Beginner questions about unstable voltage in laptop

[cook]

Re: Beginner questions about unstable voltage in laptop

- The battery was not connected during the temperature tests.
- I will check the continuity and post the results.
- I don't have a bench power supply.

[cook]

Re: Beginner questions about unstable voltage in laptop

Spider1211, sent you a PM. -- Thanks

[cook]

Re: Beginner questions about unstable voltage in laptop

The minus side of PCE6 has continuity to ground. I also checked its capacitance while in circuit, and it is.02 nf Testing across PCE6 gives about 101 ohms.

PCE6 ground has continuity to PQ19 Source (pin 1,2,3)
PQ19 D (pin 5) has continuity to PQ18 S (pin 1,2,3)

PQ19
With + on D and - on S, no continuity. It beeped slightly for a fraction of a second, but no continuous beep. 100 ohms
With + on S and - on D, no continuity. It beeped slightly for a fraction of a second, but no continuous beep. 140 ohms

Testing with + probe on D and - probe at a ground point, no continuity.
With + on S and - at a ground point, there is continuity.

PQ18
With + on D and - on S, no continuity. It beeped slightly for a fraction of a second, but no continuous beep. 148 ohms
With + on S and - on D, no continuity. It beeped slightly for a fraction of a second, but no continuous beep. 105 ohms

Testing with + on D and - at a ground point, there is continuity.
With + on S and - at a ground point, no continuity.

I have no voltage or just a millivolt at PJ5, PJ6, or PU10 on pins 1 or 5.

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

Testing with + on D and - at a ground point, there is continuity. (For PQ18)

How many ohm do you have? Having continuity could mean there is a short on either the +Vcore_IN or +VBAT. Unsolder PJ14 (make sure you clean the pads properly and there is no solder bridging the 2 contact points) and test for continuity again (black on ground), but this time check on both side of the PJ14 contacts.

What we are trying to establish here is to check if the short circuit is on +Vcore_IN side or +VBAT side.

PQ19
With + on D and - on S, no continuity. It beeped slightly for a fraction of a second, but no continuous beep. 100 ohms
With + on S and - on D, no continuity. It beeped slightly for a fraction of a second, but no continuous beep. 140 ohms

PQ18
With + on D and - on S, no continuity. It beeped slightly for a fraction of a second, but no continuous beep. 148 ohms
With + on S and - on D, no continuity. It beeped slightly for a fraction of a second, but no continuous beep. 105 ohms

The low resistance could be due to faulty/leaky mosfets. You can confirm by measuring them off circuit.

DO NOT apply power to the board if you remove any mosfet or their controller IC.

[cook]

Re: Beginner questions about unstable voltage in laptop

On PQ18, testing with + on D and - at a ground point, I have 8 ohms.

I will work on the other items and post back when I have some results.

[cook]

Re: Beginner questions about unstable voltage in laptop

PJ14, measuring from pin 1 (+vcore_in) to ground is 6.7 ohms. Pin 2 (+vbat) is 33 Mohms to ground.

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

PJ14, measuring from pin 1 (+vcore_in) to ground is 6.7 ohms. Pin 2 (+vbat) is 33 Mohms to ground.

That looks promising since it indicates the short circuit is on +Vcore_IN.

With a power supply this would be very easy to detect. But since you do not have one, visually inspect the following: PC86, PC87, PC95, PC377, PC378, PC379.

Those are capacitors, look for the one that seems discolored and/or cracked. Otherwise you need to start removing one by one until the short is gone. Although the bad cap can be left out, it is good practice to replace it.

Once you find the shorted cap, remeasure across the mosfet to check if they are still in the 100 - 150ohm range you were getting previously. If yes, replace them also.

[cook]

Re: Beginner questions about unstable voltage in laptop

Oh man, that is good news. I was wondering.... To diagnose those caps further, could I plug in the netbook power charger to the system board, and put some isopropyl on the capacitors to see if any dry out faster that the others? Would you recommend that?

[emestee]

Re: Beginner questions about unstable voltage in laptop

You could also use the thermal printer paper such as the one used in faxes, cash registers and credit card terminals. You could also just touch the caps one by one or apply an inverted compressed air bottle. I don't like using isopropyl for these tests because if, when you power it up, there's a spark inside the shorted cap, let's just say you better have insurance.

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

To diagnose those caps further, could I plug in the netbook power charger to the system board, and put some isopropyl on the capacitors to see if any dry out faster that the others? Would you recommend that?

I would avoid doing that since you do not know if you have leaky mosfets. You could try apply a low voltage on that line using a 18650 battery (3.7V nominal voltage) but make sure its not for extended period of time. Assuming you have around 6.8ohm short, that would result in about 2W power dissipated which should be visible with alcohol.

Do keep in mind that bare 18650 cell are unprotected, so be very careful using them as well as making sure the wire does not accidentally touch other circuits/components.

Since there isn't many caps on the shorted line, you could opt for the safer route. Test you luck, you might get the shorted one at first try

[cook]

Re: Beginner questions about unstable voltage in laptop

Fax paper or cash register receipt! -- Yes, that is a great ideal. And compressed air or freeze spray - both would be much safer!

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

I don't like using isopropyl for these tests because if, when you power it up, there's a spark inside the shorted cap, let's just say you better have insurance.

IPA is harder to ignite as one may think. Additionally, the amount used is minimal.

I had similar concerns in the past so I decided to test it. I recreated test scenarios and no amount of sparking ignited the IPA. Maybe other could share their experience.

[piernov]

Re: Beginner questions about unstable voltage in laptop

Haven't followed the discussion, but 6.8 ohms really sounds like CPU VCore resistance to ground. If you measure that on main power rail it means you have shorted high side MOSFET. If you inject more that 1.3V or so, you kill the CPU if it wasn't dead already.

And using a lithium battery for that sounds like a pretty bad advice to me.

[emestee]

Re: Beginner questions about unstable voltage in laptop

IPA is harder to ignite as one may think. Additionally, the amount used is minimal.

I had similar concerns in the past so I decided to test it. I recreated test scenarios and no amount of sparking ignited the IPA. Maybe other could share their experience.

I'll admit that my prejudice is due to some people (*cough* Soren *cough*) using zippo lighter fluid for this. Guess what, Zippo lighter fluid cans say "easily ignited" on them because it's a selling point.

I would be delighted if, ahem, someone else conducted these tests and published a video.

[cook]

Re: Beginner questions about unstable voltage in laptop

Spider1211,
Before I had read your post today, I plugged in the power supply and the power light came on. It has not been doing that, so removing the connection within PJ14 must have been the change that made it come on. (I have not reconnected PJ14 yet.) I have been looking at the boardview, and the schematics, and I can't figure out what exactly we did by disconnecting the circuit at PJ14. Could you explain it?

[cook]

Re: Beginner questions about unstable voltage in laptop

I looked closely at the capacitors and one of them (PC87) has a small chunk of missing tan material on the side. Is is that a classic symptom of a SMD capacitor that might be bad?

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

I looked closely at the capacitors and one of them (PC87) has a small chunk of missing tan material on the side. Is is that a classic symptom of a SMD capacitor that might be bad?

The fact that its broken is already reason enough to remove it. Most of the time these caps short internally. Although not as common as internal short, they can also fracture/crack.

Remove it and recheck continuity across its solder pads.

Post a picture if possible.

[emestee]

Re: Beginner questions about unstable voltage in laptop

Yeah, by cutting off a section of the board you also disconnected the shorting part. So that's definitely progress.

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

Spider1211,
I have been looking at the boardview, and the schematics, and I can't figure out what exactly we did by disconnecting the circuit at PJ14. Could you explain it?

Read post #24.

When you measure continuity on PQ18 and ground, there was very low resistance. This means that something on that line is shorting to ground. When you checked continuity across the mosfet, the resistance were higher (meaning it could be a leaky mosfet).

You said that PQ19 has no continuity between drain and ground. So we can conclude that PQ18 is no shorted. (If PQ18 was shorted, you would get continuity between both Drain and Source with respect to ground)

This means we can narrow our search area to where the Drain of PQ18 is connect. However, this is connected to the main 19V supply which connects to other part of the board. (Which means the fault could be anywhere on that rail).

Lucky for us, PJ14 is there. An analogy could be: Think of PJ14 as a circuit breaker in your electrical panel. You have a main feeder which branches off to different areas of your house.

Now imagine you had only 1 main circuit breaker. After the breaker, you electrical cable goes to different area of your house (say you have 3 rooms). If there is a fault in room 1, you main breaker will trip and will keep tripping until you remove the fault. However, since there is only 1 main breaker, you have to assume the fault can be in any of the 3 rooms, which make it difficult and time consuming to narrow down.

On the other hand, imagine you have the same setup but this time after the main circuit breaker each room gets their own individual circuit breakers. If there is a fault (Lets assume for sake of argument that the main circuit breaker has a lower rating than the individual room breakers), the main circuit breaker will trip. This time however, you can isolate the problematic area. You start by assuming the fault is it room 1. All you need to do is to trip/disconnect circuit breaker for room 1.

When you reset the main breaker, power is restored to the room 2 & 3 but because room 1 is now isolated, the main breaker no longer sees a fault and therefore does not trip.

Your job now is just to find the root cause of the fault in room 1. Once repaired, all you need to do is reconnect its circuit breaker.


In laptop case, the 19V (+VBat) is the "Main feeder" and PJ14 is acting as a bridge (analogous to the room circuit breaker). When you disconnected PJ14, you isolated the "room" (+Vcore_IN, where PQ18, etc is located) from the "main"(+VBat) and managed to locate on which side the fault was (see post #26).

Hope that somewhat helps

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

I'll admit that my prejudice is due to some people (*cough* Soren *cough*)

LOL! I remember waiting for the fluid to light up when I first saw his video