H.P. L1710 back lamps turning off

[alexanna]

I have a H.P. L 1740 that after about 20 minutes the back lights are going off, And a image is visible.
The B.L. on signal is not changing when the lamps go off, I have traced the signal down to the green card between the two ccfl transformers and it is good.
With the monitor dissembled it takes much longer for the lamps to turn off upwards of 2 hours or more.
I am seeing 12.5v across the inverter caps when the lamps or on and aprox.13v when the lamps are not on.
I have measured the A.C. voltage on the return lines of the ccfls when the lamps are on.
#3 is between 1.18 and 1.25 vac as measured with min. max
#4 is 1.42 to 1.58 vac
#1 is 1.70 to 1.89 vac
And #2 is 1.14 to 1.20 vac
I have tried the measurements with two different panels and voltages are very similar on the return lines.I really haven't checked voltages on the return lines enough to know if there is too much of a difference in the voltages on the return lines? I am thinking about poor solder joints in the inverter area. When I can get this to fail again is there anything else that I could check to narrow down the problem?
U1 is an oz59360n. The two other 8 leg chips are 9971gm mosfets.
Al.

[alexanna]

Re: H.P. L1710 back lamps turning off

Well this monitor is finial starting to fail more often it's been on about two hours.
I realize a dvm may not be the best tool to diagnose this problem, But I have been probing around checking some voltages, On the AP9971 mosfet when the back lamps are working I am reading 12.5vdc on both the source and the gate, and I am reading 1.63vdc on the drain.
When the lamps go off the source and gate voltages go up close to 13vdc, But The drain voltage goes down to 0vdc.
When I am reading the 1.6v on the drain of the mosfet am I measuring the voltage drop across the mosfet,and when I see the 0v is that an indication that its open and failing?
Al.

[retiredcaps]

Re: H.P. L1710 back lamps turning off

I had a similar situation with a Benq monitor. It would shut off after 15 or 20 minutes if left at the default brightness/contrast settings. If I lowered settings, the monitor would stay on longer.

I finally got an identical monitor and found out it was the backlights that were at fault.

Before I got my hands on the identical monitor, I tried everything from resoldering all the joints, replacing the caps, etc and nothing worked.

[alexanna]

Re: H.P. L1710 back lamps turning off

Well I have run into that also, But I have installed a known good panel and I am getting the same results so I don't think it's the bulbs.
Al.

[jetadm123]

Re: H.P. L1710 back lamps turning off

Sounds like a thermal problem. May I suggest taking a blow dryer and heating up parts of the board, especially the circuit in your third photo, to accelerate the shutdown process. Hopefully, you can at least narrow down the portion of the board that's causing the problem.

[alexanna]

Re: H.P. L1710 back lamps turning off

I need some help on this one.
When the back lamps are working on this monitor measuring the voltage at the drains of ap9971 mosfet I read 1.63v I also read a similar voltage at pins 1 and 8 at the oz controller, little bit higher 1.72v.
When the back lights go off the voltages of both the mosfet drain and the controller pin 1 and 8 voltages go to 0v indicating no current flow.
My questions is if the oz controller is not providing a ground to the mosfet shouldn't I be reading whatever the source voltage is at the drain of the mosfet?
Al.

[alexanna]

Re: H.P. L1710 back lamps turning off

When I measured the voltages on the return lines of the ccfls on this monitor there was a big difference in them. And when measuring current flow of the ccfls the readings I am getting are in U.A. or micro amps.#1 is reading 3.50UA #2 is 6.28UA #3 is 3.23UuA and #4 is 4.20 UA.
With the ammeter not connected this monitor will run about 2 hours until back light failure occurs, When I have the ammeter hooked up to the #1 ccfl its has the classic 2 second to black problem, when I test the other 3 ccfls with the ammeter connected the lamps stay on [no 2 sec problem].when I change the positions of the ccfls on connector from one to two the 2 sec problem occurs only when the ammeter is connected to ccfl connector #1
I am wondering if this monitor may have more than one problem, Like something wrong with the over or under voltage protection of the inverter and possibly a bad ccfl or maybe one of the poly caps in the inverter area?
Well if all this seams confusing don't worry I feel like I'm so deep in the forest on this one I can't see thru the trees! Dose this give anybody any ideas what might be wrong
Al.

[alexanna]

Re: H.P. L1710 back lamps turning off

I am sorry I mixed up the decimal point please move it one to the right.
Thanks Al.

[alexanna]

Re: H.P. L1710 back lamps turning off

I guess I am thinking out loud on this thread!
I decided to disassemble the LCD to inspect things thinking I might find a wire touching or something else weird, everything looks good almost like new as far as the ccfls go.
I have a laptop inverter tester and I am able to power up a ccfl tube and with the tester each ccfl is drawing right at 121.1ua.
I got thinking about the poly caps in the inverter area as may be causing a problem, I have no way to measure them but the markings on them are 12j 3kv and n3 3kv.
I searched n3 3kv and got some hits on these two capacitors being used together and some have been causing problems, No one could identify the cap n3 3kv.
I searched my spare parts and found two 22j 3kv, and tried them on the inverter in the area of ccfl connector #1 and#2 and it is causing a two sec to black problem immediately after powering on the monitor. I wonder if I might be on to something with the poly caps?
Does anyone know the value of the cap that's marked with the 3n? And could raising the capacitance of the two poly caps caused the back light failure to occur sooner?

[Wrog]

Re: H.P. L1710 back lamps turning off

I got thinking about the poly caps in the inverter area as may be causing a problem, I have no way to measure them but the markings on them are 12j 3kv and n3 3kv.
I searched n3 3kv and got some hits on these two capacitors being used together and some have been causing problems, No one could identify the cap n3 3kv.
I searched my spare parts and found two 22j 3kv, and tried them on the inverter in the area of ccfl connector #1 and#2 and it is causing a two sec to black problem immediately after powering on the monitor. I wonder if I might be on to something with the poly caps?
Does anyone know the value of the cap that's marked with the 3n? And could raising the capacitance of the two poly caps caused the back light failure to occur sooner?

If these are the circular blue ones, they are ceramic caps. I've never seen the N3 3KV one go bad, but I've seen a couple 12J 3KV burn out. If they go bad, you'll immediately know because it'll be physically damaged (see attachment). The 12J 3KV is a 12 picofarad 3 kilovolt (as noted in this thread here on badcaps) but not sure what the n3 3kv might be.

[alexanna]

Re: H.P. L1710 back lamps turning off

Yes they are the ceramic,And they do not look dammaged.
Al.

[alexanna]

Re: H.P. L1710 back lamps turning off

I am going to document this as go .
When I was checking amperage draw on the ccfl circuits I could get what I thought was a more meaningful reading measuring in U.A. for amps. And when I had my ammeter hooked to the #1 ccfl connector the 2 sec. to black symptom occurred almost immediately on power up of the LCD ,I could test 2-3-and 4 circuits with no problem.
Last night I tried replacing caps c14 the n3 3kv and c13 the 12j 3kv with two 22j 3kv caps and a two sec black problem occurred almost immediately. I was able to find another 12j 3kv cap and I installed it position c13 and I reinstalled the n3 3kv back at c14.
Right now I am able to install my ammeter on the #1 connector and the 2 sec to black problem has gone away.
Also when measuring current flow of each of the other ccfls the amperage reading has gone up a little and appears more stable .The reading I am get in now is between .07ma and .08ma.On all the ccfls.
I will let you know what happens after about 3 hours of running the monitor.
Al.

[alexanna]

Re: H.P. L1710 back lamps turning off

No good news yet!
Everything that I have done has caused a little bit of a change.
I have replaced the four ceramic caps in the inverter area, after their replacement the amperage reading
Of the ccfls has changed, Also the voltage reading on the return lines has changed ,I am seeing a high of .676mva To a low of .500mvac this is measured with min max and if it makes a difference the meter I am using is a rms meter .
Heat defiantly makes a difference causing the problem to occur.
I have removed the ccfl coils and changed their position on the board and the voltage/amperage readings stays with the connector not the ccfl.
Symptoms have changed somewhat ,When the monitor is in its glory, the back lamps are acting like they have been switched off, Like a kid that has just figured out what a light switch dose! A lot of the monitors that I have seen that flash like this one have been a capacitor issue, And they have been replaced Panasonic FM series.
I have been doing a lot of voltage readings and I am wondering if something is happing so fast the voltmeter is missing it, or my eye blinked just at the right time. I am unsure what to do at this point.
Al.

[PlainBill]

Re: H.P. L1710 back lamps turning off

What is the part number of U1 on the daughter board?

PlainBill

[alexanna]

Re: H.P. L1710 back lamps turning off

After my first post I have purchased a better magnifying glass, I am coming up with a different number for u1.It reads
OZ5936GN
23963.1N
0849A1
I am unable to find a data sheet.
Al.

[PlainBill]

Re: H.P. L1710 back lamps turning off

After my first post I have purchased a better magnifying glass, I am coming up with a different number for u1.It reads
OZ5936GN
23963.1N
0849A1
I am unable to find a data sheet.
Al.

Maybe OZ9936GN? That's a legitimate part number, but I can't find a datasheet either.

PlainBill

[PlainBill]

Re: H.P. L1710 back lamps turning off

Here's a hand drawn schematic of an inverter using the OZ9936GN. It's crude, but should be enough to go on.

PlainBill

[alexanna]

Re: H.P. L1710 back lamps turning off

Maybe OZ9936GN? That's a legitimate part number, but I can't find a datasheet either.

PlainBill

I realy think it is a oz5936gn,I will get my wife to help verify
she has better eyes than I do.Thanks for the effort but this damm thing has devloped some other issues.I will try to explain in my next post.
Al

[alexanna]

Re: H.P. L1710 back lamps turning off

This monitor appears to have several issues.
Initially checking the monitor out, the voltage on the 1.8 v regulator was going down to about 1.1 volts and at that time I would lose back lights .I salvaged a regulator from a spare logic board and the 1.8v stayed very steady at 1.8v all the time even when the back lamps would start the flashing .
A different problem has started now at it is very strange, the power LED has changed from green to amber as if no signal is present but the back lights are staying on, and the power button dose not respond to turn the monitor off.
The reading on the 3.3v regulator on the logic board has gone very low.
Vin is 5v. Vout is down to about 1.1v,I will try to come up with another regulator ,to get the 3.3v back and see what happens.
Al.

[alexanna]

Re: H.P. L1710 back lamps turning off

This is just a side note .I sure am glad I don't have to depend on repairing this monitor to put food on the table, Because I could probably make more money Standing on the street corner with a sign, Will work for food, And just hope people throw money at me!
It's frustrating but I am not willing to give up on this monitor just yet
Al.