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Loud hum through speakers - Yamaha CA-1000

Discussion in 'Solid State' started by rjsilva, Feb 14, 2013.

  1. mbz

    mbz Super Member

    Messages:
    2,179
    Location:
    Melbourne, Victoria
    Not much time today. Remove the function board, it should leave one connector into the rear board with a blue wire somewhere in the middle.
    Unplug the connector and measure resistance of this pin to chassis, ie, measure male end first then repeat on female end measuring at tab.
    Give both male/female a spray.
     

     

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  2. rjsilva

    rjsilva Active Member

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    Ok, I guessed at which connector you’re talking about. Only one of them had a blue wire though it was on the left side.

    In any case, with the connector unplugged there was only one pin that gave a reading, that was one of the three clear wrapped wires. None of the female connectors on the board had any reading when it was unplugged.
     
  3. mbz

    mbz Super Member

    Messages:
    2,179
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    Melbourne, Victoria
    Apology, my mistake. The clear insulated wire in the green wrap provides the GND connection to the "coupler switch", so no GND when this connector unplugged.
    ie, no GND connection to the blue wire (EM pin) or the clear insulated wires to power amp (red and white wrap).

    I need to get my head around the key symptom that there is no hum with coupler switch ON and RCA cables between L&R Pre-out/Main IN. However there
    is hum if the coupler switch is then set to off.
     
  4. mbz

    mbz Super Member

    Messages:
    2,179
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    Melbourne, Victoria
    The coupler switch only switches the L&R audio path, no switching of the GND connection. So a dirty switch should only affect the L&R audio quality
     
  5. mbz

    mbz Super Member

    Messages:
    2,179
    Location:
    Melbourne, Victoria
    Regarding that connector, can you measure the resistance between,
    - clear insulated wire in green wrap to blue wire
    - clear insulated wire in green wrap to clear insulated wire in red wrap
    - clear insulated wire in green wrap to clear insulated wire in white wrap

    The coupler switch set to off, RCA cables between preout/main in and connector plugged in. Need to measure at the metal tabs, as an example
    from your previous post, however this somewhat duplication of previous measurements. Alternatively, you could try deep cleaning of the coupler
    switch. ie, unsolder switch, dismantle, clean. Need to be very careful not to damage any of the switch gear. A dirty switch does not match the symptoms.

    upload_2018-7-12_11-57-59.png
     
  6. rjsilva

    rjsilva Active Member

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    Location:
    PA
    Thanks mbz! I really appreciate it. I’ll do the testing tomorrow morning.

    I should clarify that the coupler switch or connecting RCA cables to the preout reduces the hum but doesn’t eliminate it. And if I recall correctly, I ran the preout into a KA7100 and there was some hum in the signal there.
     

     

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  7. mbz

    mbz Super Member

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    2,179
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    from post #15
    Need to be clear on this. Where were the RCA cables plugged, preout only? preout to main in ??? Please repeat the test using RCA cables, coupler On then OFF
     
  8. mbz

    mbz Super Member

    Messages:
    2,179
    Location:
    Melbourne, Victoria
    from post #30, probably confusion on my part.

    Looking like we need to revist the power supply.
    Will wait for you to clear up the hum v coupler v RCA situation
     
  9. rjsilva

    rjsilva Active Member

    Messages:
    444
    Location:
    PA
    I’m sorry for the confusion! That original thread was from 5 years ago and I can’t remember everything clearly. But here is the current state of affairs:

    Coupler on:
    • Preout to main in with RCA - no change in hum
    • Preout but with RCA tips touching main in rings - hum reduced to about 20% (I did this accidentally)
    • Preout connected to output of external CD player (even if unplugged from outlet) - hum reduced to about 20%
    • CD player to main in - no change in hum
    Coupler off:
    • Preout to main in with RCA cables - no change in hum
    • No RCA connected - a reduction in hum (vs coupler switch on)
    Since there is a reduction in hum with the coupler switch off, is it reasonable to conclude that the hum is generated in the power supply and then amplified by the preamp? However, I do remember connecting the preout to the KA7100 and there was hum there too.
     
  10. mbz

    mbz Super Member

    Messages:
    2,179
    Location:
    Melbourne, Victoria
    The pre-out/main in behaviour is something of a red-herring, apart from suggesting the hum is comming from both the pre- and power amp stages.
    Both share a +/-50V supply. This should be checked out, ie, rectifier, caps, transistors, diodes,,,

    Measure dc voltage at +50V wire/tab on either left/right power amp board. Repeat measurement at -50V wire.

    Post a high res pic of the power supply/relay board
     
  11. rjsilva

    rjsilva Active Member

    Messages:
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    Both sides measured between +/-50.0V and 50.2V.

    I hope this image is detailed enough for you. The upload seems to reduce the resolution. Let me know if you need more or a close up of a section. Also, you can see one resistor I replaced above the group of four capacitors, there is another identical one you can’t see. CDCA56DE-FF9F-4543-ACED-3C5124FC1C07.jpeg
     
    Last edited: Jul 12, 2018

     

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  12. mbz

    mbz Super Member

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    Voltages look goodm suggests transistors/zeners ok. The blue diode pack is not OEM, however wouldn't get +/-50Vdc if issues.
    Maybe @avionic can comment on that glue above the caps, ie, ripple from larger caps fed into smaller caps/bass or TR703.

    Need to test for hum/ripple on the regulated supply without a cro, need to think about it (audio probe?)
     
  13. rjsilva

    rjsilva Active Member

    Messages:
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    Location:
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    I should say that I replaced all of the capacitors on the power supply board (a second time) when I was troubleshooting this back in 2013. I saw on that thread that doing that apparently reduced the hum a tiny bit, but I don’t actually remember.

    Additionally, I swapped out the two big capacitors with the originals and the hum was the same (I put the replacements back in after that).

    I say this since you’re pondering the power supply as a potential source of the problem.

    I should also say that it had been working fine for a few years after I recapped it and replaced many parts. I think I did that in 2009/10.
     
  14. mbz

    mbz Super Member

    Messages:
    2,179
    Location:
    Melbourne, Victoria
    The 120Hz hum in both pre and power stages suggests a problem in the regulated supply. There is a glue used by yamaha and
    others that becomes corrosive and conductive, I'm not sure if that glue was used in the CA-1000/in the pic provided. I think not.
    Ideally an oscilloscope is needed to test the regulated supply, I'm in the process of "building" an audio probe which might be
    useful for hum, give me a few days on this.

    I assume you have a cheap (not true rms) multimeter like me. What you could try is measure the "AC" ripple before and after
    the regulation transistors by,
    - Black probe connected to chassis GND, measure AC voltage at R706 as shown, Then measure at the +B solder blob on the relay board
    - RED probe connected to chassis, measure AC voltage at R705 as shown, Then measure at the -B solder blob on the relay board

    upload_2018-7-14_10-3-5.png

    Non-True rms multimeters will interpret the AC ripple on the +/-60Vdc as a large AC voltage, maybe 80-100VAC.
    The point to the test is to confirm the VAC is significantly reduced at the +/-B test points. Important that red probe
    is grounded for the -B test. Next stage would be transistor replacements TR701,2,3,4 but need some measurement
    to identify/confirm.
     
  15. rjsilva

    rjsilva Active Member

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  16. rjsilva

    rjsilva Active Member

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    Here are the results (AC).

    R706: 0.45V
    B+: 0.41V

    R705: 0.49V
    B-: 0.31V

    When I was working with this board I noticed that I had replaced TR701 and TR704.
     

     

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  17. avionic

    avionic " Black Knights " Subscriber

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    Did you replace that non-oem blue dual diode in the bridge rectifier circuit ?
     
  18. mbz

    mbz Super Member

    Messages:
    2,179
    Location:
    Melbourne, Victoria
    You've got a reasonable multimeter, wasn't confused by the dc component, unlike my elcheapo.
    I wouldn't say either +/-B was great 0.31.0.41Vac ripple... The positive rail (TR703,4) would be
    the first to try however I would like some "justification" before replacement.

    What did you replace TR701,4 with?
     
  19. mbz

    mbz Super Member

    Messages:
    2,179
    Location:
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    "Preferred" is TR701, TIP32C, TR704 TIP31C
     
  20. rjsilva

    rjsilva Active Member

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