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This is rather LENGTHY and detailed dhivi, advising you in advance.
All of the components for a laptop are crammed inside a tiny box with a poor cooling system.
The cooling system consists of a small fan and Heatsink. (Fan/Heatsink Assembly)
The Heatsink Assembly is shaped like a U for the Compaq Presario CQ40 series.
The U shaped Heatsink Assembly is composed of a finned Heatsink, Cooling Tube, and metal plates (2) attached to the Cooling Tube.
The finned Heatsink is on one end of the Cooling Tube, with the 2 plates of metal spaced around the Cooling Tube away from the Heatsink.
The 2 plates of metal sit on top of the Processor, and graphics chipset. One on the Processor, one on the graphics chipset. (GPU. Graphics Processing Unit)
The Cooling Tube is a hollow copper tube slightly flattened, and filled with Nitrogen gas.
Heat is absorbed by the plate on top of the Processor, and plate on top of the graphics chipset, and transferred to the Cooling Tube. The Cooling Tube absorbs the heat, and transfers it to the finned Heatsink.
The Heatsink then radiates the heat away. Air flow from the fan flows through the fins of the Heatsink, and helps carry the heat away.
Underneath the two metal plates is a thermal medium. For the processor, Compaq used Thermal Paste. For the graphics chipset they used a Thermal Pad.
Below is a link for disassembling a Compaq Presario CQ50, CQ60, or CQ70 series laptop. VERY similar to the CQ40 series,
(Your laptop is one of the Compaq Presario CQ40 series of Notebook PC's. The exact Product Number is on the bottom of the laptop in a white Service Tag. P/N = Product Number)
Scroll down the page, and view Steps 20 and 21. This shows you what your Fan/Heatsink Assembly looks like.
(Also a good guide for disassembling your laptop)
In Step 21 you see the Fan/Heatsink Assembly removed. Looking at the two chipsets on the motherboard,
1) The Rectangular chipset on the Left, is the Processor 2) The Square chipset on the Right, with the purple diamond in the middle, is the graphics chipset
The purple diamond is a Thermal Pad.
In my opinion a Thermal Pad is junk. Thermal compound (Paste) is interweaved in the fabric of the Thermal Pad. Thermal paste dries up after being repeatedly exposed to heat.
This is why the top of the Processor, and bottom of a Heatsink on a desktop computer, needs to be cleaned off, and new Thermal Paste applied every so often. The Thermal Paste dries up, and loses it's conducive properties. (Even if it's Artic Silver 5)
Primer: The top of a Processor, and bottom of a Heatsink are not perfectly smooth. A microscopic view would reveal, 'Pits, Hills, and Valleys'. This imperfections create air pockets when the Heatsink sits on top of the Processor.
Air is an Insulator, not a Conductor. Thermal paste fills these voids, (Imperfections), and is an excellent conductor of heat. It helps transfer the heat from the top of the Processor to the Heatsink.
Same thing for the two metal plates which sit on top of the Processor, and graphics chipset in a laptop. The top of the Processor, and the top of the graphics chipset are not perfectly smooth. Neither is the 2 metal plates which sit on them.
The Thermal Pad has dried up, and is allowing the graphics chipset to overheat. This is what causes the lines you see in the LCD screen.
The Thermal Pad should be removed, and the top of the graphics chipset Thoroughly cleaned. Also the top of the Processor. The bottom of the two plates of metal should also be Thoroughly cleaned. Then new, fresh Thermal Paste should be properly applied.
The Thermal Pad can dry up in a laptop in a years time.
To add: The graphics chipset is surface mounted to the motherboard. The Processor in a Compaq Presario CQ40 series Notebook PC is an Intel Core 2 Duo T7300.
The mounting of the graphics chipset is a surface mount called BGA. Ball Grid Array
Instead of pins on the bottom of the graphics chipset there are solder balls. The surface of the motherboard has matching Copper Pads. The graphics chipset is set on the motherboard's copper pads, and heat is applied. The solder balls melt, and solder the graphics chipset to the motherboard.
With constant overheating due to the thermal pad has dried up, or due to overheating in general, the solder joints start to melt, and the graphics chip loosens from the motherboard. This causes a poor connection, and the results may be what you are seeing with your laptop.
This is the download page for the Service Manual (Free) for the Compaq Presario CQ40 series of laptops. I chose the Compaq Presario CQ40-200 CTO Notebook PC, but the Service Manual is for all of the CQ40 series,
Click on the file name > Compaq Presario CQ40 Notebook PC - Service and Maintenance Guide After you click on the link it may take up to 30 seconds before you see the first page, and addional time for the PDF file to fully download.
If the laptop shuts down and won't boot up until it has cooled down, then the CPU is overheating and goes into thermal shutdown. This is caused by either a faulty fan or the fan and heatsink assembly is clogged with dust.
If this is not fixed then eventually the CPU will fail completely.
Check the fan to see if it spins freely if not then replace the fan.
If the fan and heatsink is clogged with dust then remove the dust from the fan and heatsink fins.
Have you tried connecting it to an external monitor? Are you sure the heatsink fan is not defective? If the screen works on an external monitor you know the video card is good. As long as the cable is plugged into the motherboard to provide power for the heatsink then it's either a bad heatsink fan or the motherboard connector for the heatsink is bad. My money is on the heatsink fan if you can get video from an external monitor.
I had the same problem on my v2000. apparently the fan shroud for the cpu cooling fan is made of plastic and can deform. In my case the deformation allowed the fans blades to strike the heatsink stalling the fan. This would lead to system overheat and shutdown. I disassembled the unit removed the fan shroud and installed .5mm O-rings onto the mounting screws moving the fan away from the heat sink and allowing normal operation. This repair is not for the faint of heart as these are tiny screws.
It appears your CPU is overheating and is shutting down.
Does the fan spin at all when the laptop is switched on?
The fan could be faulty, if it does not spin freely.
To test the fan, try removing it and connect it to a 12 volt DC power supply, if the fan spins fast then it is OK and the fault is on the laptop, possibly the power supply to the fan. In this case you need professional help.
If the fan appears OK then the problem could be insufficient too old heat sink compound between the CPU and the heatsink. You will need to remove the fan and heatsink and the old compound and put some fresh heatsink compound on the CPU/heatsink and reassemble the fan/heatsink.
Sounds more like the CPU is over heating. This is a fairly common fault on laptops after dust hair etc build up in the fins of the heatsink from the cpu.
After it blocks up, the fan can not push air through it to cool the cpu down and so the laptop goes into thermal shutdown to protect itself.