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    SMPS low Vout

    Hi there.

    A quick one - BENQ projector SMPS, according to the SM there should be 2.5V, 5V and 12V at any time ( good ol' times, when there were no eco regulations ).
    What I see though, is - +2.7V, +4.8V and -9.9V (I guess the polarity is irrelevant ).
    So, the projector starts, and shows some functionality, but is not powering up the ballast ( there is power supply, but no command I guess ). Before troubleshooting more the MB ( there was a puffing smd capacitor for which I'll upload some media in the main cap thread ) and the ballast, I'd like to get the Vs within the specs. The 2.5 and 5V seem to be within the limits, but the 12V is low.
    What I've done so far is to desolder the c707 and measure it... and it its fine - 707uF ESR 0.01.
    What's next?
    Attached Files
    Last edited by madan1; 02-09-2019, 05:00 AM.

    #2
    Re: SMPS low Vout

    could be the diode, BUT - is it running hot?
    i'm thinking something may be overloading it.

    Comment


      #3
      Re: SMPS low Vout

      Thanks for the reply... but it's my bad. I was measuring it wrong. In the SM there is no connector pinout and I assumed that it was rail-gnd rail-gnd... aligned, but it turned out that the ground are only 4 pins on one of the rows. Probably if I had paid more attention to the traces on the PSU board instead of focusing on the "usual suspects" would have saved me lot's of time .
      Now, with the correct pinout, I'm able to read 2.63, 5.07 and 11.9V when the unit is "on". By the way, all caps are nichicon and all which I have measured are at least the uF shown on their labels... and that thing is 10+ years old. Well done nichicon

      Comment


        #4
        Re: SMPS low Vout

        Originally posted by madan1 View Post
        By the way, all caps are nichicon and all which I have measured are at least the uF shown on their labels... and that thing is 10+ years old. Well done nichicon
        Just for good measure I would recommend recapping this unit specifically the capacitor that are less than 100uf
        9 PC LCD Monitor
        6 LCD Flat Screen TV
        30 Desk Top Switching Power Supply
        10 Battery Charger Switching Power Supply for Power Tool
        6 18v Lithium Battery Power Boards for Tool Battery Packs
        1 XBox 360 Switching Power Supply and M Board
        25 Servo Drives 220/460 3 Phase
        6 De-soldering Station Switching Power Supply 1 Power Supply
        1 Dell Mother Board
        15 Computer Power Supply
        1 HP Printer Supply & Control Board * lighting finished it *


        These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

        1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
        2 Hem Saw Computer Stack Board

        All of these had CAPs POOF
        All of the mosfet that are taken out by bad caps

        Comment


          #5
          Re: SMPS low Vout

          Once I get the projector going I might recap it... for troubleshooting purposes I guess it's fine at the moment.

          By the way, here is Smokey the capacitor - https://www.badcaps.net/forum/showth...404#post880404

          Comment


            #6
            Re: SMPS low Vout

            recap is most common repair on PSU so i recommend to do it as first

            especially if it is 10years old

            Comment


              #7
              Re: SMPS low Vout

              It's an old PSU indeed, but all caps are nichicon, I have desoldered some of them and they all read fine ( even more than what is stated on their labels ).

              Unfortunately I'm still not able to fix it so some assistance will be appreciated.
              OK, so the story so far - the topic label is wrong. All voltage rails were present and it was my bad not measuring them fine.

              After some testing of the unit, the PSU just popped. I guess I have shorted something, but no obvious signs - no burn marks, no smoke.. nothing, but a loud explosion and pieces of TOP247 everywhere.
              I replaced the TOP + R653 ( it was reading 40+K instead of 6.8R ). All other components seem to be fine - the resistors show the expected values, the diodes work in only one direction, the caps have their capacitance.
              In the process of testing, I broke c653. Should I replace it or it will work without it ( the default TOP247 datasheet does not show it )?
              The 20VPFC rail is missing, thus the 380V controller also does not work and there is only 320V on the highV rail.
              When powered up, the TOP247 gets hot, but that is the only sign of life I got from it.
              Is it possible that I have bought a faulty IC or there is something else?

              Comment


                #8
                Re: SMPS low Vout

                Originally posted by madan1 View Post
                It's an old PSU indeed, but all caps are nichicon, I have desoldered some of them and they all read fine ( even more than what is stated on their labels )
                that's a red flag!
                did you check ESR & Leakage, or just the capacitance?

                when caps go leaky they often start to read a higher capacitance.

                Comment


                  #9
                  Re: SMPS low Vout

                  I checked them with "transistor tester" and by "higher capacitance" I mean like 50uF and ESR 0.38 ( real values for c654 ), when it should be 47uF.
                  This is the only electr. cap in the TOP247 circuit. There are two more which I tested outside the circuit - c661 and c620 and they also showed similar values.
                  By the way, is it possible to have a leaky cap and still having capacitance detected by a multimeter?
                  Attached Files

                  Comment


                    #10
                    Re: SMPS low Vout

                    yes, but if it's leaky it will show higher capacitance.
                    the meter charges the cap and times it - the longer it takes then the higher the capacitance.
                    if it's leaky it takes longer to charge and fools the meter.

                    Comment


                      #11
                      Re: SMPS low Vout

                      In the process of testing, I broke c653.
                      I would replace the broken cap
                      This is from the pdf:
                      An external bypass capacitor closely
                      connected between the CONTROL and SOURCE pins is required
                      to supply the instantaneous gate drive current. The total amount
                      of capacitance connected to this pin also sets the auto-restart
                      timing as well as control loop compensation.

                      If R653was open I would also suspect the optocoupler may have been damaged, you should also check the resistance of the transformer between pins 9 & 10

                      And bad caps CAN show a higher value on a cap meter.
                      Last edited by R_J; 02-19-2019, 10:46 AM.

                      Comment


                        #12
                        Re: SMPS low Vout

                        Originally posted by stj View Post
                        yes, but if it's leaky it will show higher capacitance.
                        the meter charges the cap and times it - the longer it takes then the higher the capacitance.
                        if it's leaky it takes longer to charge and fools the meter.
                        Will a simple DMM resistance test show the leak?

                        Comment


                          #13
                          Re: SMPS low Vout

                          Originally posted by R_J View Post
                          I would replace the broken cap
                          This is from the pdf:
                          An external bypass capacitor closely
                          connected between the CONTROL and SOURCE pins is required
                          to supply the instantaneous gate drive current. The total amount
                          of capacitance connected to this pin also sets the auto-restart
                          timing as well as control loop compensation.

                          If R653was open I would also suspect the optocoupler may have been damaged, you should also check the resistance of the transformer between pins 9 & 10

                          And bad caps CAN show a higher value on a cap meter.
                          I guess I was looking at this:
                          "In addition to the 47 μF CONTROL pin capacitor, a high
                          frequency bypass capacitor in parallel may be used for
                          better noise immunity."
                          I will try to find one, but what do you think 0.1UM means ( quoted from the SM )? Is this 0.1uF? The cap is of the smallest smd type and is real pain in the a** to solder it in that overpacked space.

                          Comment


                            #14
                            Re: SMPS low Vout

                            Most of the time, yes you can detect leakage with a DC ohmmeter though it can be a test of patience to wait for the capacitor to charge. Ideally leakage is tested at the rated voltage which a simple ohmmeter can't do.
                            Last edited by eccerr0r; 02-19-2019, 11:37 AM.

                            Comment


                              #15
                              Re: SMPS low Vout

                              Yes you can use a .1µf, check scrapped boards, it helps if you have a schematic of the scrapped board to find a smd cap.

                              Comment


                                #16
                                Re: SMPS low Vout

                                Originally posted by eccerr0r View Post
                                Most of the time, yes you can detect leakage with a DC ohmmeter though it can be a test of patience to wait for the capacitor to charge. Ideally leakage is tested at the rated voltage which a simple ohmmeter can't do.
                                I tested all elements from the TOP247 circuit with an external power supply and did not find any leaking diodes or caps.
                                The resistors checked with DMM and again - all read fine.
                                Just tested and the optocoupler - when voltage is applied on the diode side, it changes its resistance on the other side. Is it normal to be conductive in both directions ( on the transistor(?) side )?
                                Got a new TOP247 and it shows higher resistance between S and D, so I guess the other one was faulty.
                                Unfortunately in the local shop they did not have smd caps, so for now I'll be troubleshooting without the c653.
                                Tested again out of the board c654 and c661. Used external power supply to charge them and a transistor tester. Again - the capacitance is fine, the ESR is fine, they do not selfdischarge or conduct current. Anyway, this time I'm switching them with new ones just because I'm fed up with testing them .
                                Should I desolder the transformer to test it?
                                On the board I get the following readings:

                                7-8 - 0.8ohm
                                8-9 - 0.1-0.2ohm
                                9-10 - 0.1-0.2ohm
                                8-10 - 0.3-0.4ohm

                                5-4 - 0.1-0.2ohm
                                4-2 - 0.1-0.2ohm
                                2-1 - 0.1-02ohm

                                Comment


                                  #17
                                  Re: SMPS low Vout

                                  Just tested and the optocoupler - when voltage is applied on the diode side, it changes its resistance on the other side. Is it normal to be conductive in both directions ( on the transistor(?) side )?
                                  maybe? what resitance do you get? as in xxxΩ
                                  The transformer resistances are likely ok, and not open.
                                  Are you getting any voltage at all on the secondary? even a couple volts
                                  Last edited by R_J; 02-20-2019, 04:35 PM.

                                  Comment


                                    #18
                                    Re: SMPS low Vout

                                    Originally posted by R_J View Post
                                    maybe? what resitance do you get? as in xxxΩ
                                    The transformer resistances are likely ok, and not open.
                                    Are you getting any voltage at all on the secondary? even a couple volts
                                    for the octoC
                                    when using the positive probe on pin 3 and negative on pin 4 I get between 120ohms and 38k
                                    when the positive probe is on pin 4 and the negative on pin 3 - 60ohms - 10m
                                    the values change parabolically to the applied voltage ( start high, go low and then again high with the V increase ).
                                    On the pins for the voltage output I get millivolts - 0.15-0.20.
                                    The high V line still is 320 instead of 380.

                                    Comment


                                      #19
                                      Re: SMPS low Vout

                                      If this top247 circuit is not working, you won't have 20VPFC to turn on the pfc circuit.
                                      You may want to check D681 and make sure nothing is shorted off that line,
                                      Are any of the 12,5,2.5 volt lines shorted or is the power supply removed for these tests?

                                      Comment


                                        #20
                                        Re: SMPS low Vout

                                        Originally posted by R_J View Post
                                        If this top247 circuit is not working, you won't have 20VPFC to turn on the pfc circuit.
                                        You may want to check D681 and make sure nothing is shorted off that line,
                                        Are any of the 12,5,2.5 volt lines shorted or is the power supply removed for these tests?
                                        I also thought the same about the 20V rail, but was not sure.
                                        D661 works as a diode when tested by DMM and external power supply, what other tests I can do?
                                        I also injected 16volts on c661 and got only 2.3mA leak.

                                        The psu is tested totally out of the circuit - it does not require control signal to start, so no need for the rest of the boards. There is no load on the lowV connector. Visually everything on the secondary side looks fine, but I have not tested almost anything there.

                                        I just measured some more voltages:
                                        on c681 - 0.4V

                                        on pins 1 and 2 on the optoC - 0.13V
                                        on pin 3 - 0.46v
                                        on pin 4 - 5.2V
                                        Last edited by madan1; 02-20-2019, 06:14 PM.

                                        Comment

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