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Reparing my Citation 12

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  • Reparing my Citation 12

    Hi,

    I posted the beginning of this story...

    http://techtalk.parts-express.com/fo...ion-12-failure

    ...in this thread and I could really use some advice on where to go from here

    I finally came around last night to investigate which transistors is failing and other symptoms of failure:

    See the attached Pictures:
    -One driver and one pre-driver seems to be blown after diod-testing in situ but with the PCB removed
    -The Power-transistors are OK
    -The bad channel has a 8V AC on the output, almost no DC offset
    -According to the mounting-instruction (Yes, this was a kit in the 70ies!!), the heatsink is attached to the drivers (see pic nr 2)

    Now;
    -Is AC on the output logical if the indicated transistors are blown?
    -Are there modern replacement transistors?
    -Should both drivers be replaced in this situation?
    -I Think that the BIAS-trimpot caused this trouble in the first Place, is there good replacements?

    Any advice from you electronicians (is this really a Word) would help.

    Best regards//lasse
    Perry Mason talking to his dentist:

    "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

  • #2
    How did you test the transistors?
    The schematic has voltage test points - did you test these?
    Yes bias pots are available.
    "Not a Speaker Designer - Not even on the Internet"
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

    Comment


    • #3
      Originally posted by Sydney View Post
      How did you test the transistors?
      The schematic has voltage test points - did you test these?
      Yes bias pots are available.
      Hi,
      I decided to take advantage of the fact that the PCB can very easily be removed and thus, a lot of the transistor-connections will be easily reached.
      You can see that on the schematic.

      I used a diod-test-mode on my multimeter, testing "both directions x 3"
      No, I choose to make the first set of tests, the diod-tests, without live power in the amp, but I tested the power-supply and the output voltages (+/- 41 V) are there.

      Regards//lasse
      Last edited by lasse; 03-26-2017, 07:00 AM. Reason: Forgot to say "regards" and sign the post
      Perry Mason talking to his dentist:

      "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

      Comment


      • #4
        That design doesn't have an adjustment for DC offset - the pot is for bias on the output transistors
        Did you start at the input of the board and check each resistor?
        btw I just finished a repair on one of my DH-220 - there was excessive voltage at the Q1 collector - the problem was an open R1.
        "Not a Speaker Designer - Not even on the Internet"
        "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

        Comment


        • #5
          Originally posted by Sydney View Post
          That design doesn't have an adjustment for DC offset - the pot is for bias on the output transistors
          Did you start at the input of the board and check each resistor?
          btw I just finished a repair on one of my DH-220 - there was excessive voltage at the Q1 collector - the problem was an open R1.
          Hi again,

          No, I haven´t checked the resistors. Perhaps next step?
          The voltages with the amp running?

          regards//lasse
          Perry Mason talking to his dentist:

          "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

          Comment


          • #6
            Yes - for the voltage tests - Powered with no signal.
            Negative meter probe clipped to the amplifier ground bus - the Positive probe used to verify voltages at various points.
            In my case - I was getting over 7V where I should have been getting less than a volt.
            In your amp: The + voltage rail is being reduced down to approx +.65V at the Emitter of Q702, Approx 38V at the Collector.
            The Base Input should be very small +.04V as should the Output -.06V

            The resistors can be checked without power
            "Not a Speaker Designer - Not even on the Internet"
            "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

            Comment


            • #7
              http://www.raincityaudio.us/blog/har...ower-amplifier
              "Not a Speaker Designer - Not even on the Internet"
              "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

              Comment


              • #8
                Hi,

                Yes, all the caps are replaced in my unit and the old ones measured "all over the Place", so I think that once I get the unit fixed, it will be a good addition to my "inventory" of vintage sound-gear. And a Epic unit from my high-school days when you could work extra for a year to buy a preamp...


                Thanks for the link to that positive story on Cit12!

                regards//lasse
                Perry Mason talking to his dentist:

                "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

                Comment


                • #9
                  This is a closeup of the fix to my DH-220. Note the offset pot in the right; the bias ( out of focus ) in the background.
                  This style is still available; newer replacements look like rectangular blocks with the adjustment screw at the top not the side.
                  Click image for larger version

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                  "Not a Speaker Designer - Not even on the Internet"
                  "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

                  Comment


                  • #10
                    Hi again,

                    Now, I´ve checked all the voltages in the blown channel (and compared with the good channel)

                    See Pictures below. No doubt about it, is it?
                    A number of transistors will have to be replaced.

                    But How many?
                    And while I´m at it-should I replace the ones in the goodish channel? (the voltages ar OK, but not perfect-perhaps a adjustment issue)

                    regards//lasse
                    Perry Mason talking to his dentist:

                    "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

                    Comment


                    • #11
                      That 10.3V is excessive.
                      R708, R706 are ok?
                      And Diode CR702?
                      "Not a Speaker Designer - Not even on the Internet"
                      "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

                      Comment


                      • #12
                        Hi Lasse very sorry for the delay here, I've been off the forums for a long time.
                        As I've mentioned, I built this amp from scratch as a kid so I know it well and
                        there are several threads over at DIYaudio where I've helped people fix their
                        Citation 12 amps. There is even a SPICE simulation file for it there.
                        Please let me know if I can help.
                        Are you aware that this amp is an adaptation of an old reference design in a
                        late 1960s RCA transistor manual?

                        Are the output transistors in sockets in your unit?

                        Pete B.

                        Comment


                        • #13
                          Glad to see you back Pete!
                          "Everything is nothing without a high sound quality." (Sure Electronics)

                          Comment


                          • #14
                            Thank you!

                            Comment


                            • #15
                              A few comments:
                              When you do the transistor "diode" test be sure to check ohms C-E and E-C sometimes there are shorts or
                              leakage, it should be very high impedance.

                              Notice that one end of the bias pot is open, it should be connected to the wiper since the intent of the design
                              if to have 0 - 4.7K ohms of adjustment range but if the wiper lifts we get an open circuit. With the connection
                              we get 4.7K if the wiper lifts.

                              Make sure that all the caps have the correct polarity.

                              You are probably right about those failed transistors and there are no modern replacements for those TO5's with
                              integral heatsinks. The original design in the RCA manual employed 40409/40410 drivers which are 90V 1A
                              devices, those in that circuit are 90V 2A devices - perhaps the HK engineers found it was required when
                              stressed hard.

                              Pull the drivers if you are sure that they are bad, test them again out of circuit.
                              Do you have a preference for a TO-220 or TO-126?
                              Last edited by Pete Basel; 10-13-2017, 01:10 PM.

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