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Kenwood CS-4035 Oscilloscope repair questions

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    Kenwood CS-4035 Oscilloscope repair questions

    Hola all,

    I recently picked up two dead scopes in a job lot. One had missing and blown components as well as a toasted trace on the PSU, whereas the other was complete and not burnt.
    Anyway, managed to get both PSU's working enough to switch the scopes on, with the LV rails all more less where they should be, and that is;
    +140v
    +8v
    +5v
    -8v

    Minus 8v is the adjustable reference voltage, which actually doesn't seem to adjust on either PSU. There is a KMA 01 hybrid IC with a couple of op-amps in it which the voltages pass through.

    The problems arise when the PSU is connected to the main board and immediately all the rails drop by around 20% or so.

    I'm just wondering how to go about locating the problem? I have a millivolt meter but no working scope.

    Would I be correct in thinking that considering how the fault is across all the rails and that all rails are dependent on the minus 8v, that this is where the problem should lie?


    There are 37 electrolytics on each main board. I'm planning on testing them first.


    Also enclosing the service manual, in case anyone wants a look...


    Would be great to get these working.
    Attached Files
    Last edited by Crystaleyes; 05-02-2022, 07:44 AM.

    #2
    Re: Kenwood CS-4035 Oscilloscope repair questions

    can you post a photo of the kma01 hybrids?

    i see a 33uf cap, wondering if it's tantalum or an electrolytic!

    Comment


      #3
      Re: Kenwood CS-4035 Oscilloscope repair questions

      The power supply's rails are all interdependent which makes troubleshooting really difficult.
      The -8V rail is the master and if it sags, it will upset the other rails. The +8V (thus +80V and +140V) regulators use the -8V as their reference.
      The module schematic does not look quite right, there should be a path to some reference for the top two op-amps (140V, 5V reg) but not a big deal.

      So I would watch the -8V rail for shifting, it's critical that rail does not sag when you plug in the other board.
      If the HV has problems it will overload the raw +/-10V rail which feeds the +/-8V regs. I'd think about pulling flyback transistor Q8 to stop that.

      With no load, many outputs i.e. the +80V and +140V will go too high due to the helper resistors R51, R1. This is normal. There's R5 helper as well for the +8V so it might too go higher with no load. You might have to make up some dummy loads to test the PSU stand alone. It's a big hassle.
      Attached Files

      Comment


        #4
        Re: Kenwood CS-4035 Oscilloscope repair questions

        Originally posted by stj View Post
        can you post a photo of the kma01 hybrids?

        i see a 33uf cap, wondering if it's tantalum or an electrolytic!
        Looks more like an smd electrolytic but everything is painted black
        Attached Files

        Comment


          #5
          Re: Kenwood CS-4035 Oscilloscope repair questions

          Hi Redwire

          Originally posted by redwire View Post
          The power supply's rails are all interdependent which makes troubleshooting really difficult.
          Excellent news then?

          The module schematic does not look quite right, there should be a path to some reference for the top two op-amps (140V, 5V reg) but not a big deal.
          On the forum where I found the .jpg (and also a custom replacement board design) they were also discussing this design perhaps being incorrect

          If the HV has problems it will overload the raw +/-10V rail which feeds the +/-8V regs. I'd think about pulling flyback transistor Q8 to stop that.
          Haven't been able to test the HV properly due to lack of probe, only measured on TP12 where it measured -353v which is within spec

          With no load, many outputs i.e. the +80V and +140V will go too high due to the helper resistors R51, R1. This is normal.
          You might have to make up some dummy loads to test the PSU stand alone.
          This is an area I have next to no experience. Would the dummy loads be resistors and how would I calculate the values if so?

          It's a big hassle.
          Possibly. At the moment, it is still a challenge which I'm up for attempting

          Last edited by Crystaleyes; 05-02-2022, 07:50 PM.

          Comment


            #6
            Re: Kenwood CS-4035 Oscilloscope repair questions

            that cap on the hybrid probably needs to go

            Comment


              #7
              Re: Kenwood CS-4035 Oscilloscope repair questions

              Originally posted by stj View Post
              that cap on the hybrid probably needs to go
              Cool. Thanks.

              Shall give that a go.

              Could it be that there is actually no problem on the main board and that the voltages are dropping due to the -8v rail failing under load?

              The way I had been thinking is that as something is pulling the voltages down then something is 'failing' as opposed to s dead short, so I was planning to test all the caps, followed by the semiconductors etc.


              And here is the IC off the board
              Attached Files
              Last edited by Crystaleyes; 05-03-2022, 06:52 AM.

              Comment


                #8
                Re: Kenwood CS-4035 Oscilloscope repair questions

                Actually, upon closer inspection there appears to be corrosion on the cap and on one of the op amps.



                Doing a quick test shows that only IC pin 3 has disconnected, but that is seemingly unconnected anyway.

                The cap however, gives a reading of 1430nF, an ESR of 115Ω and 32% Vloss, so that is fooked, and will delicate removal
                Attached Files
                Last edited by Crystaleyes; 05-03-2022, 08:35 AM.

                Comment


                  #9
                  Re: Kenwood CS-4035 Oscilloscope repair questions

                  It's a good find, all of my early (1990's) SMT electrolytics did not last for some reason it's like the PCB assembly heat was too hard on them.
                  I'm not sure what solvent would work to dissolve the epoxy coating, or if heat is enough. The pads are likely fragile if this is thick-film substrate with tinned metal on top. It's surely got fragile thick-film resistors printed on the board so scraping is pretty risky.
                  Can you try anything like acetone or xylene and a q-tip swab etc.

                  Comment


                    #10
                    Re: Kenwood CS-4035 Oscilloscope repair questions

                    Originally posted by redwire View Post
                    It's a good find, all of my early (1990's) SMT electrolytics did not last for some reason it's like the PCB assembly heat was too hard on them.
                    I'm not sure what solvent would work to dissolve the epoxy coating, or if heat is enough. The pads are likely fragile if this is thick-film substrate with tinned metal on top. It's surely got fragile thick-film resistors printed on the board so scraping is pretty risky.
                    Can you try anything like acetone or xylene and a q-tip swab etc.
                    Didn't need anything other than a quick blast of heat after running a craft knife along the edges

                    The pads look a bit oxidised but as I am in the humidity of Rio at the moment it's not so surprising on a 30+ year old pcb, and the 33uF cap now repeatedly measures at 43pF!!!

                    Just gonna connect up a standard electrolytic to test and I'll see about tidying things up properly if it works
                    Attached Files
                    Last edited by Crystaleyes; 05-03-2022, 11:46 AM.

                    Comment


                      #11
                      Re: Kenwood CS-4035 Oscilloscope repair questions

                      The cap is across the 5.1V reference zener, so I would expect 5.1V if it's working. Note it's -ve wrt GND, a -5.1V ref. so (+) goes to GND.

                      Comment


                        #12
                        Re: Kenwood CS-4035 Oscilloscope repair questions

                        Originally posted by redwire View Post
                        The cap is across the 5.1V reference zener, so I would expect 5.1V if it's working. Note it's -ve wrt GND, a -5.1V ref. so (+) goes to GND.
                        Thank you for that. I hadn't noticed that minor detail in the polarity





                        So on a more serious tip, why is the cap reversed polarity? What job is it performing being that was around?
                        Last edited by Crystaleyes; 05-03-2022, 11:59 AM.

                        Comment


                          #13
                          Re: Kenwood CS-4035 Oscilloscope repair questions

                          The cap is part of the -5.1VDC reference for the -8V regulator. It then makes the master reference for the other rails. Just to save money, use one low drift zener reference instead of a bunch, like one per for each rail. It's also a single calibration at the factory.
                          I would use a variac to power it up after, just to make sure the -8V rail is working.

                          Back in the 80's these Japanese scopes were copied by the Koreans and made a lot cheaper.

                          Comment


                            #14
                            Re: Kenwood CS-4035 Oscilloscope repair questions

                            i would put an smd tantalum cap across the pads

                            Comment


                              #15
                              Re: Kenwood CS-4035 Oscilloscope repair questions

                              Unfortunately changing the cap didn't resolve the lack of -8v adjustment and therefore neither any of the voltages on the main board, although I suspect there is something else bringing the voltages down once that board is connected.

                              With that board disconnected the voltages are not too far out and the tube has a bright spot in the middle of the screen.


                              Anyway, whilst the Hybrid was off the board, I took the opportunity to take measurements based on the schematic, including the zener and all the resistance combinations which all measured fine.

                              Therefore, if this problem 'is' on the hybrid, I imagine it to be on the TO 072 op-amp, which judging by the capacitor, shouldn't be such a problem to replace. Just need to find one now...

                              Thank you so much though for the support and suggestions.

                              Any fresh ideas on where to head next will still be appreciated

                              Comment


                                #16
                                Re: Kenwood CS-4035 Oscilloscope repair questions

                                Can you measure the DC voltages on the SIP module's pins, just a quick check to see if anything looks off. Probably better with the scope board connected to the PSU board.

                                Comment


                                  #17
                                  Re: Kenwood CS-4035 Oscilloscope repair questions

                                  Originally posted by redwire View Post
                                  Can you measure the DC voltages on the SIP module's pins, just a quick check to see if anything looks off. Probably better with the scope board connected to the PSU board.
                                  I'm presuming that the SIP module is this hybrid IC, and I think you meant to ask to see what 'isn't' off.

                                  Enclosing the schematic again for quick reference

                                  The voltages with the main board 'disconnected' are as follows...
                                  (Schematic listed voltages are in brackets)

                                  Pins;

                                  1: +158.8v (+140v)
                                  2: NC
                                  3: -0.3v (+0.6)
                                  4: +7.33v
                                  5; +3.3v
                                  6: 0.0v
                                  7: +8.6v (+8v)
                                  8: +8.09v (+0.33)
                                  9: +4.84v (-0.33)
                                  10: -9.47v (-8v)
                                  11: -0.65v
                                  12: 0.0v Gnd_


                                  Seems tto me that the op amp has failed internally. (Noticable above are pins 1 and 10, which were 142v and 8.2v at the last time of reading)



                                  Then with the main board 'connected'.. Things get curiouser and curiouser...


                                  1: +100v
                                  2: NC
                                  3: -0v
                                  4: +0.6v
                                  5; 3.25v
                                  6: -0.7v
                                  7: +2.33v
                                  8: +3.45v
                                  9: +3.15v
                                  10: -5.7v
                                  11: -1.26v
                                  12: Gnd_0.0v

                                  It's perhaps a good thing that I have two of these scopes...

                                  Speaking of which I pulled the KMA 01 from the 2nd scope and once again the cap measured bad. This one had 77% VLoss and 110Ω ESR
                                  Attached Files
                                  Last edited by Crystaleyes; 05-03-2022, 08:52 PM.

                                  Comment


                                    #18
                                    Re: Kenwood CS-4035 Oscilloscope repair questions

                                    Originally posted by stj View Post
                                    i would put an smd tantalum cap across the pads
                                    I'll bear that in mind for once these are fixed.

                                    One question, "would the failing electrolytic be enough to fry the op-amp?"

                                    Comment


                                      #19
                                      Re: Kenwood CS-4035 Oscilloscope repair questions

                                      Can you measure voltage across the new cap, things are acting like the 5.1V zener is pooched or the op-amp driving pin 9 is not working, or 10V zener D5 or Q5 has failed, connecting to pin 9.
                                      I'm guessing if the 5.1V zener reference is not there (shorted or open), the op-amp can look bad but it's really just making the wrong output based on a bad ref.

                                      What really doesn't make much sense is pin 11 (-8V feedback pot) reads more negative when pin 10 is more positive. That's either a bad measurement or that op-amp has a failed input.

                                      Comment


                                        #20
                                        Re: Kenwood CS-4035 Oscilloscope repair questions

                                        Originally posted by Crystaleyes View Post
                                        I'll bear that in mind for once these are fixed.

                                        One question, "would the failing electrolytic be enough to fry the op-amp?"
                                        no idea
                                        but op-amps fail a lot anyway

                                        btw, TO 072 is probably supposed to be TL072

                                        Comment

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