# Compaq C700 screen problem



## pnguyen (Sep 25, 2008)

My Compaq laptop is not working properly. The problem is that I cannot see anything on the screen. It appears black but until a well-lit area I can see my login underneath the dark screen but I can't see it clearly. When I turn on my laptop, I can hear everything start up properly, it's just I can't the screen. The night before when I used it, it was fine but today when I turned it on, I can't see anything on the screen. What is wrong? And how do I fix it? Please help.


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## TyDeacon (Aug 11, 2007)

The back light appears to have failed. You will have to replace the LCD panel to fix this issue.


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## TriggerFinger (Jan 30, 2008)

If you can see your Desktop beyond the dark screen then it could be the backlight or inverter.


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## danb28 (Mar 26, 2011)

Hi,

I have the same problem, however I know its not the black light as i have tested the screen on another laptop.

The laptop wouldn't boot at all when i got it.
I know about the Nvida chip over heating issues and usuly perform the reflow on them, however as this is an intel chipset i wasn't sure if it would work, Now i have the dull screen. My initial though was that it was the backlight but after further test this is not the case.

My next thought is the GPU is broke. Any other ideas?

Thanks
Danny


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## Techie19 (Feb 10, 2008)

TriggerFinger said:


> If you can see your Desktop beyond the dark screen then it could be the backlight or inverter.


I agree. From my recent experience with a Dell laptop with same problem (very dim screen), it turned out to be a bad connection between the motherboard and video card. Took it apart and reseated the video card and it was good to go.

First thing you should do is connect your laptop to an external monitor to confim that you're graphics are ok (video card, integrated graphics chip).

Then I would start of by checking your connections between the motherboard and the screen (or video card if applicable). And video card and motherboard. Even if eveverything looks to be ok, I suggest you disconnect and reconnect to make sure that everything is plugged in properly.

Next I would check your inverter is ok. The inverter is what provides power to the backlight. So if its not putting out the power needed, you can get the symptoms you are experiencing. The inverter is the cheapest fix, compared to replacing the backlight or LCD screen. 

If the new inverter doesn't fix the problem then the backlight can infact be the problem. Backlights are a pain to replace. You gotta know what you're doing because they are very fragile. But really just a pain because they are kinda "built in" with the LCD. Usually if the backlight needs replacing, its cheaper and better off to just getting a new/used LCD.

Hope this info helps. Good luck. Post your findings.


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## danb28 (Mar 26, 2011)

Hi,

Thanks for your post, we are on the same wave length.

I have a dell xps m1530 and the nvida GPU needs a reflow once in a while as the chip comes lose.

As this is an Intel and i have never had a GPU issue with an Intel card i wasn't sure if it would work the same. However i tried it and it came on to dull screen (as if no back light)

Thankfully i have 2 laptops that are more or less the same and i have tested the LCD and the transformer on the good laptop and they both work fine.

so it comes back to the GPU. I have not had any experience removing and re-seating a GPU. Is it not soldered on? - I will have a look at youtube....

Oh and yes the external TFT worked fine.

Many thanks
Danny


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## Mojojojo3698 (May 20, 2011)

I have a Compaq C700 and had a screen problem as well. It would intermittently go very dim (you could barely see it only when a bright light was pointed at it) and then eventually it would always be dim. I thought it was a back light problem and was ready to replace the entire screen. But I saw a suggestion to replace the LCD power inverter. This is a long narrow circuit board located in the narrow enclosed section at the bottom of the screen, in between the hinges. I found this part new online for $15 so I gave it a try. It resolved the problem. I have no relation to this site nor am I trying to sell LCD power inverters. But this is what the part looks like. See link. 

Compaq Presario C700 LCD Power Inverter (407800-001) - MachinaElectronics.com


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## dorriGa (Jun 20, 2011)

I have a compaq presario c700, first dont work, and the problem was a mosfet burned. MOSFET P#: FDS6675BZ.

In schema are PQ1 and PQ2, for my lap was just PQ2. And for faileture in charge batteri is the another mosfet PQ3.
HP compaq Presario C700 LAPTOP NO Power - Page 3 - HP Support Forum
Spanish with pics Presario c700 muerto!! - Foro de usuarios de HP

















You can temporarily welded pin 3 to pin 6, if you can´t buy a new mosfet (2 USD$). No easy available in my country .

The another problem with the screen.
When I was testing I had to buy a new inverter and backlight lamp (both was broked) and the screen still black..... but the problem is in the GPU. 
GPU dont give a signal for power up the inverter.









The pins in the inverter are 7, 2 for grown (1 and 2 pin), signal for open or close laptop (pin 3, 0v off and 1v on), ON/OFF inverter (pin 4, 0v=off, 5v=ON), brightness ( pin 5, 2v to 3.5v) and power 18.5v (pin 6 and 7).
The GPU from my laptop in pin 4 get 0.3v when is power on, and 0v in power off and this fail to turn ON the light in screen, the inverter need over 5v (I tested 3v and work).

The solution is get 3v no always power , 3v than is active when the power is on and not all time when is conected the motherboart with battery or external ac power. 
There is a IC from you can get 3v (there is too another for 5v see the schema) no always power, and solder to pin 4 on inverter. This work for me.
The ic is in the schematic *U20* solder to pin 1,2 and 3.
I cut the cable (just the cable to inverter pin4) between inverter and motherboard, near to mobo for no charge 3v to GPU justo to inverter.

If do you want the shcema this url can help; hp compaq presario C700 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

The ideal solution for all, is buy a mosfet for fix internal power faileture, and for the black screen resoldering GPU with a professional equipement.

Sorry for my engrish... if this is useful or do you want more info, PM me.


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## dorriGa (Jun 20, 2011)

You can temporarily welded pin 3 to pin 6 in PQ2. sorry


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## Cornie85 (Sep 6, 2012)

Hi guys, I have the same problem. I've changed the inverter and thought I'd solved it, but on further inspection:

It works perfectly fine running off the battery, but once it's plugged into the wall, it works for 2-3 minutes and then does the old 'dull screen' again.

At the minute, my only option is to charge it whilst shut down and run it off the battery - can't run it off socket at all, the dull screen returns every time. Strange that it charges fine when shut down though?

Probably something wrong with my adaptor? loose connection? something surging in there somewhere? I have no idea...


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