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    #1

    Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

    I recently got an older pc system with an Asus P4S800 Socket 478 motherboard and a P4 3GHz processor. The system runs rock solid but has one quirk: the RTC stops keeping time any time the PC is unplugged from 120v AC.

    When I first got the pc, it had been stored in a closet for months and the CR2032 CMOS battery was depleted, so I replaced the battery. Now the CMOS is retained when I unplug the power so that part is working.

    But if I unplug the power cord from the pc for 4 hours and then plug it back in, the RTC will loose, you guess'd it, about 4 hours.

    Do any of you motherboard repair guys have a schematic for this board to tell me if the RTC on this motherboard has an internal battery that may be depleted. If you can provide the part number and RefDes, I can order one up and replace it and maybe remedy to only quirk this pc seems to have.

    Thanks in advance for any help you guys can provide!
    Tags: None
    #2
    Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

    are you sure the battery is good?
    they can be pretty low and still hold cmos.
    anything over 3v is good

    Comment

      #3
      Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

      and a p4 3ghz is NOT old.
      The great capacitor showdown!

      Comment

        #4
        Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

        [Reply for Chris1992]
        The system is probably 18 months old, which is very NEW for me but OLD for the guy I got it from.


        [Reply for kc8adu]
        I bought a brand new CR2032 and used a voltmeter to check the voltage before installing it into the motherboard. It was around 3.1v. I can pull it out and check it again today.

        If the battery is low today, what could be killing it? There should not be anything pulling that much draw on this battery, especially since the PC stays plugged in the 120v AC 99% of the time.


        Which is why I'd like to know where the RTC is located -- so I can check the Vcc pins when the PC is unplugged to see what voltages are present.

        Keep the replies coming... ...and Thanks!

        Comment

          #5
          Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

          Old for the guy you got it from?! He's RICH. 2 years is NEW for me.
          The great capacitor showdown!

          Comment

            #6
            Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

            There's a 32768Hz crystal on (almost) every board - a small metal cylinder with 2 pins near the RTC (near south bridge today as RTC is integrated there). I'd check if that crystal is not damaged or replace it. I have a 486 board where the RTC did not work - even on the Standard CMOS SETUP screen the time did not advance (it seemed to work in DOS/other OS as all OSs have their own way to keep time). I found that two chipset pins, where the crystal was connected, were bent and shorted together. After fixing that, it worked (and still works) fine.

            BTW. The battery should last 5 years (or even more) if not shorted accidentaly (by putting the board on a metal surface).

            Comment

              #7
              Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

              Yeah, his new computer is definitely a DREAM Machine!

              And like I said, this P4S800 system is Awesome to me, since it is going to replace my Athlon 900/KT133A that I build back in 2001, once I get all the kinks worked out of it.

              If worst comes to worst, I'll just have to buy a new(er) Socket 478 motherboard but I don't think it will come to that (at least, I hope not).

              That's why I'm begging and pleading for a bit of help so I can track down this reasonably small issue. I have searched all over Google and haven't had any luck tracking down anyone else with this particular problem -- at least, not yet.

              Comment

                #8
                Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

                Thanks Rainbow,
                I'll look for the crystal. I know exactly what type of crystal you are talking about.

                That's also a good idea to Ohm out the BATT to GND connections with the battery removed (and power disconnected) to see if there are any low impedance paths I need to troubleshoot.

                Comment

                  #9
                  Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

                  of course, the issue is sufficiently minor that you could just leave the computer plugged into AC.
                  The great capacitor showdown!

                  Comment

                    #10
                    Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

                    True, as long as it's not an early symptom of a more pronounced failure -- plus it's not supposed to work that way.

                    Comment

                      #11
                      Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

                      pool.ntp.org

                      that is my full answer
                      "The one who says it cannot be done should never interrupt the one who is doing it."

                      Comment

                        #12
                        Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

                        I checked the CR2032 battery over lunch and it was 3.0v so it hasn't been drained yet. I don't know of a good place to measure the voltage of the battery while it is installed, though.

                        I Ohm'd out the battery socket (with the battery and 120V AC removed) and got aroun 580 Ohms. I realize that's only an estimate, but it shows there isn't a direct short between BATT and GND. And doing the math: (3 volts / 580 Ohms = ~5mA). That seems like a reasonable load to me (anybody know anything different?).

                        I looked for the 32KHz crystal but I didn't see one of the oblong ones labeled with that frequency, but did find what looks like a 25MHz and 12MHz close by. The chipset IC closest to the battery is a BGA -- I don't know if this is the South Bridge, but I'm guessing it is since the other IC is covered up by a heatsink (North Bridge?). In any case, I can't look at the solder joints easily -- and I sort of doubt that is the problem.

                        Per Hansson,
                        Thanks for the suggestion on the ntp server. That is a good ban-aid.

                        But I'm still trying to determine what the root cause it...

                        Comment

                          #13
                          Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

                          I tested my dead Abit NF7-S and I have 649 Ohm... The battery is only at 2,3v though...

                          But of course since the board is dead I have had it disconnected for like a year so maybe that is just normal...
                          "The one who says it cannot be done should never interrupt the one who is doing it."

                          Comment

                            #14
                            Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

                            Originally posted by abernax
                            I checked the CR2032 battery over lunch and it was 3.0v so it hasn't been drained yet. I don't know of a good place to measure the voltage of the battery while it is installed, though.
                            Negative wire of the meter to any ground on the board (like PS/2 connector shield) and positive to the top of the battery.

                            I looked for the 32KHz crystal but I didn't see one of the oblong ones labeled with that frequency, but did find what looks like a 25MHz and 12MHz close by.
                            Something like this http://ytkesheng.cn.alibaba.com/athe...-20952848.html

                            Comment

                              #15
                              Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

                              Duh!!! I should have remembered that the positive terminal is on the upward facing side of the CR2032! Good call, Rainbow.

                              Yes, I did see one of those barrel canisters near the battery, but there was no writing on the side that was facing up so I didn't know if it was a crystal or what. I will try to check and see if it is oscillating when 120v AC is removed.

                              Comment

                                #16
                                Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

                                look for a cylinder abut 1/8 in da x1/2 long.
                                or a hc49s smt cased rock. these are like an inverted bathtub.
                                all these are 32.768 khz except a very rare few boards.

                                Comment

                                  #17
                                  Re: Asus P4S800 RTC won't keep time? (and "do you have a schematic?")

                                  Okay, I checked the voltage of the CR2032 battery while installed in the P4S800 motherboard.

                                  With 120v AC cord plugged in, the CR2032 read 3.02V.
                                  With 120v AC cord unplugged, the CR2032 read 2.98V.

                                  My guess is that 2.98V should still be enough to run the RTC, right?

                                  I didn't have a scope available last night to see if the 32KHz output differs depending on whether the 120v AC cord is plugged in or unplugged. That is my next task...

                                  Comment

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