerances were too tight.

PlainBill

[CapBlown]

Re: HP w20 LCD Display Inverter, running out of options.

Update: This monitor has been working now for more than 750 backlight hours withour any problems since it was fixed. Strange that by removing parts you can actually fix something

[blackbox]

Re: HP w20 LCD Display Inverter, running out of options.

Hello guys, it's time for me to ask for help with this monitor. I read the thread and thanks to PlainBill and CapBlown I got my display working again. Previously it had the same problem like the CapBlown's but there was a smell like burnt. And the cause of this was the capacitor C17 (27pF/3KV) shown on the picture - it changed it's color to dark brown. The red line is the connection to the transformer.
I removed Q23 and voila the backlight turned on. It worked some 4-5 days (10 hours a day) and started to smell again. Then I decided to put the two bridges (C19 and C25). When I switched the display on there was no backlight. I heared the high frequency noise for a while and then nothing.
One important thing - I haven't changed any part on the board.
Well, I will try to repair it. Thank you.

[selldoor]

Re: HP w20 LCD Display Inverter, running out of options.

Hmm read the thread some complex circuit stuff in there. More simple things I can help with. Did you test the secondary windings on the inverter transformers. Have you anything else you can check the bulbs on?

[blackbox]

Re: HP w20 LCD Display Inverter, running out of options.

Thank you for answering, selldoor. I paid for my impatience and for not reading carefully. Now I'm not living in the mainland and there are not many possibilities to find the 3KV caps. In the first page of this thread there is an explanation of the case with these caps but I decided not to wait and anxiously bridged C19 and C25 with the hope that something will change. It changed ... but for bad
So I got what I deserved. Later I will try to investigate the problem and will communicate. Thank you.

[sivads]

Re: HP w20 LCD Display Inverter, running out of options.

It's L106. Some sort of choke perhaps?

test

[deni-m]

Re: HP w20 LCD Display Inverter, running out of options.

Hello,

I need help. I'm want to update my hp w2007v from ccfl to led.
I've already buy the led stripes and led drivers.
Can somebody help me where to find 12V for drivers on this board.
If need any info please tell me.
Pictures attached.

Thanks

[Agent24]

Re: HP w20 LCD Display Inverter, running out of options.

Honestly if you're going the the trouble, you might as well just use new CCFLs.

Considering the amount of LED backlight failures in cheap TVs these days, I don't have much faith in LED strips.
LED strips have multiple LEDs. Each one is a point of failure. A single CCFL is much more reliable.

[budm]

Re: HP w20 LCD Display Inverter, running out of options.

See picture for the B+ for the LED driver board.
You still need to locate the BL_ON signal and the Dimming signal to control your LED backlights.