SA-9900 - Preamp low volume hum

Discussion in 'Pioneer Audio' started by Jerry's TV, Feb 4, 2018.

  1. Jerry's TV

    Jerry's TV Member

    Messages:
    67
    Location:
    Waterloo, Ontario Canada
    Hi all - this is a repost to start a new thread on the SA9900 ... "sa-9900-2sc1451-ksc3503-causes-dc-on-volume-potentiometer".

    http://audiokarma.org/forums/index....on-volume-potentiometer.805352/#post-11288903

    I used LesE's component list (thanks a bunch!!) and have worked through the power supply cards AWR-070B; AWR-077, the input equalizer amp AWK-038, switch card AWS-080, control amp AWG-031 and protection circuit AWM-0701. No changes to Tone Switch AWR-079. Rebuilding the Control Amp eliminated a “pop” with the S9 Tone on/off and clicking through the volume control pot. AWR-070 power rails are +-58V; +- 49.1; Pin 15 -12.85; Pin 17 + 12.45 (a little high).

    I have not changed the main power caps or worked on the power amp yet. It seems to be performing well. I’ll do this next once the pre-amp is fully sorted.

    Audio sounds good and things are working as expected… except for one annoyance. I have a subtle 60 hz background hum on both channels. It is not effected by the volume control, input selection or any other front panel controls. If I set preamp bypass switch to “bypass” for power amp only, the hum goes away. (so it is coming from the pre-amp). When I connect a multi-meter to measure the + 58V or -58 supply on power supply AWR-070B the hum will stop. Coupling issues?

    Control Amp AWG-031 - If I ground the audio pins 4 and 19 input to S9 Tone Switch, I can turn the hum on/ off with tone switch; Tone Off – no hum; Tone On = hum. If I ground pins 3 and 20 after S9 the hum stops and is not switched by S9. Grounding pins 2 and 21 output of Control Amp stops the hum as well.

    If the problem was Tone Switch Board AWS-079, which S9 activates, I should be able to turn the hum on/off with S9 at any time. Re-grounding pin 12 of AWS-079 does not help.
    So, this leads me to think S9 Tone on/off switch is the issue? It is mounted on Switch Circuit Assembly AWS-080, schematic shows no other circuity for S9, just the switch.

    Thoughts and suggestions? I am away for the week so thought I’d put this out to the smart minds on AK for discussion, then come back to it next weekend.
    Brian T
     
  2. Jerry's TV

    Jerry's TV Member

    Messages:
    67
    Location:
    Waterloo, Ontario Canada
    Mark - the unit is assembled with side panels on, no bottom or top covers or cage on the power supply. I am missing the shield plate p/n ANH-233A between the control amp and the power transformer. I fashioned a piece of metal to insert in the gap as a temporary shield and gounded the metal, no change in the characteristic of the sound. I'll try putting all the covers on and see if that changes the hum.
     
  3. LesE

    LesE 110284 Subscriber

    Messages:
    677
    Location:
    Ontario, Canada
    Mark makes a very good point regarding the covers. I seem to recall getting some hum with the covers off so it is important to have the amp completely buttoned up before evaluating it's background noise.

    Regarding AWR-070 pin 17, I belive that the 10V indicated in the SM may be incorrect. The voltage on pin 17 is used to mute the audio output of the preamp during power up. Upon turn on, Q7 is on and pin 17 is around -48V. After a few seconds, C20 charges sufficiently to turn off Q7 allowing pin 17 to swing positive and turn on JFETs Q7 & Q8 on the Switch Assembly which allows audio to pass through to the power amp. With Q7 off, the voltage on pin 17 will be set by the voltage divider formed by R20 & R21 which should be around 18V.

    Next time in, I am planning on revisiting Q11 & Q12 on the control board. I haven't yet decided if I will replace R41 & R42 or go with KSC1845s with TO-92 heatsinks for insurance. The gain of the C1845 will result in a somewhat higher operating point than the 5V that you get with the C1451 but I don't imagine that it would be an issue.

    Les
     
  4. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

    Messages:
    20,889
    Location:
    Bensenville,Illinois
    When a whole interim post I was working on for this thread recently disappeared
    (did you rename the thread or delete it and start over?)
    I felt like quitting. All that typing LOST. But NOT the research...

    Ground the volume pot's wipers - that is the input to the circuit that you are muting the output of for testing.
    By ground I mean earth, to a section of the chassis that does not have ground currents flowing through it.
    Perhaps pin 10 to pin 11 which is supposed to be earthed, and pin 13 to pin 12.
    OF COURSE the volume control should be set at minimum volume.
    Try first with pins 11 and 12, THEN if hum, try a separate wire to a ripple free part of the MAIN chassis that is part of the star ground, if you cannot visually trace the solid metal of the sheet to the ground point, don't trust it..
    Grounding is a science that too many treat as a black art. Or magic....

    I will be very interested to see what that does. I am casting a gimlet eye to the biasing setup used for the current sources Q7 and Q8. They are part of the voltage amplification stage in this feedback amplifier configuration (Q1,Q3,Q5,Q7,Q9) for the
    They both share a base connection for bias - a voltage divider (R25 1K to -48v reg through a series diode and R26 100K to ground.)
    that is grounded, unlike EVERYTHING else in that circuit running on +48v dc reg and -48v dc reg.
     
    Last edited: Feb 4, 2018
  5. Jerry's TV

    Jerry's TV Member

    Messages:
    67
    Location:
    Waterloo, Ontario Canada
    Hi, this weekend i will try inserting a new shield cover between the power transformer/ control amp and replacing all covers. If not successful i'll ground the volume wipers.

    Mark -- is grounding pins 4 and 19 of the Control Amp and being able to switch the hum on/off with S9 achieving the same as grounding the volume control wipers that are located further back in the circuit? (and yes i did shuffle the original post, sorry for the confusion)

    thanks for the ideas.
     
  6. Jerry's TV

    Jerry's TV Member

    Messages:
    67
    Location:
    Waterloo, Ontario Canada
    Hi -- I installed the power supply cage cover, top and bottoms and sides. No change to the hum. I still need to fab a metal cover ANH-233 between the power transformer and control amp, but I have tested the control amp out, laying horizontal, as well as installed in place. I have installed a temporary shield to no effect. I'll get that shields cleaned up, but I think I still need to find the leak.

    I grounded the volume control wiper at Control Amp AWG-031 pins #10, #13 - no effect on the hum.
    Here is a summary of chasing the hum through the control amp...
    Ground pins #4,#19 - hum will be switched on/off with S9 Tone Switch
    Ground pins #3,#20 - hum is off always. output of tone amp and input of buffer amp.
    Ground #10, #13 - no effect - volume control wiper input.

    I also grounded pins #7& #12; #1&#2 and #11&8 of tone switch card AWS-079 - no effect to hum on any of these; with or without S9 Tone Switch engaged.
    When I operate the amp on a 45 watt DBT the hum is still there, but at an acceptable level. I think regulated 58v drops to ~ 48v on this DBT.

    Next step is to craft and install the missing shield, as it’s needed anyway.
    Any comments welcome.
     
  7. Jerry's TV

    Jerry's TV Member

    Messages:
    67
    Location:
    Waterloo, Ontario Canada
    HI - I am a little bit closer. I found chassis grounding pin #21 of the Switch Circuit AWS-080 made a significant difference. Currently, pin #21 ground wire routes to the pre/main amp breakout connector, which is routed to the control amp grounds on pins 11 and 13. I added a new ground wire from AWS-080 #21 to chassis ground. I'm guessing this is not intended to be chassis grounded... I'm OK?

    One note - chassis grounding the other ground pins on AWS-080 #14, #17, #28 did not make any difference to the hum, only Pin #21.
    The ground wire from pin #21 is unsheilded and routes near the 110VAC power switch so is suspect that is the culprit.

    Now when i slide a grounded shield (i use the amp's aluminum face plate) between the power transformer and the control amp for the missing shield ANH-233 - it gets better again.
    Now when i ground Volume wiper pins #10 to #11 and #13 to #12 as suggested above, the hum goes away completely.

    I will build my permanent shield for ANH-233 and install.
    Then I'll button it back up with all covers and see where I am at.
     
  8. LesE

    LesE 110284 Subscriber

    Messages:
    677
    Location:
    Ontario, Canada
    Brian ... According to the schematic, AWS-080 Pin 21 should be connected to chassis ground. If you don't already have continuity between Pin 21 and the chassis, then it would be appropriate to provide it as you have done. You will also want to make sure that there is continuity between the chassis and Pin 28. Pins 21 & 28 should also be connected together through the AWS-080 circuit board traces.
     
  9. Jerry's TV

    Jerry's TV Member

    Messages:
    67
    Location:
    Waterloo, Ontario Canada
    Hi Les, I made a new ANH-233 shield and buttoned it back up. It is still there, but much better and only noticeable if you're listening for it. Low level music covers it fine.
    I am going to move on to rebuilding the power amp. I'll let you know how that goes.

    If anything else comes to mind to try, let me know.
     
    Last edited: Feb 12, 2018
  10. nbndtrain

    nbndtrain Active Member

    Messages:
    411
    Location:
    Denver, CO
    Just an idea.Try a good pair of rca cables between pre-out and amp in and see if it helps. I am chasing some strange noise/ground issues on an SA-9100 and believe something is not well with all the RCA jacks.
     

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