Beginner questions about unstable voltage in laptop

[cook]

Re: Beginner questions about unstable voltage in laptop

Ditto!!!!!

[cook]

Re: Beginner questions about unstable voltage in laptop

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

Yes, that explains it. Very nice explanation.

[cook]

Re: Beginner questions about unstable voltage in laptop

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.

My phone does not do closeups very well but I will see if maybe we have a camera that will work.

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

My phone does not do closeups very well but I will see if maybe we have a camera that will work.

That's fine. Just remove the cap and check for short. DO NOT reconnect PJ14 until you are sure the mosfets (PQ18 & PQ19) are not leaky. (check if they still read 100 - 150ohm as was the case previously). If they are, replace them.

[cook]

Re: Beginner questions about unstable voltage in laptop

I am having issues with my soldering iron, so it will might be a few days before I can get that cap removed.

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

I am having issues with my soldering iron, so it will might be a few days before I can get that cap removed.

Use your hot air station. Just mask off any plastic material nearby that could get blasted with hot air as well as covering the CPU (if its nearby and isn't already covered by the heatsink).

Just practice on a scrap board if you are not feeling too confident yet then move to the actual motherboard.

[cook]

Re: Beginner questions about unstable voltage in laptop

Yes, I want to practice on a scrap board first, one that has lead-free solder on it. My practice thus far was using a leaded board, and that went well, but when I tried on lead-free, I could tell it was not working, and I think my hot air was not calibrated right. It was not hot enough. So I'm trying to 'calibrate' it just enough so that I can melt lead-free solder on a practice board. Tomorrow I will be practicing that, so hopefully I will have that cap off tomorrow.

[cook]

Re: Beginner questions about unstable voltage in laptop

I removed PC87!! Finally. It was shorted.
New values for PQ18 and 19 are below.
PQ18
In circuit testing of continuity showed no continuity at all.
In circuit testing of resistance...
+D -S = nothing
-D +S = 11Mohms
In circuit testing of continuity on PQ19
+D -S = 85 ohms
-D +S = 112 ohms

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

PQ18 seems to test OK.

The low resistance continuity test on PQ19 could be due to the CPU (although I would expect lower resistance on that line due to CPU). Are you comfortable with removing PQ19 and testing out of circuit? If not, try removing PL7 and redo continuity test (DO NOT apply power to the board when any of the components are off the board)

If PQ19 still shows low resistance out of circuit, replace it.

Once you confirmed everything tests OK and is soldered back (pay special attention to position/orientation of the components when resoldering, especially the MOSFET) on the board, do another continuity check across them as well as with black probe on ground (as you did during initial troubleshooting). If everything checks out, reconnect PJ14 and turn on the laptop.

[cook]

Re: Beginner questions about unstable voltage in laptop

I just now picked up 2 boards that I can use to practice my hot air soldering skills with. So hopefully between now and sometime tomorrow, I will remove PQ19 and test. PL7 is right next to the plastic connector for the battery, so I am thinking I will not remove that until my skills are better. I think PQ19 will be safer to remove.

[cook]

Re: Beginner questions about unstable voltage in laptop

PQ19 removed from the board!! Testing of PQ19...
Continuity mode
+D -S = no continuity
-D +S = no continuity
Resistance mode
+D -S = Continually bounced between no continuity and 3.3 KOhms
-D +S = .73 KOhms

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

PQ19 removed from the board!! Testing of PQ19...
Continuity mode
+D -S = no continuity
-D +S = no continuity
Resistance mode
+D -S = Continually bounced between no continuity and 3.3 KOhms
-D +S = .73 KOhms

No continuity sounds good, but am not liking the resistance values (it could also be that you partially turned it on when handling the mosfet also). Make sure your probes are making proper contact with the pins as sometimes when you desolder there can be a thin film of flux that solidifies on its surface.

Do a check in diode mode. You should have no continuity on +D-S and a diode reading on -D+S. And also remeasure the resistance value (usually we do only continuity/diode and do not measure resistance, but they should have very high resistance in OFF state).

You should properly ground yourself when working with these components. MOSFETs fairly sensitive to ESD.

[cook]

Re: Beginner questions about unstable voltage in laptop

After I removed the chip I cleaned the contacts with IPA, so we should be OK there.
For these measurements, I did ground myself taking the measurements. Also, I switched to better test leads on my DMM.
Some of my results are different than what I has originally posted.
Diode Mode
+D -S = no continuity
-D +S = .132
I waited about 5 minutes before testing resistance..
+D -S = 43K
-D +S = .732 K ohms
Then when I tested +D and -S again, the result was different and it continually bounced between no continuity and 3.3 K ohms

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

Continuity and diode test looks good, but am a bit skeptical about the resistance value. I'ld rather change it (especially given its in the circuit generating CPU voltage).

Replace it with similar or an equivalent. Check on your scrap board (assuming they are laptop boards).

Try this:
Put your meter in diode mode.

(1) Put black probe on gate (G) of mosfet and Red on drain (D). You should not have continuity.

Now measure continuity/resistance at +D-S and -D+S (you should not have continuity)

(2) Next put Red probe on gate (G) of mosfet and Black on drain (D). You still should not have continuity.

But now you should have continuity across +D-S and -D+S. (also measure resistance while at it)

(3) Repeat step 1.

If you still have the kind of resistance you described, its better you change the mosfet.

[cook]

Re: Beginner questions about unstable voltage in laptop

I got the same results, so I am looking for a replacement mosfet. I have already found the data sheet for the mosfet in my board. And I found the one for the mosfet in my scrap board too. They are from different companies, so I am studying their specs to be sure they are basically the same thing. My scrap board is another HP Mini, although a different model, it looks really similar to mine. I think it is a bit newer board, but the 2 mosfets are right there next to the battery connector just like they are on my board. As long as volts are the same, amps are the same or higher, and resistance is fairly close, is that all I need to be concerned with for these mosfets?
Original mosfet: 30V, 14.8A, 6.0 m-Ohm (FDMC 7672S)
Replacement: 30V, 24A, 24 m-Ohm (AON 7410)

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

AON 7410 should be fine

[cook]

Re: Beginner questions about unstable voltage in laptop

[cook]

Re: Beginner questions about unstable voltage in laptop

After a bit more of practicing soldering, I replaced the bad mosfet PQ19.
Testing at PJ14...
Pin 1 to ground = 16 m-Ohms and climbing
Pin 2 to ground = 12 m-Ohms and climbing
Is it OK to test the power using the power charger to see if I have 19V at the power connector?

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

Testing at PJ14...
Pin 1 to ground = 16 m-Ohms and climbing
Pin 2 to ground = 12 m-Ohms and climbing

mOhm = milli Ohm
MOhm = Mega Ohm

I assume you meant 12 & 16 Mega Ohms?

Is it OK to test the power using the power charger to see if I have 19V at the power connector?

You mentioned in post #36 that the power light came on, so we already know there is power at the connector.

Connect PJ14 and try with the adapter.

[cook]

Re: Beginner questions about unstable voltage in laptop

Sorry. Yes, I meant M, not m.
I will reconnect PJ14 and test with adapter.