Kenwood Basic C2 Earth Hum

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by Chazola, Jun 2, 2018.

  1. Chazola

    Chazola New Member

    Messages:
    4
    Hi, I have a C2 that works well except it has a 50hz earth hum through the output- headphone and main out- it's set level, i.e. doesn't increase/decrease with the volume control. I've disconnected all connections except mains power obviously and it's still there.
    Checked for dry joints etc all over the boards and re-soldered some of the RCA connections and I've re-capped it, still hums. The balance control was knackered of course when I got it so I did the simple jumper by-pass.
    It stops humming if I turn off the output using the front switch.
    I've seen some C2s have a capacitor on the rear panel output RCAs, mine doesn't have this...
    Any ideas appreciated! :)
     

     

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  2. mbz

    mbz AK Subscriber Subscriber

    Messages:
    1,956
    Location:
    Melbourne, Victoria
    Did you confirm with an oscilloscope or other that it's 50Hz? Could it be 100Hz?
    50Hz will be due to mains "ripple" finding its way into the audio path, normally something like a bad ground connection (wire or solder) outside
    chance would be failed rectifier.

    100Hz will be ripple after rectification, again it could be bad GND connections (RCA GND's etc...) Could also be tired caps (eg, filters), transistor....

    Hum on both channels?
     
  3. Chazola

    Chazola New Member

    Messages:
    4
    Thanks, not confirmed with an oscilloscope but with 50hz generated in a sound editing program to compare, plus I'm in the UK with 50hz mains so I guess you could say an educated guess! Hum is steady on both channels, reasonably low-level and only noticeable when near the speakers.

    I'm wondering if it could be to do with taking out the balance control as I just linked the pins and didn't add the resistors shown in other threads, though others have said they did the same with no issues...
     
  4. mbz

    mbz AK Subscriber Subscriber

    Messages:
    1,956
    Location:
    Melbourne, Victoria
    50Hz hum means the "leak" is occurring before rectification. If rectifiers
    were tired I would expect the hum to be more noticable.

    However, looking at the sm shows a simple rectifier/single diode and
    power supply. Single diode means half wave rectification ie, 50Hz "pulses".

    Since both channels affected points to power supply, could be either pos
    or neg voltage rail. Candidates are,

    Q29, Q30
    C79,80(47uf/16V)
    C83,84(470uf/25V)
    C85?(??/25V)
    Q25,26
    C77,78(1000uf/35)
    C71,72(100uf/25V)

    Really need an oscilloscope or maybe an audio probe to track this down.
    If the C2 is unrestored then you could take a punt, most likely candidates being
    C77,78 (1000uf/25V), C71,72(100uf/25V) and Q25,26, these are the main components
    for the opamp power supply
     
  5. Chazola

    Chazola New Member

    Messages:
    4
    Just an update on this- turned out to be the headphone amp- bypassed it completely as I never use headphones, and all nice and quiet :)
     

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