A friend with a HP Compaq CQ61 laptop had an issue with the laptop not turning on. They had given me a clue by saying that the charger did not seem to be working. However they had tried another charger from one of their friends that was supposedly the same model and it still didn't work.
So I plug the laptop charger in to the mains and then plug the charger into the laptop. Sure enough there was no life in the laptop. The battery had also run down so no blinking flashing leds, screens or anything.
Crack out the trusty meter and check to see if there was any volts coming out of the charger and going into the laptop. Nope there wasn't.
Do we have mains going into the charger? Change the meter to AC and measure the voltage. None again. Perhaps the fuse has gone. Unplug from the mains to check and..... well see for yourself
Total lack of the live pin on the plug. Well this will be easy, just change the plug.
Plug changed, does it work? No. Damn!
Perhaps the lead is broken between the charger and laptop connector. Crack open the charger using this Instructable (if you try it don't go blaming me if you electrocute yourself or burn your house down etc). Case cracked open and low voltage lead checked, no broken wires. Damn and blast!
Right lets check this other laptop charger. Hmm not an original HP. Well plug it in and the mains light on the laptop comes on for 2 secs and then goes off. Oh dear.
Doing a bit of Googling on the subject of these laptops throws up problems with overheating video chips, reflowing solder joints, bad southbridge chips, turn the laptop upside down whilst powering it up (yes really). This isn't looking good.
Lets clutch at a few straws. I wonder if the CMOS battery has gone bad? Take the CMOS battery out and check with meter, 3v. Hmmmm
Put it all back together. Plug in charger. Whats that flashing light? It's the battery charging.
After leaving the battery to charge for a while, so it was not critically low, the laptop turned on. Now we are getting somewhere.
All seems to be working except the power icon does not show that the laptop is plugged in to the mains and charging, it just shows the battery level. I think this could be due to the charger not being an original HP charger. There is an extra lead in the lead from the charger to the laptop which is an ID lead. It seems that if there is a problem with the ID lead the laptop does not recognize that there is a charger plugged in. This can cause the laptop to think it is running on batteries when it is on mains. This could cause the laptop to slow down trying to save power, depending on the power options that are set.
Oh well at least it turns on now. I will probably recommend a new charger, an original HP one!