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    #61
    Re: Beginner questions about unstable voltage in laptop

    The test went well and the power light came on. -- Thank you again!!!
    How important is it to replace PC87? It is a 10uF, 25 volt X5R. The schematics show one just like it on the right side of PC87. And on the left side of PC87 is a 100nF X7R 25V. And there are 3 others of different sizes near it, but I am not sure if they are really populated on my board or not.

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      #62
      Re: Beginner questions about unstable voltage in laptop

      Originally posted by cook View Post
      The test went well and the power light came on. -- Thank you again!!!
      Congrats on fixing your laptop! Is it booting properly in windows, etc?

      How important is it to replace PC87? It is a 10uF, 25 volt X5R. The schematics show one just like it on the right side of PC87. And on the left side of PC87 is a 100nF X7R 25V. And there are 3 others of different sizes near it, but I am not sure if they are really populated on my board or not.
      Its not critical, they are filter caps that filters out noise on that rail. Although it is not super critical if you do not replace it, it is good practice to do so.

      You should be able to find similar one on your donor board. It does not necessarily have to be in the same circuit as you board (CPU in this case). Just search "10uF_X5R_25V" and locate them on your donor board. 10uF 25V will be the bigger one.

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        #63
        Re: Beginner questions about unstable voltage in laptop

        I am still attaching everything to the motherboard so I have not booted anything yet.
        Regarding PC87, the closest size that my HP donor board has is 4.7uF, 25V. Is that close enough? If that is not a good replacement I will keep looking at 2 other donor boards I have.

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          #64
          Re: Beginner questions about unstable voltage in laptop

          Its a good idea to test before you reassemble. Put the RAM and connect the display just to make sure you atleast get image.

          You could use the 4.7uF but would ideally want similar values as replacement.
          Last edited by Spider1211; 05-11-2020, 11:37 AM.

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            #65
            Re: Beginner questions about unstable voltage in laptop

            Spider1211,
            I connected the keyboard and display and it works! The fan came on and the display worked. It really works! Thank you. What a great feeling that it worked!! I may put the drive in to see if that works too, and then I will see about finding the right capacitor. Thank you again. Now I want to go back through everything we did to be sure I understand the thought process behind what it was doing, and what you had me check. --- Thanks

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              #66
              Re: Beginner questions about unstable voltage in laptop

              Glad to hear! If it booted, then it should also boot and load from HDD without issue. The feeling of fixing something never gets old

              Yes, its good to keep notes about issues encountered and steps taken to resolve it/them. It will be nice if you can post it here so that we can cross check as well as it helping others in similar situation.
              Last edited by Spider1211; 05-11-2020, 01:48 PM.

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                #67
                Re: Beginner questions about unstable voltage in laptop

                REPAIR SUMMARY

                HP Mini 110-3735DX
                The motherboard is a Foxconn Edwards_1_HPC_XS MP_MB_V2

                When plugged into the charger, the computer does not do anything; no lights, no fan, no video. Testing the charger when not plugged into the laptop showed 19 volts. The charger was tested under load and it worked fine, so it did not appear that the charger was at fault.

                The system board was removed, and everything was removed from it (keyboard, display, RAM, Wi-Fi, etc.). The battery was not connected to it, and remained disconnected for all of the following tests.

                When the charger was connected to the system board, the voltage at the power connector on the system board quickly looped through these 3 voltages; .9, 1.3, and 3.4. When connected to the charger there were no power lights on the board. The cycling of the voltage at the connector indicated that the main 19V rail was not working. Something was taking all the power which caused the 19V rail to shut down, and then it tried to turn on again. This is a classic symptom of a short somewhere on the board and capacitors are frequently the component that cause this.

                The resistance between the plus and minus pins at the power connector was .480 M-Ohms. This value indicates there is not a short for at least the first part of the board. Looking at the schematic narrows it down and we can say there is no short for any components that are between DC+ and PQ1 (the first mosfet). This means the focus for the fault needs to be after PQ1.

                I noticed that 1 or 2 capacitors on the board seemed warmer than other components. They were located close to each other near the power supply connector.

                Testing for shorts to ground on components located around the warm spots found some shorts. But the focus of further testing was on the components that were getting warm. A schematic and boardview were used to help identify components on the board and how they connected to each other.

                These are the warm/hot temperature 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. I did not realize it at the time, but these are not part of the circuit that is after PQ1, so they would not be the initial area to focus on to narrow down the failing part(s).
                * 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

                These are the temperature results from the back side of the board. The back side does not have the main power connector on it. 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

                We will focus on the hottest components first, and we will use the schematic to determine the circuit they were in. Doing this will will help us narrow down the search area. The hot components on the back side of the board are after PQ1, so they will be our first area of focus.

                The following components were checked with a DMM: PQ18, PQ19, PCE6 (I also checked PJ5, PJ6, and PU10 just to verify there is no voltage there.)

                PQ18
                * With the + probe (red) on D and the - probe (black) on S, no continuity. 148 ohms
                * With + on S and - on D, no continuity. 105 ohms
                * With + on D and - at a ground point, there is continuity. 8 ohms <<Having low continuity like this could indicate a short on the D circuit, which is either +Vcore_IN or +VBAT because they are the only 2 circuits that D on the mosfet connects to. >>
                * With + on S and - at a ground point, no continuity.
                <<The low ohm resistance across D and S could indicate a faulty or leaky mosfet. That will be checked later.>>

                PQ19
                * With + on D and - on S, no continuity. 100 ohms
                * With + on S and - on D, no continuity. 140 ohms
                * With + on D and - probe at a ground point, no continuity.
                * With + on S and - at a ground point, there is continuity.
                <<The low ohm resistance across D and S could indicate a faulty or leaky mosfet. That will be checked later.>>

                PCE6
                * Testing across PCE6 gives about 101 ohms.
                * The minus side of PCE6 has continuity to ground.

                PJ5, PJP6, PU10
                Although these were not hot, they were tested anyway.
                I connected the charger and tested voltages...
                * PJ5 and PJ6 had no voltage (or just a milivolt). A good board should have 3.3VA at PJ6 and 5VA at PJ5
                * PU10 (pin1 and pin5) had no voltage. A good board should have +V5 and +V3.3 at those pins.
                By testing PJ5, PJ6, and PU10, we have re-confirmed that the 5 volt rail is not working and the 3.3 volt rail is not working. And it also re-confirms that we have a short on the 19V power rail because the 5 volt and 3.3 volt rail is powered by the 19v rail.

                The test results for PQ18 showed a potential short on +Vcore_IN or +VBAT. The schematic fortunately showed a jumper that would isolate +Vcore_IN from +VBAT, jumper PJ14.

                PJ14 was desoldered and a careful visual inspection made to be sure there was no bridging between the 2 pads. PJ14 connects the 19 volt rail (+VBAT) to +VCore_IN. This is also the exact place on the circuit that has the two mosfets that were hot, PQ18 and PQ19. By disconnecting +VCore_IN from the 19V rail, a resistance test can help indicate if there is a short in the +VCore_IN circuit or not. Testing PJ14, pin 1(+Vcore_IN) resistance to ground was 6.7 ohms. Pin 2(+VBAT) resistance to ground was 33 M-Ohms. The low resistance for +VCore_IN indicates there may be a short to ground in the +VCore_IN circuit.

                Note: At this point, the only thing that has been done to the board is to desolder PJ14. Since no chips had been removed, it was OK to plug in the charger. When it was plugged in, the power light came on. This confirms that disconnecting PJ14 removed some or all of the faulty circuit and the power circuit was sensing a stable 19 volts. The charger was then disconnected.

                Thus far, the testing has focused on the area of the hottest two components, PQ18 and PQ19. The circuit they are on was isolated by desoldering the jumper PJ14, and further testing confirmed that most likely the issue is only in that part of the board (+VCore_IN). A simple test of the mosfets indicate they might be bad, but first the short needs to be found and removed before it can be determined if the mosfets are really bad or not. So the next step is to look at the capacitors on +VCore_IN between PJ14 and the chip MAX8796GTJ+T. (Note: Although a bench power supply could have been used to narrow down the failing components, one was not available.)

                A visual inspection of PC86, PC87, PC95, PC377, PC378, and PC379 was done to see if any seemed discolored and/or cracked. (If they all appeared OK, it would be easy enough to start removing them one by one until the short was found. Once the shorted cap was found and removed, I needed to remeasure across the mosfets to check if they are still in the 100 - 150ohm range I got previously. If yes, they would need replacing too.) After close inspection, it was noticed that PC87 had a small chunk of missing tan material on the side. My camera does not focus on an item this small so I could not get a picture of it. The fact that it had tan material missing on it was enough reason to start with that chip and remove it. It was removed and it was bad. And once it was removed, the other caps next to it all tested good now. (Before PC87 was removed they all appeared shorted.)

                Now it was time to retest the mosfets to see if they still looked bad or leaky. If they still read 100 - 150ohm (as was the case previously) they would need to be replaced.

                In circuit testing of continuity of PQ18 and PQ19 showed no continuity at all.

                Testing PQ18 in circuit looked OK....
                +D -S = nothing
                -D +S = 11 M-Ohms

                Testing PQ19 in circuit
                +D -S = 85 ohms
                -D +S = 112 ohms
                The low resistance continuity test on PQ19 could be due to the CPU. The easiest way to be sure is to remove it from the board and test it, so that was done. It was removed, flux was cleaned off, and it was tested. See posts #51, #52, #53, and #54 above for the details of how it was tested.

                Here are the test results for PQ19 out of the circuit.
                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

                Although continuity and diode test looked good, the resistance value did not seem right, so it was replaced with a similar mosfet.
                * Original mosfet: 30V, 14.8A, 6.0 m-Ohm (FDMC 7672S)
                * Replacement: 30V, 24A, 24 m-Ohm (AON 7410)

                Then PJ14 was re-tested with good results...
                Pin 1 to ground = 16 M-Ohms and climbing
                Pin 2 to ground = 12 M-Ohms and climbing

                PJ14 was reconnected, and the power light came on when the charger was plugged in.

                Further testing was successful, so it was put back together. The capacitor that was removed will be replaced at a later time.

                Thanks to Spider1211 for all his guidance in this repair.

                Comment


                  #68
                  Re: Beginner questions about unstable voltage in laptop

                  The resistance between the plus and minus pins at the power connector was .480 M-Ohms. This value indicates there is not a short for at least the first part of the board. Looking at the schematic narrows it down and we can say there is no short for any components that are between DC+ and PQ1 (the first mosfet). This means the focus for the fault needs to be after PQ1.
                  Just to add another piece of info. There are 2 input protection mosfet, PQ1 and PQ2. It can happen that either gets shorted.

                  In your case, you would have seen a short circuit to ground if you measured at Source (S) of PQ2 but no short if you measured at Drain (D) of PQ2.
                  Last edited by Spider1211; 05-28-2020, 10:10 AM.

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