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Asus GU502DU - CPU fet dead, now no battery charging

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    #1

    Asus GU502DU - CPU fet dead, now no battery charging

    I normally am able to search and repair by myself, but not on this one. Nice Asus laptop had a short on one of the dual fets feeding the cpu, lucky customer! The board works, boots ok, but the battery does not charge. I don't have a Laptop Battery Analyzer yet so I ordered a replacement. Now I'm 3 replacements deep and I'm out of ideas.

    Here are the measurements around BQ24780s that I know to get. this is on my bench supply set at 19.5 and the latest, best battery I have:

    ACN/P are both 19.5 and
    ACDRV is 25.46

    ACDET is present at 2.654
    ACOK is really high at 5.97V (too high? It's resistors measure ok so maybe the chip?)
    REGN is at 6.03 which seems SOMEWHAT normal to me? I am not sure.

    BAT+ is present at 13.10V
    bat clock is 3.3 and seems to go to 2.9 every half second. pulsing low?
    bat data is similar, pulsing as it were.
    BATDRV stays low at 13.13 although that voltage is higher than it was 2 minutes ago... is this a wait and come back thing... it's only pulling 20mA.

    I've looked for shorts everywhere I can think of.... I'm out of ideas but am ready to take measurements if anyone has some advice.
    Boardview and schematic are attached.

    13.15v now.

    [MOD EDIT] Schematic & boardview moved to new thread --> https://www.badcaps.net/forum/troubl...atic-boardview
    Tags: None
    #2
    Re: Asus GU502DU - CPU fet dead, now no battery charging

    ACOK is often pulled up to the REGN pin.

    Measure the voltage to ground of the SRN and SRP pins.

    Post the measurements.

    Comment

      #3
      Re: Asus GU502DU - CPU fet dead, now no battery charging

      You have checked PQ8901/8902 right? One of these goes short after a 19V rail overload, which causes battery charging to be disabled.

      Comment

        #4
        Re: Asus GU502DU - CPU fet dead, now no battery charging

        Originally posted by mon2 View Post
        ACOK is often pulled up to the REGN pin.
        Got it, that makes sense. Good to check.
        Measure the voltage to ground of the SRN and SRP pins.

        Post the measurements.
        SRP - ~40M to ground
        SRN - ~40M to ground
        .471 on diode mode SRN/P

        You have checked PQ8901/8902 right? One of these goes short after a 19V rail overload, which causes battery charging to be disabled.
        As much as I know how to check them. The resistances across pins are all megaohm range. Gates, sources, drains all seem fine in all directions. Diode mode (red on ground) shows .397 on AC_BAT_SYS which is right after PQ8902, which seems low-ish?
        Last edited by down1nit; 04-11-2023, 10:02 PM.

        Comment

          #5
          Re: Asus GU502DU - CPU fet dead, now no battery charging

          some times short on main rail, overload battery and damage battery circuit

          Comment

            #6
            Re: Asus GU502DU - CPU fet dead, now no battery charging

            I tried a couple other bq24780s from dead boards, they showed different symptoms that I didn't like so I put the original back on.

            I'll toy around with different chips, wouldn't be the first time a chip was partly broken internally. Thank you.

            Comment

              #7
              Re: Asus GU502DU - CPU fet dead, now no battery charging

              Reread what they said, yeah, on my 4th battery right now. Also, found a used BQ24780s and it has the exact same symptoms as the OG BQ did.

              Comment

                #8
                Re: Asus GU502DU - CPU fet dead, now no battery charging

                Well battery should be charging unless it's being disabled via the EC through SMBUS.

                Maybe take a look at the DC jack as this is a common failure of Acer/Asus laptops. This one has an extra connection that indicates the jack is fully inserted. On other designs, this connection must be grounded to enable ACDET. However this one has some additional circuitry via PQ6004A/B to detect adaptor insertion. In fact, there is a short table there indicating charging is disabled if this is not present.

                So check ADP_INSERT_NG#. If it's low, then charging is disabled and system is throttled. When the jack is crook, you don't hear the short 'click' when you push it fully in. You could just short this line to ground to see if the battery will charge if the jack is U/S.
                Last edited by reformatt; 04-12-2023, 12:38 AM.

                Comment

                  #9
                  Re: Asus GU502DU - CPU fet dead, now no battery charging

                  The posted readings for SRN and SRP are resistance readings to ground and their diode mode readings, not voltage readings to ground.

                  Meter in dc volts mode (30v or higher is ok).

                  Apply the power adapter.

                  Measure the following carefully as the logic board is powered.

                  Red meter probe to SRN pin on the charger IC. Black meter probe to ground. What is the measurement?

                  Repeat with SRP pin on the charger IC and ground.

                  Then remove all power and with no battery.

                  Meter in lowest resistance mode.

                  Place one meter probe on SRN pin and other meter probe on the SRP pin. What is the total current path resistance for this battery charge circuit?

                  Post each measurement.

                  Comment

                    #10
                    Re: Asus GU502DU - CPU fet dead, now no battery charging

                    Originally posted by reformatt View Post
                    Well battery should be charging unless it's being disabled via the EC through SMBUS.

                    Maybe take a look at the DC jack as this is a common failure of Acer/Asus laptops. This one has an extra connection that indicates the jack is fully inserted. On other designs, this connection must be grounded to enable ACDET. However this one has some additional circuitry via PQ6004A/B to detect adaptor insertion. In fact, there is a short table there indicating charging is disabled if this is not present.

                    So check ADP_INSERT_NG#. If it's low, then charging is disabled and system is throttled. When the jack is crook, you don't hear the short 'click' when you push it fully in. You could just short this line to ground to see if the battery will charge if the jack is U/S.


                    This was it. Hopefully my experience will allow other techs to find this circuit which was clearly printed, but ignored by me. I put a blob in between pins 1 and 2 on PQ6004 and the (third) battery is now charging at 3 amps and climbing. Reformatt, thank you. This is the first I've seen this circuit, I suppose my shop is not as high volume?!?

                    mon2 I love you.

                    Thank you as well. You asked me to plug in the adapter and I did not. Apologies. I do not have an adapter yet.


                    Notes:
                    =======
                    I have been using my bench power supply the whole time, I scraped away some of the coating by PC6007 to solder my wire to. I have not been using the customer's power supply, as they did not bring it, and I do not have one with this barrel and pin configuration. I definitely do not have an official Asus branded adapter, and as such I will remedy this. I'll also grab a generic one to salvage the barrel, so I can cut it off and then clip into it with a power supply. Oftentimes the circuitry for Adapter detect can be found in the barrel, even on generic ones, however having a BRANDED OEM [super powerful] version of whichever barrel your customer's laptop takes is a good idea as well. I will remove the blob before testing with the customer's power adapter as it seems a useful circuit.

                    Comment

                      #11
                      Re: Asus GU502DU - CPU fet dead, now no battery charging

                      I wasn't aware you were bypassing the connector entirely by soldering to the board. I must admit I didn't read your first post in depth, so missed the bench power supply reference. Had I registered, I would have been straight up recommending you don't go any further without a known good adaptor. HP's for example are really picky, so you need to go this route on future repairs as a sanity check.

                      The adaptor detection in this model is simply a connection shorted to ground via the barrel when the adaptor is fully plugged in. There are no smarts in the adaptor, unlike the Dell's which have a one-wire circuit. Some models have a FET that shorts out ACDET till this connection is grounded and turns off the FET. In both cases, it prevents full current/surges till the adaptor is fully seated in the plug. That's my assumption anyhow.

                      Anyway, trap for young players.

                      Comment

                        #12
                        Re: Asus GU502DU - CPU fet dead, now no battery charging

                        You're all right mate. It's a set up you've read hundreds of times, you were looking for the familiar or unfamiliar. I actually should know better, I'm not really new at this and I am embarrassed (but proud to have made an example for google searchers).

                        The adapter is absolutely part of the "equation" of board repair, in that it's sometimes variable.

                        Also I just read that sometimes the onewire resistor is in the tip, sometimes it's in the transformer box. This true? Guess I was lucky with mine.

                        Comment

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