23" HP FN747AA no Backlight (board# 715G2824-1-7)

[houndazs]

Ok so i have a 23" LCD TFT HP monitor, with a backlight problem. The backlight comes on for about a second, then goes off. It does this a second time about 45sec. to a minute later and then the screen goes into stby mode. The LCD panel itself works, as i can see images when i hold a flashlight up to the LCD. I have changed all the caps in the power circuit, even though none of them showed signs of being bad. I have tested power at the, large filter cap, and at the bridge rectifier, both test good. Im new to the LCD repair and can not seem to find the Power IC on this board to test, if someone could point it out that would be great! Also, i tested all 4 backlight ports with a know good backlight i use for testing. I have included a picture of the board and the model number sticker of the monitor. any sugestions on where to go from here would be very helpfull. thanks in advance for your help!

[retiredcaps]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

The backlight comes on for about a second, then goes off.

This sounds like "2 seconds to black" from your description.

Possible causes are

1) bad caps
2) bad ccfl and/or wiring
3) bad inverter transformer
4) bad inverter sense circuitry

Since you already replaced the caps and verified the ccfls are okay, that leaves 3 and 4.

For 3, measure the resistance of the secondary inverter transformer pins for T601 and T602. Report the ohms readings. They should be within 3% of each other.

[houndazs]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

This sounds like "2 seconds to black" from your description.

Possible causes are

1) bad caps
2) bad ccfl and/or wiring
3) bad inverter transformer
4) bad inverter sense circuitry

Since you already replaced the caps and verified the ccfls are okay, that leaves 3 and 4.

For 3, measure the resistance of the secondary inverter transformer pins for T601 and T602. Report the ohms readings. They should be within 3% of each other.

Thank you so much for the reply ill do that tonight and get back to you with the results!

[houndazs]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

Thank you so much for the reply ill do that tonight and get back to you with the results!

OK so i tested the 2 pins on the primary side and both transformers tested at .46 on the 20k ohm setting. is this good? i will assume so since they both tested the same. what next?

[retiredcaps]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

OK so i tested the 2 pins on the primary side and both transformers tested at .46 on the 20k ohm setting.

We are interested in the secondary side. Put your multimeter on 2000 (2k) ohms, if manual.

edit: The secondary side are the ones "going into" the blue caps.

[retiredcaps]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

Also, i tested all 4 backlight ports with a know good backlight i use for testing.

Hmm, I just re-read this part and may have mis-intrepreted the test that you did.

If you tested just the ccfl ports, that means you didn't test the HP ccfls. Is that correct?

"2 seconds to black" could be a ccfl dying or poor ccfl wiring. See the following pictures for examples.

http://s807.photobucket.com/albums/y...klight%20lamp/

[houndazs]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

Hmm, I just re-read this part and may have mis-intrepreted the test that you did.

If you tested just the ccfl ports, that means you didn't test the HP ccfls. Is that correct?

"2 seconds to black" could be a ccfl dying or poor ccfl wiring. See the following pictures for examples.

http://s807.photobucket.com/albums/y...klight%20lamp/

No i tested the CCFL with a know good lamp and had the same results....~2sec to black.

[houndazs]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

We are interested in the secondary side. Put your multimeter on 2000 (2k) ohms, if manual.

edit: The secondary side are the ones "going into" the blue caps.

Im sorry i did test the secondary windings as you described above. there are six pins on the side closest to the 2 caps, and 2 pins closest to the 2 blue caps.

[houndazs]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

I think i am going to need an "O" scope and a flyback tester to troubleshoot the rest. i figure if i am going to be repairing a bunch of these might as well. was thinking of getting one of those $80 handheld "O" scopes. i have read that they are fine for LCD monitor repair, but i do run into the occasional LCD TV. will probably go with the "blue Box" for a flyback tester. so what is the next step for this LCD? i would love to use it as my new gaming monitor as it was given to me for free......

[houndazs]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

I think i am going to need an "O" scope and a flyback tester to troubleshoot the rest. i figure if i am going to be repairing a bunch of these might as well. was thinking of getting one of those $80 handheld "O" scopes. i have read that they are fine for LCD monitor repair, but i do run into the occasional LCD TV. will probably go with the "blue Box" for a flyback tester. so what is the next step for this LCD? i would love to use it as my new gaming monitor as it was given to me for free......

OK so i believe i found the power IC its the only IC on the board and i am reading ~85vDC from the VCC pin on the Power IC(pin at dot) to hot ground of large filter cap. i have read that this should be between 16-20vDC. have i found something or am i measuring wrong?

Posted a pic am i missing components in the red square? BTW this was one of those "white van" monitors......it came from a old co-worker of mine.....

[jsog]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

What IC are you looking at? There might be surface-mount chips on the bottom of the board.

That red-outlined section appears to be for an optional audio amp circuitry. CN601 would be the headphone jack.

[houndazs]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

What IC are you looking at? There might be surface-mount chips on the bottom of the board.

That red-outlined section appears to be for an optional audio amp circuitry. CN601 would be the headphone jack.

i did not even think of looking on the bottom of the board.....here are two pictures of the bottom of the board. i believe the IC on the left is my power IC as it is under the large filter cap.....Is this correct?

[Rtech]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

Thats would seem to be it, but as you have LED, and can see images, then it it should be fine, try to find the datasheet if you really want to look closer at it. Have you meaured to output of the Schottky diode, providing the inverter voltage ?

[houndazs]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

Thats would seem to be it, but as you have LED, and can see images, then it it should be fine, try to find the datasheet if you really want to look closer at it. Have you meaured to output of the Schottky diode, providing the inverter voltage ?

no i have not....not sure which one it is, but have a good idea its the one at the top of the board on the right between the two caps correct? it has a pink component(resistor) above. am i correct?

[Rtech]

Re: 23" HP FN747AA no Backlight (board# 715G2824-1-7)

It will be one of them at the top fixed to a heatsink.The right hand one is possibly the 5volt one, and the one nearest to the inverter transformers at the top, probably the 12volt one.