Beginner questions about unstable voltage in laptop

[cook]

I am diagnosing an HP netbook (model 110-3735DX) that has a 19 volt external power supply/charger like most laptops have. I get 19 volts from the power supply/charger when it is not connected to the computer.

The system board has been completely removed and there is no RAM in it and the battery is not connected.

When I read the voltage at the power supply connector on the system board, I get a reading that changes quickly with these values; .9, 1.3, 3.4, and it basically loops like this. The voltages are usually no less than .9 and nothing usually over 3.5. I do not get any lights on the system board.

I assume this indicates a problem with the 3.5 voltage section which is trying to start but keeps shutting down for some reason. Does that seem like the most likely problem? Is there anything else that symptoms like this would typically indicate?

I have also noticed that 1 or 2 capacitors on the board are much warmer than anything else on the board. They are both very close to each other are get to a temperature of 102F. They are located close to the power supply connector. Would it make sense that one of these is most likely cause the issue, or could they just be a symptom of something else that is failing?

I am a beginner in electronics. Although I have watched and learned much from many repair videos, and I have only successfully diagnosed and replaced a few capacitors on a few older boards.

I hope this is the right forum for this topic.

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

Can you locate your test points on a schematic and post here? Power cycling could be caused by a short circuit on that power rail. Measure its resistance to ground to confirm. Capacitors are usually common causes for short circuit.

[cook]

Re: Beginner questions about unstable voltage in laptop

I am still searching for a schematic for this system board. I have recently asked for the schematics on the 'schematics request' forum of this site. I do see various test points on the board and I will measure them to see if any of them give a big difference in resistance.

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

Post a high definition picture of the board as well as a closeup of the problematic area. Label the places you tested and the results you got on there.

[cook]

Re: Beginner questions about unstable voltage in laptop

It may take a day or 2, but I will get the picture and resistance results.

[R_J]

Re: Beginner questions about unstable voltage in laptop

Have you tried another power supply adaptor? I would do that first before ripping into the laptop. It could be the adaptor can't supply any current and is shuttiing down

[cook]

Re: Beginner questions about unstable voltage in laptop

I will try a different power supply and post my results.

[cook]

Re: Beginner questions about unstable voltage in laptop

I tested the power supply that came with the laptop by connecting it to a standard incandescent light bulb that had a resistance of 17 ohms. I got a faint steady glow from the bulb, and the power was stable at 19 volts. That sounds like the power supply does remain steady even under about 1 amp of load.

Today I hope to start testing the resistance at the test points.

[cook]

Re: Beginner questions about unstable voltage in laptop

I am using the board view files from this post to locate my test points:
https://www.badcaps.net/forum/showpo...6&postcount=15
Let me know if a close up photo is needed

1. The first screen shot from the board view shows the two areas that I tested.
2. The second screen shot is a close up of the test points that are very close to where the power charger connects to the system board and the connections to the battery are located. It is the same area of the board where the CPU is located, although the CPU is on the other side.

Test Point Results - resistance to ground
TP3, 5, and 7 - Not connected to ground
TP4: .454 M ohms
TP6: 5.50 M ohms
TP2: .451 M ohms

3. The third screen shot is for the test points around the center of the board.
TP14: .440 M ohms
TP10: .439 M ohms
TP11: .428 M ohms
TP15: .442 M ohms
TP19: .426 M ohms
TP18: .429 M ohms
TP39: .397 M ohms
TP1: .396 M ohms
TP13: .431 M ohms
TP40: .438 M ohms
TP16: .428 M ohms
TP17: .428 M ohms
TP21: .425 M ohms

I don't know if any of these results point to a specific problem or not. Or maybe they indicate that some areas of the board might be OK?

Next I will be seeing if I can identify exactly what part is getting warm. Something is getting warm in the area of second screen shot.

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

You measured various test points available on the board. By test point we do not necessarily mean a literal test point (TP). It could be any point/pin on the board.

I saw your post on the schematic request. Can you confirm if the schematic you found there matches you motherboard? Check the parts such as CPU, GPU, Embedded Controller (EC), etc on the laptop and compare with the schematic and the boardview. It'll be easier to proceed then.

Measure & check if you have 3.3V on the power button and post back. Also measure the resistance on the line you said the voltage was fluctuating.

[R_J]

Re: Beginner questions about unstable voltage in laptop

I tested the power supply that came with the laptop by connecting it to a standard incandescent light bulb that had a resistance of 17 ohms. I got a faint steady glow from the bulb, and the power was stable at 19 volts. That sounds like the power supply does remain steady even under about 1 amp of load.

Today I hope to start testing the resistance at the test points.

As soon as you applied the voltage to the lamp the resistance will go up and the current draw will go down. So I hope the power supply is ok

[cook]

Re: Beginner questions about unstable voltage in laptop

Thanks R_J. I am still getting 19 volts on the power supply when it is not connected to anything. I did a bunch or searching to see if there is an easy to put a load on the power supply, but I did not come up with a really simple way. I will keep this in mind as I continue to trouble shoot the circuit and will not take it for granted that it is working OK.

[cook]

Re: Beginner questions about unstable voltage in laptop

Spider1211,
I looked closely at the pdf file for the Edwards series of the Atom netbook system board, and at the same time the board view files I mentioned in post #9 above. I mainly focused on the initial power supply components. And for that area my board does seem to match both the pdf and the board view files.

I am getting the fluctuating reading at the main power connector on the system board. The resistance between the plus and minus pins at the power connector is .480 megaohms. (I have a similar vintage Atom CPU netbook from a different vendor, and it has 20.70 megaohm resistance at the power connector.)

Since I don't have stable power at the power connector, I did not measure the voltage at the power button.

I will be testing various points around the power connector and checking for shorted capacitors too. I will post the results once I have them.

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

Can you show us on the boardview/schematic which caps you said were getting warm.

An acquaintance had an almost similar issue once, he said the area near power connector was getting warm. Turned out to be a loose connection to the motherboard. Might be worth a look.

[cook]

Re: Beginner questions about unstable voltage in laptop

These are the results from the front side. This is the side that has the main power connector on it. These readings were taken after being plugged in for about 4 minutes.
* The hottest parts were FBB2, RB21 and RB18 at about 98 degrees
* PR14, PR3 and PC4 were 97 degrees
* DB4, DB5, DB6 were 95
* PC94, PC95, PC97 were 95
* PQ5, PQ6, PQ7, PD5 were 93
* PL7 was 92

[cook]

Re: Beginner questions about unstable voltage in laptop

These are the results from the back side of the board. The back side does not have the main power connector on it.
Backside Notes- These readings were taken after being plugged in for about 4 minutes
* PQ18 - 103 degrees
* PQ19 and PCE6 were 97
* UH1 was 95
*The area in between UH1 and PCE6 had no components but was around 96 degrees
* PC2, PC3, and CB30 were around 91 degrees

[cook]

Re: Beginner questions about unstable voltage in laptop

Spider1211,
I started following the power circuit on the board starting at the power connector. I did simple tests for a short and found that everything seemed OK until I came to the first mosfet, and it does not seem right, but I am not sure. This mosfet is one of the components on the board that was getting warm, but it not the hottest component.
In diode mode, I only get a reading with plus on the drain and ground on the source, and the reading is .408. I did not remove it from the system board. All the other testing on it showed no connection at all. It does not show a short to ground. Am I right in thinking that this mosfet is damaged?
The mosfet is PQ5. It is P-channel - PMV65XP
The picture below shows the mosfet.

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

These are the results from the front side. This is the side that has the main power connector on it. These readings were taken after being plugged in for about 4 minutes.

Was the battery connected or just the power adapter?

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

These are the results from the back side of the board. The back side does not have the main power connector on it.
Backside Notes- These readings were taken after being plugged in for about 4 minutes
* PQ18 - 103 degrees
* PQ19 and PCE6 were 97
* UH1 was 95
*The area in between UH1 and PCE6 had no components but was around 96 degrees
* PC2, PC3, and CB30 were around 91 degrees

Check continuity across these components "PQ18, PQ19 and PCE6" with your meter on continuity mode. Also measure continuity (with black probe on ground) Drain, Source and PCE6. If there is continuity on both side (on PQ19 & PCE6), try removing PCE6 and re-measure.

I hope that if there is a short, its the PCE6 otherwise that could mean you have a dead processor.


Please check if you have 3.3VA (Measure on PJ6) & 5VA (measure PJ5) [Page 34 of schematic] as well as +V5AL and +V3.3AL (on PU10)

[Spider1211]

Re: Beginner questions about unstable voltage in laptop

In diode mode, I only get a reading with plus on the drain and ground on the source, and the reading is .408. I did not remove it from the system board. All the other testing on it showed no connection at all. It does not show a short to ground. Am I right in thinking that this mosfet is damaged?

This actually means the MOSFET is good.

The 0.408 is due to the internal body diode of the MOSFET. With the meter in diode mode, Red (plus) probe on Drain and Black (ground) on source the internal diode is forward biased and you get the voltage drop across it (depending on your meter, you might get [0.408] or [ 408])*. When you reverse the probes, the internal diode is reverse biased and would show as an open circuit(depending on the meter you use, you might get [1 ] or [ OL ]).

*0.408 means 0.408V while
408 means 408mV = 0.408V

Typical body diode reading range from around 0.2 to 0.6V. Datasheet of actual component will give you its typical value.


Do you have a bench power supply (that can put out at least 20V and 2A)?