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    Astec AA16990 PSU failure

    Hello crews,

    I have an ancient Astec AA16990 PSU from the first generation Power Macintosh (about 1993). The computer was fully functional until the last month when I noticed that the floppy drive has suddendly stopped working. This brought me to the idea to check the PSU first.

    The cover lists the following output voltages to be supplied:

    +5Vdc, 11.6A (red cable)
    +12Vdc, 2.5A (yellow cable)
    -12Vdc, 0.5A (blue cable)

    However, a quick multimeter check shows the following output voltages:

    red cable --> +4.4Vdc
    yellow cable --> +7Vdc
    blue cable --> -4,4Vdc

    All voltages seem to be out of expected range which could explain the floppy drive failure.

    I bet the PSU need a major recapping due to the age of the unit that still seems to contain original components from 1993.

    I counted 12 electrolytic capacitors by Nichicon on the PCB, with no visible traces of bulging, leaking etc (see attached pictures).

    Because I have a rather limited experience with PSUs, I'd to ask the experts here about what to do in order to fix it?

    Thank you in advance!
    Attached Files
    Last edited by powermax; 05-28-2017, 01:27 PM.

    #2
    Re: Astec AA16990 PSU failure

    the ones on the left of second pic near the output cables are the key ones.
    got a side-view foto so i can see what's written on the caps?

    Comment


      #3
      Re: Astec AA16990 PSU failure

      It's quite difficult to capture these 6 caps at once. I therefore made two pictures showing 3 of 6 caps clearly.

      The biggest cap in the middle of the left image is 2200uF 10V Nichicon LXF Serie. The next one to the right is obscured by glue. The small one to the left is 470uF 10V Nichicon PL(M) Serie.

      On the right image:

      the tallest one: 1000uF 16V Nichicon SXE Serie
      the small one in the middle: 10uF 10V Nichicon
      and there is one more cap just behind the heat sink not shown on the pictures: 25uF 25V Nichicon LXF Serie
      Attached Files

      Comment


        #4
        Re: Astec AA16990 PSU failure

        Have you checked the batteries in your meter?
        Because I would not expect the computer to work with voltages so much out of whack...
        "The one who says it cannot be done should never interrupt the one who is doing it."

        Comment


          #5
          Re: Astec AA16990 PSU failure

          I just swapped the battery in the meter and tested the voltages again. Now I'm getting:

          +4,5V (red)
          +7V (yellow)
          -11,9V (blue)

          Comment


            #6
            Re: Astec AA16990 PSU failure

            Would you recommend to replace all electrolytic caps in that PSU? If yes, should I choose new caps to match the original Nichicon series or is there smth better I could use?

            Comment


              #7
              Re: Astec AA16990 PSU failure

              change everything other than the huge mains smoothers.

              make a full list so we can lookup the specs

              Comment


                #8
                Re: Astec AA16990 PSU failure

                First of all, those SXE series capacitors are Nippon Chemicon (or United Chemicon), not Nichicon. The date code format was the first clue. Given the 24 year old age of the P/S, replacing all the electrolytics would probably be a good idea. Chances are, all the various series of parts are obsolete, possibly excepting that Nichicon cap on the output side (if it's a PL series part, the PM series is the "same", except for being lead-free).

                Because modern parts may have ESR so low that stability problems could result, I would suggest replacing the output caps with Nichicon PM or PS series parts, or maybe Nichicon PJ or Nippon Chemicon LXV series parts.

                I notice the PCB is very browned. That suggests a problem with the P/S fan, if it has one, with the system fan (if ...), or that the system cooling vents were partially blocked for quite a while. So I would consider replacing those as well (they are really old, too), and even using somewhat higher airflow fans.
                PeteS in CA

                Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
                ****************************
                To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
                ****************************

                Comment


                  #9
                  Re: Astec AA16990 PSU failure

                  Originally posted by PeteS in CA View Post
                  I notice the PCB is very browned. That suggests a problem with the P/S fan, if it has one, with the system fan (if ...), or that the system cooling vents were partially blocked for quite a while. So I would consider replacing those as well (they are really old, too), and even using somewhat higher airflow fans.
                  Thank you for pointing that out! Yes, the PSU has a built-in fan (see the attached images). According to the part description, it's a NMB PEWEE BOXER 3108NL, 12VDC, 0.19W fan motor. I don't have any clue how old it is but it seems to work flawlessly.

                  Any suggestions for a replacement?
                  Attached Files

                  Comment


                    #10
                    Re: Astec AA16990 PSU failure

                    Fans are a mechanical part with a finite life. 24 years is really old for a fan, so replacing it could be a good idea, even if it's nice and quiet right now. NMB is a very good brand and NMB still makes that fan, but I don't know what you have easily available. So if you can't get the same part at a reasonable price, get the dimensions, "Max. Air Flow", and "Max. Static Pressure" numbers from the linked NMB webpage and find an equivalent from another good vendor.
                    PeteS in CA

                    Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
                    ****************************
                    To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
                    ****************************

                    Comment


                      #11
                      Re: Astec AA16990 PSU failure

                      I would say replace all of the really small caps first. The bigger caps, although very old, should still be okay. I have many caps from the 80's and early 90's that are still in spec and running fine in the equipment they are in.

                      And the primary input caps shouldn't need replacement in at least 5-10 more years if they are a quality brand (seeing that this is an Astec PSU, all of the caps inside it are quality Japanese caps).

                      If the problem is not related to the very small caps, I would also inspect all of the solder joints. Both the 5V and the 12V rails seem low... but especially the 12V rail. So check the rectifiers and their joints too.

                      Originally posted by PeteS in CA View Post
                      I notice the PCB is very browned. That suggests a problem with the P/S fan, if it has one, with the system fan (if ...), or that the system cooling vents were partially blocked for quite a while.
                      PCB looks fine to me. Just looks like the cheap Phenolic brown stuff, that is all.

                      As for the fans: I wouldn't replace any fans, unless they are either starting to get noisy or stick / not rotate freely.

                      Most sleeve bearing fans are serviceable if they have a removable rubber plug on the back.

                      Ball bearing fans, on the other hand, are not serviceable once they develop bad bearings... that is, unless you replace the ball bearings with new ones.

                      NMB fans like the one above rarely have a rubber plug, and thus are typically impossible to service (unless you drill them out)... but I have not found a need to replace one yet. These fans will rarely seize up and are much better quality than a new cheap fan.
                      Last edited by momaka; 06-27-2017, 07:06 PM.

                      Comment


                        #12
                        Re: Astec AA16990 PSU failure

                        Originally posted by momaka View Post
                        I would say replace all of the really small caps first. The bigger caps, although very old, should still be okay. I have many caps from the 80's and early 90's that are still in spec and running fine in the equipment they are in.

                        And the primary input caps shouldn't need replacement in at least 5-10 more years if they are a quality brand (seeing that this is an Astec PSU, all of the caps inside it are quality Japanese caps).

                        If the problem is not related to the very small caps, I would also inspect all of the solder joints. Both the 5V and the 12V rails seem low... but especially the 12V rail. So check the rectifiers and their joints too.


                        PCB looks fine to me. Just looks like the cheap Phenolic brown stuff, that is all.

                        As for the fans: I wouldn't replace any fans, unless they are either starting to get noisy or stick / not rotate freely.

                        Most sleeve bearing fans are serviceable if they have a removable rubber plug on the back.

                        Ball bearing fans, on the other hand, are not serviceable once they develop bad bearings... that is, unless you replace the ball bearings with new ones.

                        NMB fans like the one above rarely have a rubber plug, and thus are typically impossible to service (unless you drill them out)... but I have not found a need to replace one yet. These fans will rarely seize up and are much better quality than a new cheap fan.
                        Ultimately, it does come down to the risk-benefit balance. That balance will be very different for a computer used constantly for a critical application, vs. a machine only used occasionally. Astec used good components with good skill, as a whole.

                        Good point about the FR-2 paper-phenolic PCB material, of which I'm not a fan (don't love paper-epoxy, either). The brown color and light/shadow in the pix may have fooled my eyes.
                        PeteS in CA

                        Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
                        ****************************
                        To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
                        ****************************

                        Comment


                          #13
                          Re: Astec AA16990 PSU failure

                          Originally posted by momaka View Post
                          If the problem is not related to the very small caps, I would also inspect all of the solder joints. Both the 5V and the 12V rails seem low... but especially the 12V rail. So check the rectifiers and their joints too.
                          Well, I attached my first attempt at schematics drawing for the secondary side of the PSU of interest. All electrolytic caps (6) are marked with red circles. The part in the green box looks like a fan speed control circuitry. Unfortunately, I have no clue how this L7905CV IC is used for controlling the built-in NBM fun which operates in the voltage range of 6.0 - 13V.

                          Would you kindly take a look at the schematic and tell me which components could eventually cause too low voltages so I could make further investigations?
                          Attached Files
                          Last edited by powermax; 07-05-2017, 08:05 PM.

                          Comment


                            #14
                            Re: Astec AA16990 PSU failure

                            Originally posted by momaka View Post
                            As for the fans: I wouldn't replace any fans, unless they are either starting to get noisy or stick / not rotate freely.
                            I tested the fun with a 9V battery. It works well with no mechanical noise. I therefore ain't going to replace it for now.

                            Anyway, kudos to PeteS in CA for finding out that NMB still makes this fan! Any replacement should be very easy now.

                            Comment


                              #15
                              Re: Astec AA16990 PSU failure

                              Originally posted by powermax View Post
                              1 Well, I attached my first attempt at schematics drawing for the secondary side of the PSU of interest. All electrolytic caps (6) are marked with red circles. The part in the green box looks like a fan speed control circuitry. Unfortunately, I have no clue how this L7905CV IC is used for controlling the built-in NBM fun which operates in the voltage range of 6.0 - 13V.

                              2 Would you kindly take a look at the schematic and tell me which components could eventually cause too low voltages so I could make further investigations?
                              1 The L7905CV is a 3-terminal -5V regulator, with the "Ground" pin connected to ground. The 2SD467 is used as a temperature dependent resistor, so that the voltage to the fan is 5V plus the voltage across the 2SD467.

                              2 Your partial schematic shows the output rectifiers and filters and the fan control circuit. It lacks the feedback circuit which has components that affect regulation.
                              PeteS in CA

                              Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
                              ****************************
                              To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
                              ****************************

                              Comment


                                #16
                                Re: Astec AA16990 PSU failure

                                Originally posted by PeteS in CA View Post
                                2 Your partial schematic shows the output rectifiers and filters and the fan control circuit. It lacks the feedback circuit which has components that affect regulation.
                                That's right - the circuit on the other side of the transformer T1 is quite complex. There is a lot of stuff there including an extra PCB with SMD components. I omitted them in the hope that my problem is resided solely on the output side.

                                I'll see what I can do.

                                BTW, this PSU is declared to be an "auto-configuring" one - i.e. it has a circuit that automatically adapts to the input voltage instead of having a 115/230V switch. How such a functionality is usually implemented?

                                Comment


                                  #17
                                  Re: Astec AA16990 PSU failure

                                  Originally posted by powermax View Post
                                  1 That's right - the circuit on the other side of the transformer T1 is quite complex. There is a lot of stuff there including an extra PCB with SMD components. I omitted them in the hope that my problem is resided solely on the output side.

                                  I'll see what I can do.

                                  2 BTW, this PSU is declared to be an "auto-configuring" one - i.e. it has a circuit that automatically adapts to the input voltage instead of having a 115/230V switch. How such a functionality is usually implemented?
                                  1 That large daughter card that spans the output side and the input side ("secondary" and "primary", respectively) has much of the regulator circuitry, may be all of it. I suggest getting the part numbers of the various ICs and see what their functions are. Also, one very commonly used IC is a TL431, which looks like a small plastic transistor, but is used as an error amplifier.

                                  2 The auto-ranging designs I worked on back in the late 1980s used a sensing circuit and relays to perform automatically the function of the manually set switch. I did not see any relays on this Astec power supply. I vaguely remember hearing of a power IC (SGS-Thomson or STMicro? Same company, name changed over time) that sensed the input voltage and perform the function of the manual switch. I never worked with that power IC, though.
                                  PeteS in CA

                                  Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
                                  ****************************
                                  To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
                                  ****************************

                                  Comment


                                    #18
                                    Re: Astec AA16990 PSU failure

                                    Originally posted by powermax View Post
                                    Would you kindly take a look at the schematic and tell me which components could eventually cause too low voltages so I could make further investigations?
                                    Unless the caps on the output rails themselves are bad (which I doubt, as Nichicon PL is quite reliable - I have an Astec PSU of a similar age, and its caps are absolutely fine), I don't think anything on the schematic you posted would be bad.

                                    Speaking of the schematic you drew: good job, looks pretty good.

                                    Originally posted by powermax View Post
                                    That's right - the circuit on the other side of the transformer T1 is quite complex. There is a lot of stuff there including an extra PCB with SMD components. I omitted them in the hope that my problem is resided solely on the output side.
                                    That daughterboard is indeed quite complex, but you need not worry about the circuits on it (yet, at least). When I suggested to replace all of the small caps, that's where I meant for you to start - replace all of the small electrolytic caps on the daughterboard. Keep the old ones, too, though. I suspect at least some of them will be good.

                                    Also, Pete mentioned a TL431 shunt regulator, and that's a very good point. A lot of PSUs use TL431 shunts for regulation, as they are simple, easy to use, dirt cheap, and thus extremely common. I believe Astec made their own ICs back in those days, so you may not see a "TL431" but an "AS431" instead. In any case, it would look like a small TO-90 transistor. It's worth replacing it, just to eliminate that as a possibility for the problem.

                                    If you have any spare junk PSUs, look in them - they often have at least one TL431 shunt regulator in them (typically near the 5VSB, but also another one if the PSU uses a mag-amp circuit for the 3.3V rail).

                                    Originally posted by PeteS in CA View Post
                                    2 The auto-ranging designs I worked on back in the late 1980s used a sensing circuit and relays to perform automatically the function of the manually set switch. I did not see any relays on this Astec power supply. I vaguely remember hearing of a power IC (SGS-Thomson or STMicro? Same company, name changed over time) that sensed the input voltage and perform the function of the manual switch. I never worked with that power IC, though.
                                    I have a newer Astec PSU from the Pentium 3 era (have not posted its pictures on badcaps.net yet), and it also has the automatic voltage selection feature. No relays in mine either, but it does have two BTB12-800B TRIACs and an LM2903 dual comparator.
                                    Last edited by momaka; 07-07-2017, 08:58 AM.

                                    Comment


                                      #19
                                      Re: Astec AA16990 PSU failure

                                      Originally posted by momaka View Post
                                      Unless the caps on the output rails themselves are bad (which I doubt, as Nichicon PL is quite reliable - I have an Astec PSU of a similar age, and its caps are absolutely fine), I don't think anything on the schematic you posted would be bad.

                                      Speaking of the schematic you drew: good job, looks pretty good.
                                      Thank you!

                                      Originally posted by momaka View Post
                                      I believe Astec made their own ICs back in those days, so you may not see a "TL431" but an "AS431" instead. In any case, it would look like a small TO-90 transistor. It's worth replacing it, just to eliminate that as a possibility for the problem.
                                      Bingo! That's indeed an "AS431". And that daughter board does contain another Astec IC - "AS3842" - which is a PWM controller, IIRC.

                                      Originally posted by momaka View Post
                                      I have a newer Astec PSU from the Pentium 3 era (have not posted its pictures on badcaps.net yet), and it also has the automatic voltage selection feature. No relays in mine either, but it does have two BTB12-800B TRIACs and an LM2903 dual comparator.
                                      Marvelous! There is neither a relay nor "power IC" in my PSU. But it does have another small PCB with "LM339N" and a dozen discrete components on it. That PCB is connected on the one side to the main rectifier bridge (KBL08) and on the other side to a component designated as "TR1". That looks vaguely similar to BTB12-800B!

                                      Comment


                                        #20
                                        Re: Astec AA16990 PSU failure

                                        Originally posted by momaka View Post
                                        When I suggested to replace all of the small caps, that's where I meant for you to start - replace all of the small electrolytic caps on the daughterboard.
                                        There is just only ONE electrolytic cap on this board - 10uF, 25V, Nichicon. Replace?

                                        Originally posted by momaka View Post
                                        It's worth replacing it, just to eliminate that as a possibility for the problem.
                                        Hmm, it looks like it will be difficult to get a new "AS431". Ok to replace it with a "TL431"?

                                        Comment

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