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Reparing my Citation 12

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  • lasse
    replied
    A thought struck me:

    Apart from all the electrolytics that I replaced before, is it a good idea to replace ALL the components on the board on the Cit12?

    That way, the perhaps not so perfect fit-ins for the old transistors could be compensated for?
    Is there a design such as this somewhere already done??

    Regards//lasse

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  • lasse
    replied
    Hello Pete,

    Nice to hear from you.
    Sorry for not observing your reply in this thread as the bringing my Cit12 "back to life" has been on hold for a while.
    I have been busy bringing old speakers back to life the last month or so.

    Soon, I will pick the cit12-project from where I left it.

    Will be back!

    Best regards//lasse

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  • Pete Basel
    replied
    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, 02:10 PM.

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  • Pete Basel
    replied
    Thank you!

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  • ernperkins
    replied
    Glad to see you back Pete!

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  • Pete Basel
    replied
    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.

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  • Sydney
    replied
    That 10.3V is excessive.
    R708, R706 are ok?
    And Diode CR702?

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  • lasse
    replied

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  • Sydney
    replied
    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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  • lasse
    replied
    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

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  • Sydney
    replied
    http://www.raincityaudio.us/blog/har...ower-amplifier

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  • Sydney
    replied
    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

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  • lasse
    replied
    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.

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  • Sydney
    replied
    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.

    Leave a comment:


  • lasse
    replied
    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, 08:00 AM. Reason: Forgot to say "regards" and sign the post

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