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Old 09-17-2011, 11:44 PM   #1
geester22
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Default HP L1906 monitor problem

Hello, I have an HP L1906 monitor that died recently. When powered up, it flashes on for a second or two and then goes blank. The power LED stays on. It doesn't appear to be a backlight issue, as there is no display (it is completely blank). Does anyone have a schematic or service manual for this unit? I have read some postings about electrolytic cap replacement, but I want some detailed info in case there is a faulty IC or other discrete component gone bad. Any one out there seen some common failures?

Thanks!
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Old 09-18-2011, 01:49 AM   #2
ipman
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Default Re: HP L1906 monitor problem

Finding bad caps is easy with ESR meter.
Finding bad diodes is easy with a multimeter. Also finding voltage presence.
Begin with troubleshooting with the above, you won't need a schematic for these. The power supply protects itself if there's no "good power" at the output.
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Old 09-18-2011, 06:51 AM   #3
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Default Re: HP L1906 monitor problem

If you have backlights coming on and going off, and you have a power LED that indicates its seeing a signal, I would suspect you have what we call 2 sec to black.
The next thing I would do is to identify the ENA/BL on pin at the connector from the power supply to the signal card, see if it has a voltage of around 3 volts when a signal is applied to the monitor. If you see 3 volts it means the signal card is asking for backlights to be on and you have a 2 sec to black problem. If you have trouble identifying the BL on pin
Post pictures of the boards a good overall shot straight down with no glare, and in focus using managed attachments
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Old 09-18-2011, 06:13 PM   #4
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Default Re: HP L1906 monitor problem

Hi ipman, Thanks for your reply. I have an LCR meter and will check some of the 'lytics after I make several in circuit tests. I did not see any obvious signs of cap problems, popped components, or thermally stressed PCB zones. I'll let you know if I find any issues with the discretes after testing.
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Old 09-18-2011, 06:33 PM   #5
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Default Re: HP L1906 monitor problem

Hi alexanna, Thanks for your reply. I don't know which pin has the ENA/BL signal, can you tell me? Is this a DC level?

It does seem like Q808 and Q811 might be thermally stressed a bit. What are these devices?

I will try to post pictures with my reply.

Thanks!
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Old 09-19-2011, 09:00 AM   #6
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Default Re: HP L1906 monitor problem

I think I can make out Q811 ,but I am unable to see Q808.If they look burned or have a bubble on them they are probably damaged, can you make out any numbers stamped on them?
Note the staggered pin numbers on the 11 pin connector; I think the backlight on pin is either pin #1 or #2, when the signal card recognizes a signal and asks for backlights to be on it drives the voltage high on one of these pins to around 3vdc.When no signal is present the voltage goes low down to 0 vdc.
Try to identify the on pin and see if the voltage stays high when the back lamps go off. You may have to probe each pin on the connector and then power on and off the monitor and look for a voltage to go high when the back lights are on.
There is a 20 pin IC in the center of the board just below the white line, what is the number stamped on it?
What are the brand and series of the capacitors used; there are some that are considered to be replace on sight.
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Old 09-22-2011, 08:31 PM   #7
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Smile Re: HP L1906 monitor problem

Hi alexanna,
Sorry for the late reply, I have been working some late hours the last couple of days.
Yes, there is a 3vdc signal at CN902-2 (pin 2) when the video signal is present.

Q808, Q809, Q810, and Q811 are all the same device. There is no manufacturing logo, the numbers are:
4512C
AA6GL

IC801 (20 pin device) also has no logo, the numbers are:
OZT1060CN
5256744
0550C

Caps are Elite PW series, Chemi-con KY series, Elite ED series

Which caps are considered replace on sight? Can you tell me an easy source for these? IC801 might also be thermally stressed. Do have a source for this?

Thanks!
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Old 09-23-2011, 09:32 AM   #8
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Default Re: HP L1906 monitor problem

Quote:
Originally Posted by geester22 View Post
Hi alexanna,
Sorry for the late reply, I have been working some late hours the last couple of days.
Yes, there is a 3vdc signal at CN902-2 (pin 2) when the video signal is present.

Q808, Q809, Q810, and Q811 are all the same device. There is no manufacturing logo, the numbers are:
4512C
AA6GL

IC801 (20 pin device) also has no logo, the numbers are:
OZT1060CN
5256744
0550C

Caps are Elite PW series, Chemi-con KY series, Elite ED series

Which caps are considered replace on sight? Can you tell me an easy source for these? IC801 might also be thermally stressed. Do have a source for this?

Thanks!
There is no problem with the delay,as long as i'm on the green side of the grass,i've got all the time in the world

There are few problems with the UCC caps, I would think the Elite would be suspect.
Because you get a good PLED and the fact you notice a good backlight enable signal of 3.2v even when the back lamps go off, I would pursue a 2 second to black problem.
The 2sec problem can be caused by a number of issues.
The most common are capacitors, a shorted or open inverter transformer, or the CCFLs themselves.
I am going to suggest, if you can to rule out a problem with the CCFLs first.
The quickest way would be to install another panels working backlights and see if the lamps stay on, do not disassemble any panels just plug in the good CCFL leads into the inverter you are working on.
If another panel is not an option, there are other things we can try.
I found a data sheet on 20 pin backlight controller IC801, but so far I have not had luck with the Q MOSFETs.
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Old 09-29-2011, 09:35 PM   #9
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Default Re: HP L1906 monitor problem

Hi alexanna,

Thanks for the data sheet. It is useful for looking at the signals on the device as well as some operational notes. I was able to capture some waveforms using an o'scope (attached). Below is a description of what I saw:

Probe 1: IC801-3 enable, approx 4-5 vdc, always present when powered up
Probe 2: IC801-2 OVP, 0vdc for approx 120 ms, then rises to 1 vdc
Probe 3: IC801-20 NDR_B, approx 75 khz, 4 vdc for 230 ms, then drops out
Probe 4: IC801-11 NDR_D, approx 75 khz, 5 vdc for 480 ms, then drops out

Unfortunately, I do not have another panel to try.

To me, it seems like the OVP signal may be causing an ungraceful halting of the N-FET D gate signal. It is difficult to tell though, it could be the other way around. Also, the P-FET gate signals are the same. By the way, VCC on the device looks clean...

Any ideas?

Thanks!
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Old 09-29-2011, 09:43 PM   #10
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Default Re: HP L1906 monitor problem

OVP is 'Over Voltage Protection'. That indicates an open CCFL or wire to a CCFL, or possibly a bad component in the sense circuitry. Don't try looking at the output directly with your scope; the voltage is probably over 1000 volts.

I note that this test hasn't been tried yet. Disconnect all but one CCFL, Turn the monitor on. If you see a flash from the panel, disconnect the first CCFL and hook up the second. Turn the monitor on again, looking for a flash. Continue through all 4 CCFLs. If one CCFL does not flash, you have found the problem.

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Last edited by PlainBill; 09-29-2011 at 09:54 PM..
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Old 09-30-2011, 08:52 PM   #11
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Default Re: HP L1906 monitor problem

Hi PlainBill,

I tried your suggestion and all four connections flashed on. Other than shot-gunning the FETs and perhaps the PWM device, what else can I try?

Thanks,

Guy
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Old 09-30-2011, 09:08 PM   #12
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Default Re: HP L1906 monitor problem

Quote:
Originally Posted by geester22 View Post
Hi PlainBill,

I tried your suggestion and all four connections flashed on. Other than shot-gunning the FETs and perhaps the PWM device, what else can I try?

Thanks,

Guy
You could try tracing the OVP lines from the individual CCFLs. This is extremely tedious, and I've only had a couple of successes. To be brutal, the success rate was so low I no longer am willing to assist with it.

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Old 09-30-2011, 09:34 PM   #13
geester22
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Default Re: HP L1906 monitor problem

Thanks for your reply. At this point, it's starting to sound like repairing this thing isn't worth the effort... I might try my hand at replacing the CCFLs if they are readily available and not too expensive.

Fixing this is more for educational value, does anyone have a schematic?
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Old 10-01-2011, 06:18 AM   #14
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Default Re: HP L1906 monitor problem

Quote:
Originally Posted by geester22 View Post
Thanks for your reply. At this point, it's starting to sound like repairing this thing isn't worth the effort... I might try my hand at replacing the CCFLs if they are readily available and not too expensive.

Fixing this is more for educational value, does anyone have a schematic?
That's the rub. With a schematic, fixing something like this would be easy. I've looked, and didn't come up with anything from the usual sources. Substituting known good CCFLs is the best way to troubleshoot this problem.

If you want to experiment, try substituting caps for the CCFLs. I have not tried this, but the concept is interesting.

PlainBill

Last edited by PlainBill; 10-01-2011 at 06:22 AM..
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