HP Presario C700 Laptop- Power socket flash on for 1 sec only- Dead- M/B question

[electronpusher]

Hi, i have the C700 which wont turn on and no lights, screen ect- totally dead- PS is good. Switch is good and 3v there shorting when pressed. When plugged in the blue light at the power socket flashes blue for a sec only . PS socket resistance is 8000 ohms Pulled it all apart and 19v is through the input mosfets and sense resistor. 3V isnt at bios chip next to super IO if it is the bios chip? no volts on any leg. pulled all parts off board to see if they are causing short or drag down Tried my PS and NO amps are being drawn and with power switch pushed on too. Fan dosent twitch. CPU ect are cold as no amps drawn Tried resistance to ground of many parts and cant find any shorts and PS wasnt drawing amps anyway. So slightly ON but not fully?. Super IO fault? Also screen disconnected in case that was faulty and 19V is at connector to screen
No lights on front and battery is not being charged.- Checked carefully board and noting burnt or out of place
Any one can help, i have schematic.- where i should look for trouble or is it one of the main chips likely and have to do a full reball ect.

thanks

 

[Admin]

Hi, i have the C700 which wont turn on and no lights, screen ect- totally dead- PS is good. Switch is good and 3v there shorting when pressed. When plugged in the blue light at the power socket flashes blue for a sec only . PS socket resistance is 8000 ohms Pulled it all apart and 19v is through the input mosfets and sense resistor. 3V isnt at bios chip next to super IO if it is the bios chip? no volts on any leg. pulled all parts off board to see if they are causing short or drag down Tried my PS and NO amps are being drawn and with power switch pushed on too. Fan dosent twitch. CPU ect are cold as no amps drawn Tried resistance to ground of many parts and cant find any shorts and PS wasnt drawing amps anyway. So slightly ON but not fully?. Super IO fault? Also screen disconnected in case that was faulty and 19V is at connector to screen
No lights on front and battery is not being charged.- Checked carefully board and noting burnt or out of place
Any one can help, i have schematic.- where i should look for trouble or is it one of the main chips likely and have to do a full reball ect.

thanks

you need to check the 3.3V at pin 8 of your Bios when your power on the motherbaord if you found it than try to use a New Bios Dump .

 

[electronpusher]

you need to check the 3.3V at pin 8 of your Bios when your power on the motherbaord if you found it than try to use a New Bios Dump .

Hi, i applied power and theres 19V on the 2 mosfets (8v on those gates) and at sense resistor 19V to main rail. So their gates are switched on somehow. no operating lights at all power button on or off. Just flash of socket light when plug in. Seen tons of repair videos and not seen this symptom.
Theres 2 8 pin chips next to the ENE super IO Chip? and nothing on pin 8 or any of the 16 pins of both. So 3.3v is supposed to be there all the time isnt it ready for bios on?
Also i dont have a bios flash gizmo after removing bios chip. I a learning all this,so what is
a Bios dump? i have am only flashed them through the bios and windows.
If the bios chip was corrupted or dead, wouldnt the power lights at least come on or some life even if no bios post? I cant see any bios page on the schematic
Also with power connected, i cant find any voltage in any of the big coils or caps inc around the CPU and thats cold anyway. I followed the 19V rail and it seems to go to CPU and resistor and mosfet and 19V on this side and nothing on other and nothing on gate of 2 mosfets there and both gate zero and they are fed from a large density chip per the schematic and no output for those gates, so that chip not on? Thats as far as i got and the 19V stops at that point. How do i upload the schematic as cant see usual file option? Page 33 has input mosfets top left. Page 37 has the density chip PU12 feeding gates for pq27 & 30, those 2 mosfets t said at the end of the 19V rail.
Except for that 19V a the sense resistor, theres no lights or volts anywhere on the board and i can find any shorts to ground from the big items as well, although there could be and turning off power , so why is power going through first 2 mosfets with gates on but no powerlights in front? and a single flash of socket blue light at socket when plug in?

thanks

 

[Admin]

Check U18
pin 9 22 33 96 111 do you get any voltage on those pin .

 

[electronpusher]

Check U18
pin 9 22 33 96 111 do you get any voltage on those pin .

NO, nothing on those pics on U18 which is the ENE chip super IO? Also nothing on the tiny caps ect around that chip. So no voltage feeding in?
Question- checking the obvious first- The power connector has that little Blue light/led and yellow and black wire feed it to the socket. That blue light only flashes when plugged in.
Could it be faulty and cause a wrong feedback to board through that M/B pin? Obviously the power red and black are good as 19V through first mosfets , but that light? unless the board isnt watching for feedback or the light flash indicates a real problem on the M/B
Also, the M/B has some 20uf caps all over and a few around the CPU measured only 15 ohms to earth on the pos side. Could this be correct anyway as when off, the CPU is supposed to have low resistance in case of static shock? Theres no hard shorts on the board just so low resistances to ground in places, so i suppose thats normal?
I fixed electronics before moving to laptops boards so always used to seeing high ohms to ground everywhere. so low ohms is normal and a learning experience on laptops?

 

[electronpusher]

NO, nothing on those pics on U18 which is the ENE chip super IO? Also nothing on the tiny caps ect around that chip. So no voltage feeding in?
Question- checking the obvious first- The power connector has that little Blue light/led and yellow and black wire feed it to the socket. That blue light only flashes when plugged in.
Could it be faulty and cause a wrong feedback to board through that M/B pin? Obviously the power red and black are good as 19V through first mosfets , but that light? unless the board isnt watching for feedback or the light flash indicates a real problem on the M/B
Also, the M/B has some 20uf caps all over and a few around the CPU measured only 15 ohms to earth on the pos side. Could this be correct anyway as when off, the CPU is supposed to have low resistance in case of static shock? Theres no hard shorts on the board just so low resistances to ground in places, so i suppose thats normal?
I fixed electronics before moving to laptops boards so always used to seeing high ohms to ground everywhere. so low ohms is normal and a learning experience on laptops?

I just unplugged power cable and fed 3V to that light through yellow and black and blue light is perfect, SO, that flash is from the M/B it seems. Real problem somewhere on board. always worth checking the obvious easy stuff than chasing tail.
Power on for a sec, then OFF? except for those 2 mosfets gates on 8V and screen connector 19V is present, rest of m/b seems dead and ENE chip U18 those pins are zero

 

[Admin]

Check PL6 and PL5 if there is any short circuit and how much voltage you get on those coils when you plug-in your ac-adapter.

 

[electronpusher]

Check PL6 and PL5 if there is any short circuit and how much voltage you get on those coils when you plug-in your ac-adapter.

Ok, we are making progress! its all out on chip PU5
PL6- is only 37 ohm to earth - No volts when powered and PL5 too- Zip
PL5- is 22K
Per the schematic- plugged in now
Pin 28 on PU5 should be 3.3 as should be 24 & 26
Pin 28 is 4.68V
Pin 26 is Zip- this feeds gate 4 to PQ16 where pins 5,6,7,8 have 19.32V waiting-ok
Pin 24 is 5V- this feeds gate 4 to PQ36 where gate has 5V- nothing on all other pins.
Pin 27 should also have 3.3V- Zip
Pin 15- 5V- Zip
Pin 19 -5V- ok
Pin 14- 5V- ok
Pin 16- 5V- Zip
Pin 20- 19V- ok

 

[electronpusher]

Ok, we are making progress! its all out on chip PU5
PL6- is only 37 ohm to earth - No volts when powered and PL5 too- Zip
PL5- is 22K
Per the schematic- plugged in now
Pin 28 on PU5 should be 3.3 as should be 24 & 26
Pin 28 is 4.68V
Pin 26 is Zip- this feeds gate 4 to PQ16 where pins 5,6,7,8 have 19.32V waiting-ok
Pin 24 is 5V- this feeds gate 4 to PQ36 where gate has 5V- nothing on all other pins.
Pin 27 should also have 3.3V- Zip
Pin 15- 5V- Zip
Pin 19 -5V- ok
Pin 14- 5V- ok
Pin 16- 5V- Zip
Pin 20- 19V- ok

Also the pins for the SIO U18 which had no voltage on them. I put 3.3 v on the resistor R382 and the power and charge light came on and the power socket blue light too!
So the Sio isnt getting power it seems so should be ok.
Are the 3 and 5 volt generators working on this board as the power switch has 3 volts and shorts when pressed but the bios chip has no 3 V on any pin unless thats not and bios is part of U18?

 

[electronpusher]

i'm thinking chip PU5 which is MAX8743A- power supply controller for 3.3 and 5v output
could be bad? replace that or is it something else supplying this chip with wrong signals causing the wrong output voltages i listed?

This M/b is certainly a good leaning experience!

 

[Admin]

You have found PL6- is only 37 ohm to the ground that mean you have a short circuit on the line of 3.3 v try to found from where it came .

 

[electronpusher]

You have found PL6- is only 37 ohm to the ground that mean you have a short circuit on the line of 3.3 v try to found from where it came .

Yes, that reading bothered me too but i here to learn so wasnt sure. Tricky as i cant send current down that resistance easily to heat up a part without too much voltage so will cut the coil to isolate the track then theres only a couple of tiny parts that could be shorted each side. it could also be the chip too as all the paths go back into it.Anyway i need to try all this and get it going again. Leaking more than shorted 37ohms? Anyway, all good leaning .

Thanks

 

[electronpusher]

Yes, that reading bothered me too but i here to learn so wasnt sure. Tricky as i cant send current down that resistance easily to heat up a part without too much voltage so will cut the coil to isolate the track then theres only a couple of tiny parts that could be shorted each side. it could also be the chip too as all the paths go back into it.Anyway i need to try all this and get it going again. Leaking more than shorted 37ohms? Anyway, all good leaning .

Thanks

Another question. How do i trace a SOFT Short. Voltage injection wont work as the circuit can only handle 3V? and that produces almost no amps against 37 ohms and i need 2-3 amps to heat a part up. Any ideas or technique would be appreciated. I have only dealt with hard shorts and they easy to find. Looking at the schematic, it joins to a 3VALWP? rail on the right so could it be anywhere away from this section to ? This could be a challenge!!