Yamaha C-60 Woofer Pumping Problem

Discussion in 'Yamaha' started by kermit_xc, Apr 10, 2018.

  1. kermit_xc

    kermit_xc Member

    Messages:
    84
    Location:
    Austin, TX
    ... and loose output RCA

    god, this preamp sounds amazing (paired with a matching M-60).

    But - this weird thing happens where random channels (sometimes both) start this woofer pumping thing. When this starts happening, the volume adjustment makes no difference, but the Loudness knob definitely does, it increases the amplitude of the pumping to where the sound definitely becomes audible. If I don't touch the loudness knob all is fine - but the meters on the amp keep a steady noise floor, without the meters I probably would never notice the thing

    So - what would cause this issue and how do I go about troubleshooting it ?

    I tried cleaning the volume pots with deoxit (through the 3 holes in plastic housing) but the issue prevails - I cleaned up some glue mess around the voltage transistors (see before shots below).

    DC offset: R:2.1mV; L: 1.9mV

    IMG_1250.JPG IMG_1251.JPG

    cheers!
     

     

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  2. OMGCat!

    OMGCat! AK Subscriber Subscriber

    Messages:
    2,420
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    Probably cruddy solder joints in a whole bunch of spots, especially on those pass regulators stuck to the corroded heat sinks.
    Remove those and clean up the corrosion. There should be holes where you can use screws to properly mount the sinks. The resistors in front of the sinks are probably corroded as well (47k I think) and should be replaced.

    I'd grab a magnifier and see if you can identify any other cracked joints and clean those up as well.
     
    Oldsansui441 likes this.
  3. KingBubba

    KingBubba "Too Much Stuff"

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    Are you using this with a turntable. If so, you are getting feedback from the turntable.
     
  4. kermit_xc

    kermit_xc Member

    Messages:
    84
    Location:
    Austin, TX
    yeah - one resistor is definitely charred. Will replace that
    any source unfortunately - it persists with no signal once it starts happening
     
  5. kermit_xc

    kermit_xc Member

    Messages:
    84
    Location:
    Austin, TX
    I went through the backside and couldn't find any obvious offenders in terms of bad solder joints. One of the resistors is pretty badly charred - it still measures properly though (assuming i did it right).
    No shorts on the transistors themselves. Any chances I get lucky with swapping just that one resistor ?
     
  6. avionic

    avionic " Black Knights " Subscriber

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    Pick up a couple 4.7KΩ 1/4 watt film resistors. They are glue damaged. They sit right in front of those heatsinks.
    [​IMG]
     
  7. kermit_xc

    kermit_xc Member

    Messages:
    84
    Location:
    Austin, TX
    I am wondering about the continuous loudness board - I can't easily access it for visual inspection. Would the regulator resistor answer the sysmptom
    Resistors replaced, volume control assembly deoxit’d ... issue persists. The odd thing is that no matter the input ... the woofer keeps pumping, and turning the loudness only increases the effect. Changing the volume does nothing ... but the right channel meter on the m-60 stays at 1/4th of the scale (2ohm setting) while listening. Super aggravating and I want this resolved without a expensive trip to the audio shop.

    HELP please
     
  8. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    Do you know your way around a schematic ? We need to verify if your power supply voltages are in the ball park and getting where there suppose to go.
     
  9. kermit_xc

    kermit_xc Member

    Messages:
    84
    Location:
    Austin, TX
    I'm a fast learner :) talk to me

    I have the B1 and B2 readings in the meantime

    SM calls for B1 +/- 20V, B2 +/- 16V

    -B2 = -15.9V
    +B2 = 16.1V
    -B1 = -14.7V <=== this is a problem, correct ?
    +B1 = 19V
     

     

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  10. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    Yes it is.It should be alot closer to the +B1 voltage.
     
  11. avionic

    avionic " Black Knights " Subscriber

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    Where did you measure these voltages at ? Copper buss bars ?
     
  12. kermit_xc

    kermit_xc Member

    Messages:
    84
    Location:
    Austin, TX
    This is what I have done so far:
    - replaced R271 and R272 in the T149 and T150
    - visually inspected all joints and components
    - cleaned out all pots and switches with deoxit
    - tried adjusting DC bias on rec out - but the trim pots are impossible to work with

    what's next boss ?
    - measure all transistors in circuit ?

    Screen Shot 2018-04-13 at 10.20.52 PM.png
     
  13. kermit_xc

    kermit_xc Member

    Messages:
    84
    Location:
    Austin, TX
    yessir

    - black probe on ground copper bus bar
    - red on -B/+B

    Screen Shot 2018-04-13 at 10.26.55 PM.png
     
  14. avionic

    avionic " Black Knights " Subscriber

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    Check the resistance of R276 and compare it to R275. In circuit may suffice. We are looking for 2.2 Ω ballpark. These are 1/4 watt fusible resistors.( flameproof carbon film )
     
    Last edited: Apr 13, 2018
  15. avionic

    avionic " Black Knights " Subscriber

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    If you measure greater than 2.2Ω remove or isolate from circuit and remeasure it.
     
  16. avionic

    avionic " Black Knights " Subscriber

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    Location:
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  17. kermit_xc

    kermit_xc Member

    Messages:
    84
    Location:
    Austin, TX
    2.6Ω and 2.6Ω in / out of circuit
    IMG_1484.JPG
     
    Last edited: Apr 15, 2018 at 1:53 PM
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  18. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    Super, no issue there.. Excellent photo..
     
  19. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    Location:
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    What voltages are you measuring at these two resistors ?
     
  20. kermit_xc

    kermit_xc Member

    Messages:
    84
    Location:
    Austin, TX
    R276 289mV DC
    R275 5mV DC
     

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