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Fixing a multimeter based on VC9805 schematic

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    #41
    Re: Fixing a multimeter based on VC9805 schematic

    Thank you for your reply. The m6 smd I marked in the photo and diagram was a short circuit. I don't have an m6 transistor right now. What should I do to operate the multimeter?
    Attached Files

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      #42
      Re: Fixing a multimeter based on VC9805 schematic

      Ok, that transistor matches the schematic for Tp2, controlling battery power to the multimeter, for the auto-power off.
      If it was shorted, the multimeter would stay powered on, it would not auto-shut off. The on/off switch would still work.
      It would be rare for that transistor to fail short-circuit. It seems strange. Unless there is another shorted part on the board, like a C2 or an IC.

      You can just short C-E with a jumper and the multimeter would power up and work, just no auto-shutoff feature.

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        #43
        Re: Fixing a multimeter based on VC9805 schematic

        auto-off is a pain in the ass anyway.

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          #44
          Re: Fixing a multimeter based on VC9805 schematic

          Yes, it doesn't turn off automatically. you guys are great. thank you. good people like you will save the world... so why can't I measure voltage and current? only ohm and diode (buzzer) are working. I couldn't find any other short circuit, contactor wires are in place. Could you please give an idea for this as well?

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            #45
            Re: Fixing a multimeter based on VC9805 schematic

            Now the multimeter is working but it doesn't measure voltage and current. Do you have any idea about it?

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              #46
              vc9805 multimetre arızası

              hello, I have the same vc9805 multimeter, whose photos and schematic I have attached below. After replacing the battery only ohm and diode (buzzer) measurements work, other measurements do not work voltage, current ... none. Only the number 1 remains on the screen. When I measured the components, I couldn't find any problem parts. If anyone has any idea, what should I do?
              Attached Files
              Last edited by poker; 06-24-2022, 10:24 AM.

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                #47
                Re: vc9805 multimetre arızası

                check all the transistors for shorts - some are used for input protection.

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                  #48
                  Re: vc9805 multimetre arızası

                  I checked all of them, unfortunately I can't find it, I don't understand where am I doing wrong?

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                    #49
                    Re: Fixing a multimeter based on VC9805 schematic

                    did you move any of the contacts in the rotary switch?

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                      #50
                      Re: Fixing a multimeter based on VC9805 schematic

                      Post #28 I said to use a multimeter and measure some voltages "On DCV 20V range, measure the A/D voltages: IN HI, IN LO, REF HI+, REF LO-, COM wrt V-"
                      That is 5 readings to take.

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                        #51
                        Re: Fixing a multimeter based on VC9805 schematic

                        I will measure and reply immediately. Sorry, I missed your message.

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                          #52
                          Re: Fixing a multimeter based on VC9805 schematic

                          Originally posted by redwire View Post
                          Post #28 I said to use a multimeter and measure some voltages "On DCV 20V range, measure the A/D voltages: IN HI, IN LO, REF HI+, REF LO-, COM wrt V-"
                          That is 5 readings to take.
                          sir,
                          buzzer = 2.90 v
                          temperature = 38.1 millivolts
                          in low- in hi= 0.00
                          vref+-vref- =1
                          v- v+ = battery voltage 9.0
                          v- and com varies 179 mv, 190, 110

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                            #53
                            Re: Fixing a multimeter based on VC9805 schematic

                            Display shows 1___ with those readings?
                            Pin 40 COM voltage is too low, it needs to be at least 1V above V-. On the schematic it would be QC4,QC3 to check- that would short COM below 1V. On your board check whichever two are the same transistors as those. 9Q1 maybe?

                            There should be two protection transistors between COM and V-. If your meter does not have them, the IC can get damaged on pin 40.

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                              #54
                              Re: Fixing a multimeter based on VC9805 schematic

                              Originally posted by redwire View Post
                              Display shows 1___ with those readings?
                              Pin 40 COM voltage is too low, it needs to be at least 1V above V-. On the schematic it would be QC4,QC3 to check- that would short COM below 1V. On your board check whichever two are the same transistors as those. 9Q1 maybe?

                              There should be two protection transistors between COM and V-. If your meter does not have them, the IC can get damaged on pin 40.
                              sir, If the multimeter will measure the battery at 20 volts dc, which ways do the contactor wires touch in this diagram?
                              Attached Files

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                                #55
                                Re: Fixing a multimeter based on VC9805 schematic

                                I solve the problem. I change the u2 integrated diagnostic tl062 in the diagram and the multimetrem started working as it old. If anyone experiences the same problem with me, remember this and benefit from this information. Thank you so much to the good people who answered my questions here...

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                                  #56
                                  Re: Fixing a multimeter based on VC9805 schematic

                                  So does the transistor test and temperature ranges are working?

                                  Originally posted by poker View Post
                                  vref+-vref- =1
                                  So you mean both these two pins are 1V WRT GND or is the difference between the two 1V? If it's the former, we need to look at how these voltages are being generated. If it's the latter, throw away the meter and buy another one...

                                  ------

                                  Ah okay looks like you fixed it, excellent. If it's easier just to replace parts until it works, it's still much faster than doing systemic debug... hence replacing whole meters is sometimes easier.
                                  Last edited by eccerr0r; 11-11-2022, 12:12 PM.

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                                    #57
                                    Re: Fixing a multimeter based on VC9805 schematic

                                    Originally posted by eccerr0r View Post
                                    So does the transistor test and temperature ranges are working?



                                    So you mean both these two pins are 1V WRT GND or is the difference between the two 1V? If it's the former, we need to look at how these voltages are being generated. If it's the latter, throw away the meter and buy another one...

                                    ------

                                    Ah okay looks like you fixed it, excellent. If it's easier just to replace parts until it works, it's still much faster than doing systemic debug... hence replacing whole meters is sometimes easier.
                                    I couldn't find a fault anywhere, then I wanted to replace this part and yes it works as before now, temperature, voltage, all... thank you very much for your help...

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