How to find defective component on laptop motherboard, HP vd7 laptop - Page 1
Forum section: electronics / beginners

author: chaxy
author_rank: Newbie
timestamp: December 06, 2012, 03:22:57 pm
content: I I have a Hp dv7-1170us laptop. My charger short-circuit and damaged motherboard. There is a small led next to DC jack. I used to be on (green) whenever charger is connected. Right now led is not lighting and laptop is dead. I got AC replacement but it doesn't work. Also laptop is dead if i put working battery. I also found this post :http://mayohardware.blogspot.com/2011/01/dv7-dead-no-lights-will-not-charge.htmlI followed instruction. I located SI4835 and checked with multimeter and it is good. My question is:How to find out what is wrong, how to located defective component. I check DC power jack and cables that run from DC jack to motherboard and it seems to be fine. What to do next? Is there any website or video where I can see how to do step by step check of each component. Please help.Thank you.

author: G7PSK
author_rank: Super Contributor
timestamp: December 06, 2012, 04:20:42 pm
content: Are you sure the power input socket is good, very often the center pin comes loose and burns itself out. So first check that you have power at the back of the socket, also check the replacement power supply I have had new one come in DOA. recently I had two one after the other first one dead a replacement sent out and that was also dead. Never trust nuffink until you have tested it yourself.

author: chaxy
author_rank: Newbie
timestamp: December 06, 2012, 04:53:48 pm
content: DV socket on laptop is good. Also DC socket is connected to motherboard via cables about 10" long and they are all good. Power is delivered to motherboard however led on DC jack is off. I found out that dv7 has unusual build where there is no 5V coming to board but only 19.5V and then it returns back to light led. So power is delivered to motherboard , but what next?

author: chaxy
author_rank: Newbie
timestamp: December 07, 2012, 01:07:56 pm
content: Any ideas?

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 07, 2012, 01:40:00 pm
content: There is no fixed rule for these kind of errors, specially without knowing what happened with your charger and without a proper circuit diagram.So what happened to the charger? You say short-circuit but what do you mean? If the output was shorted did the laptop do that? Or did it output a too high voltage? At the moment we can only guess. Even with all the information the answer is probably going to be that you will have to measure and reverse engineer the circuit to find out what the problem is, unfortunately service manuals or even circuit diagrams are hard to find for these things. Your only hope to get a better answer is to find someone who had the same problem and already did the work on this model.

author: Psi
author_rank: Super Contributor
timestamp: December 07, 2012, 01:58:01 pm
content: I agree, these kind of problems are extremely hard to figure out without a service manual including circuit diagrams.However, the first thing i would check is all the power diodes and transistors around the power section of the board.Shorted fets/bjt's can be detected in circuit, as can diodes which have gone open in both directions.Also keep a look out for anything that might be a SMT fuse or any PCB tracks that are a different color (melted pcb track)When a device is damaged by a faulty switchmode powerpack the most common cause is overvoltage.If you're lucky the mainboard had overvoltage protection which has tripped and blown a fuse or fried a diode.If the laptop was the cause of the powerpack failure then things are more complicated.

author: cwalex
author_rank: Frequent Contributor
timestamp: December 08, 2012, 08:43:13 am
content: Quote from: PA0PBZ on December 07, 2012, 01:40:00 pmYou can download service manuals for HP notebooks directly from HP which is nice but I don't think they will have circuit diagrams and I don't think it will help with repairing a motherboard. They are mostly useful for basic troubleshooting and disassembly guide.

author: amyk
author_rank: Super Contributor
timestamp: December 08, 2012, 10:33:19 am
content: If I remember correctly the dv7s were made by Compal. What's the OEM model on the motherboard? The Compal ones are usually LA-xxxx where the xxxx are digits. If you search for the OEM model number you might find a schematic. Once you read enough of those you'll understand the power distribution system of laptops in general and have an idea of what to look for.DC in and the battery B+ is gated via several MOSFETs to the main system power bus. This is at adapter voltage when running on AC, battery voltage when not. It is always live. The battery charger is connected across here. The main bus feeds a bunch of DC-DC converters that generate all the other main rails (5, 3.3, 2.5, 1.8, 1.5, 1.2, 1.05, 0.9, Vcore, etc. --- many laptops will have all of these voltages, each for a specific subsystem; the 1.x are usually things like DDR termination, GPU, ...) When the unit is off these are all turned off. The main bus also supplies a few low power linear regulators (usually only 3.3 and 5v), and these power the embedded/keyboard controller. The EC is what controls the other DC-DC converters and commands them to turn on when you push the power button.So in your case, check that all the standby rails are OK. If any of them are dead then nothing more will work.

author: Unknown
author_rank: Unknown
timestamp: December 08, 2012, 11:21:46 am
content: I'm writing this on a dv-7. One of their well known problems is that they fail to boot after power interruption. The solution is to remove power and battery and wait for some time. (I sometimes short circuit the two larger tabs on the battery connection.)Another peculiarity is that the power connection has 3 terminals not 2. The outer barrel is actually made from two pieces - one outer and one inner, center pin being the third connection. I had to fiddle with these when the original power brick died.

author: G7PSK
author_rank: Super Contributor
timestamp: December 08, 2012, 06:32:25 pm
content: Sometimes with laptops that have the 3 wire connection (the third is a sense wire) the laptop does not like what it sees when a new power supply is used, I have found that sometimes unplugging the bios battery as well as the supply and main battery is required to force the computer to recognize the new supply unit.

author: chaxy
author_rank: Newbie
timestamp: December 10, 2012, 03:30:51 pm
content: I will try to remove BIOS battery today and I will let you know.

author: Hoosier Newman
author_rank: Newbie
timestamp: July 22, 2014, 04:50:06 pm
content: Quote from: chaxy on December 06, 2012, 03:22:57 pmThis may help someone. There is additional informtation on following the voltages athttp://mayohardware.com/Animation/DV7.htmlclick on image to see the other side volta

author: paulhm81
author_rank: Regular Contributor
timestamp: July 23, 2014, 10:41:30 am
content: Without the small pin from the power supply the notebook should still power on but it will not charge the battery. Also if you start searching for a short on the board remember to remove the cpu because it keeps the mosfets in the on state.

author: amyk
author_rank: Super Contributor
timestamp: July 23, 2014, 12:09:37 pm
content: Quote from: paulhm81 on July 23, 2014, 10:41:30 amYou mean the protection diodes on it will conduct...?Good idea to remove it anyway - modern CPUs are made with submicron processes not designed to handle more than ~1V.