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    Re: Need Help with this Circuit Please

    Originally posted by tfs2015 View Post
    the 112Jfet's are working because they are brand new I just put them in a couple weeks ago.
    But are they being Bias ON and OFF (no Gate Voltaeg applied) by the control circuit to mute the signal. Put in new parts does not mean they are working if it is not getting the Voltages. It is like putting in new light bulb but you still have bad light switch.
    Those FETs work like as I explained before about the input of the power amp being grounded out (by the FET) to mute the input signal to GND.
    You have one FET per channel (Q201, and Q217) for muting, both FETs are being controlled by the same control line as you see in the circuit page 25. Since both channels are making noise then it has to be something in common which will be the control line to both, but if one channel does not make the thump then it is not the control problem but the FET or the components connected to that muting circuit for other channel that is bad.
    You need to check the DCV between the Source and Gate to see the Voltage level changes when the unit is powered up and when powered down.
    J112 is the N-CH JFET.
    Page 2:
    Low RDS on at zero gate voltage.


    No GATE Voltage means the audio is Grounded out.
    Attached Files
    Last edited by budm; 02-13-2016, 11:18 PM.
    Never stop learning
    Basic LCD TV and Monitor troubleshooting guides.
    http://www.badcaps.net/forum/showthr...956#post305956

    Voltage Regulator (LDO) testing:
    http://www.badcaps.net/forum/showthr...999#post300999

    Inverter testing using old CFL:
    http://www.badcaps.net/forum/showthr...er+testing+cfl

    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
    http://s807.photobucket.com/user/budm/library/

    TV Factory reset codes listing:
    http://www.badcaps.net/forum/showthread.php?t=24809

    Comment


      Re: Need Help with this Circuit Please

      ok will check voltages and report back tomorrow. Thank you for the advice.
      Computer and Small electronics troubleshooting and repair

      Comment


        Re: Need Help with this Circuit Please

        I guess I ran out of time to edit the above so here is the edited version.

        I can not find Z201 on the schematic can you ohm this out to find what it is connected to? If you go to the component layout of the power Amp and use both the copper side and component side it looks like they the Z201 anode side is connected to where D201 anode and R206 connects. The cathode side of Z201 is connected to where D204 cathode and Z204 anode connect. It is very difficult to switch back and forth turning the view and increase the magnification so you can verify this first by Ohming those points out and also by checking me by going the the component/copper layout of the board and verifying for your self that this is where those runs and components are. Lastly, I would verify all of the J numbers on both component/copper layout of the power amp are there by make sure all the jumpers on both sides of the board are in place and that they are not shorted out to the runs they lay on top off.

        Comment


          Re: Need Help with this Circuit Please

          BTW, both JFETs for the MUTING circuits were replaced because they were bad?
          Never stop learning
          Basic LCD TV and Monitor troubleshooting guides.
          http://www.badcaps.net/forum/showthr...956#post305956

          Voltage Regulator (LDO) testing:
          http://www.badcaps.net/forum/showthr...999#post300999

          Inverter testing using old CFL:
          http://www.badcaps.net/forum/showthr...er+testing+cfl

          Tear down pictures : Hit the ">" Show Albums and stories" on the left side
          http://s807.photobucket.com/user/budm/library/

          TV Factory reset codes listing:
          http://www.badcaps.net/forum/showthread.php?t=24809

          Comment


            Re: Need Help with this Circuit Please

            Originally posted by budm View Post
            BTW, both JFETs for the MUTING circuits were replaced because they were bad?
            This is a mystery as the Z201 does not appear on the schematic. Wonder if it was put in the wrong place? Maybe it took out the JFETS. Or the board designer has layout the board wrong?

            Comment


              Re: Need Help with this Circuit Please

              Based on the PCB layout of the power amp board, this is how Z201 is connected.
              Attached Files
              Never stop learning
              Basic LCD TV and Monitor troubleshooting guides.
              http://www.badcaps.net/forum/showthr...956#post305956

              Voltage Regulator (LDO) testing:
              http://www.badcaps.net/forum/showthr...999#post300999

              Inverter testing using old CFL:
              http://www.badcaps.net/forum/showthr...er+testing+cfl

              Tear down pictures : Hit the ">" Show Albums and stories" on the left side
              http://s807.photobucket.com/user/budm/library/

              TV Factory reset codes listing:
              http://www.badcaps.net/forum/showthread.php?t=24809

              Comment


                Re: Need Help with this Circuit Please

                If I connect the left and right inputs to the power amp and then ground that to the audio signal GROUND, I am still getting a little bit of audio going through the speakers but a lower level. Looking at page 24 in the manual, CN5 pin 3 is fed by -15vdv signal that connects to the anode of Z2. The cathode of Z2 connects to R143 (mixer circuit board) and that connects to CN5 pin 3, I am assuming that is control line that you have been talking about. Z201 only has .690 vdc across it. According to the parts list for the amp board, Z201 is rated for 6.8 volts so that is what I should read across it correct?
                Computer and Small electronics troubleshooting and repair

                Comment


                  Re: Need Help with this Circuit Please

                  Originally posted by budm View Post
                  Based on the PCB layout of the power amp board, this is how Z201 is connected.

                  You are right BudM thanks for the correction on the one side.

                  Comment


                    Re: Need Help with this Circuit Please

                    Originally posted by tfs2015 View Post
                    If I connect the left and right inputs to the power amp and then ground that to the audio signal GROUND, I am still getting a little bit of audio going through the speakers but a lower level. Looking at page 24 in the manual, CN5 pin 3 is fed by -15vdv signal that connects to the anode of Z2. The cathode of Z2 connects to R143 (mixer circuit board) and that connects to CN5 pin 3, I am assuming that is control line that you have been talking about. Z201 only has .690 vdc across it. According to the parts list for the amp board, Z201 is rated for 6.8 volts so that is what I should read across it correct?
                    So diode 4148 which it shows on the component layout is not a zener diode. If your diode is marked 4148 it is wrong. So, you also should check the direction of the diode according to BudM's drawing and that it is connect where BudM's drawing says it is.
                    Attached Files

                    Comment


                      Re: Need Help with this Circuit Please

                      ok I really cannot figure this out. Z201 tested fine out of circuit with 6.5 voltage drop. In circuit it only shows a voltage drop of 670 mV something here just does not seem correct to me.
                      Computer and Small electronics troubleshooting and repair

                      Comment


                        Re: Need Help with this Circuit Please

                        Originally posted by tfs2015 View Post
                        ok I really cannot figure this out. Z201 tested fine out of circuit with 6.5 voltage drop. In circuit it only shows a voltage drop of 670 mV something here just does not seem correct to me.
                        Have you confirmed the direction as per BudM's drawing? Also, what is the number on the diode? They need to be reverse biased to operate. If all this is OK then this is telling you that the current flow is in the forward bias dirrection with the 0.670 Volts.

                        Comment


                          Re: Need Help with this Circuit Please

                          Go back to the Theory of operation on page 6 to get an understand of how the diode is working in the circuit. I believe if BudM's drawing is correct the zener is in backwards and the component layout is wrong. So to reverse bias the diode as the negative rail increase, the anode needs to be attached to the negative rail, when the negative rail gets to -6.8 volts across the zener the anode will be at -6.8V and the cathode will be 0V. This would reverse bias the zener and make it conduct.

                          Comment


                            Re: Need Help with this Circuit Please

                            So does that mean that it is installed wrong on the board?
                            Computer and Small electronics troubleshooting and repair

                            Comment


                              Re: Need Help with this Circuit Please

                              Turned around Z201 and guess what less pop on Left and Right channels However it is not as bad as before. Reading 6.62 vdc across Z201. I Guess with this circuit you cannot mute out all of transient pop noises during power off or is there something else that I need to check?
                              Computer and Small electronics troubleshooting and repair

                              Comment


                                Re: Need Help with this Circuit Please

                                Originally posted by tfs2015 View Post
                                So does that mean that it is installed wrong on the board?
                                I believe it is and that is why you are reading 0.6 volts on the board and out of the board when you reverse bias it you read 6.8 volts. If you forward bias a zener diode it will work just like a regular silicon diode. You should verify this for yourself. Take the drawing that BudM has so helpfully made and verify with an visual sight and an Ohm meter exactly how the diode is polarized in the board and that it is connected to components and voltage rail. Understanding for a Zener diode to operate correctly it need to be reverse bias for the 6.8 volts to drop across it. Then read the Theory of Operation to confirm how it is working in the circuit. Confirm one side on the diode is connected to the negative rail. Understand that the anode of the diode needs to be 6.8 volt negative as respect to the cathode side to be reverse bias. You should always confirm for yourself this operation. Anyone, no matter who they are, can make mistakes.

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                                  Re: Need Help with this Circuit Please

                                  Perhaps another component is damage or even the diode is damaged. So you may not be done with the muting circuit. I would still check out the other components in the circuit as the circuit was connected wrong.

                                  Comment


                                    Re: Need Help with this Circuit Please

                                    ok will do. We need to get part of this thread moved to a new one. Especially the troubleshooting and theory part. I have learned a lot from working on this system. Fortunately according to Fender the new PA systems are designed better then the old ones.

                                    Many thanks for the help and the knowledge passed on.

                                    I guess I will have to check D201 next.

                                    Thanks,
                                    Tom S. Wells, MN
                                    Computer and Small electronics troubleshooting and repair

                                    Comment


                                      Re: Need Help with this Circuit Please

                                      One other thing, you might what to look up the tolerance of that zener diode to see if it is in tolerance. Also, you might what to check the voltage across the other Zeners that BudM has circled in his drawing to verify the voltages and also look them up for tolerances. I think BudM also, gave you a link to a speaker protection device that you might look into.

                                      Comment


                                        Re: Need Help with this Circuit Please

                                        Well for some reason it is working fine now. Might have been a loose connection somewhere. I will have to do some more looking.
                                        Computer and Small electronics troubleshooting and repair

                                        Comment


                                          Re: Need Help with this Circuit Please

                                          Another troubling thing is on the schematic C230 and C215 are marked as 4.7 uF caps with a polarity and on the parts sheet they are marked 47 uF caps with no indication of polarity. I would not want polorized caps in this place. See what you have at these points. I would also get a hold of the board design engineer at Fender and ask him what he had intended for these caps.

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