SX-750 Hum

Discussion in 'Pioneer Audio' started by DJH33, Mar 5, 2018.

  1. DJH33

    DJH33 New Member

    Federal Way, WA
    I purchased a Pioneer SX-750 a few months ago in good condition. Both channels operate on all inputs, lights were all out, generally dusty and dirty but in good shape. I was informed by seller and verified in testing that both channels have a low level hum like a ground loop buzz. I decided to continue the restoration thinking I could isolate and fix the problem. I replaced all the lights with LED's, cleaned all pots , cleaned and polished everything, re-veneered the wood and now the receiver looks and sounds great except for the buzz. I went through all the subassemblies looking for the issue. I thought I had isolated to the Power Supply and Protection Assembly. I replaced 8 caps on that board, checked all grounds and even bypassed the protection circuit and relay altogether but the buzz remains. So ... here is the symptoms and the question is, has anyone run into this and solved it. Like many things mass produced, you find one unit with a problem and there are more than likely many more with the same issue.

    1-Buzz appears in both channels at a low volume level that is only heard during quiet.
    2-Buzz is present at all volume levels but does not increase or decrease with the volume control.
    3-If the receiver is powered up with volume at 0, buzz is most often gone. Turn the volume up, play something and then turn back to 0 and buzz is present.

    Any help is appreciated!


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  2. mbz

    mbz Super Member

    Melbourne, Victoria
    Below is my basic response for hums, help get you started. Since the hum is not always present I'd check GND solder and wire connections.

    Hum can be caused by many factors so they are difficult to track down.
    Some causes are,
    - bad connection/break in the ground circuit causing one part of the GND
    circuit to be at a higher voltage than the rest. Suggest resistance check
    key points in the GND circuit to ensure continuity/0-1 ohms. Normally
    results in significant hum.
    - Tired main filter caps. Increased voltage ripple (120Hz) finds its way
    into the audio path via the output transistors. Problem noticable in power
    amp stage mostly. Expect minor hum unless the caps or possibly rectifiers
    are dead.
    - Failure in regulated voltage supply. Again increased voltage ripple (120Hz)
    is fed into preamp/phono stage, caused by failed component diode, transistor,,,
    - non-ideal grounding design. A star configuration is widely accepted as the best,
    with separate paths for analogue/digital GND or divided how you wish. A lot of
    the old designs have a contiuous GND circuit from RCA/s through preamp then power
    amp, psu terminating at the main filters. Minor improvements can be made by
    converting, to some extent, to a star configuration
    - Incorrectly grounded source, problem evident only when source connected via RCA's
    - environmental/external interference/noisy power supply
    - and, and,,,

    Many units do have some hum and it's considered "normal", you have to decide
    if it's excessive and requires your attention.

    First step would be to check all ground connections with a multimeter, amp powered off/unplugged.
    Expect maybe 1-2 ohms (leed resistance...). You should visually check solder/wire connections.
    Reflow if in doubt.

    If hum present when no wires (RCA's/speakers,,,) connected then rule out external source.

    Try and test amp as separate, ie, standalone PRE then as a power amp (use ipod as pre?)
    Is hum present in both pre and power, also on both channels?
  3. DJH33

    DJH33 New Member

    Federal Way, WA
    mbz ,
    Thanks for your reply. That is about where I was at with the grounding. I checked most of the grounds that I think would be the problem and still no luck. I guess its just a matter of persistence until I find it, give up and live with it or ebay it again. To make this even stranger, if I tap the delay relay with the volume turned all the way down, the hum goes away until the volume is turned up again. I thought the relay might be the cause so I basically removed it temporarily from the circuit. The damned annoying hum remained. Its really unfortunate because the receiver looks like new now and sounds great aside from the hum. I guess I will just keep looking.
  4. petehall347

    petehall347 the brandy coffee man Subscriber

    uk.. the middle bit
    if all that fails try shorting plugs into input selected .
    Fredrik 1977 likes this.
  5. DJH33

    DJH33 New Member

    Federal Way, WA
    IMG_1812 (2).jpg I finally got rid of the hum. It turned out to be the power supply filter caps which makes sense and I should have checked these right away. The tapping on the relay threw me off the scent. As it turns out, that also makes sense. Tapping the relay momentarily caused the output to disconnect and allowed what was left of the filter caps to charge up until called upon again. Turning the volume back up, discharged the caps and they could no longer keep up. At least thats my theory and results seem to back this up. At any rate, the SX-750 now looks and sounds like new. I replaced all lights with LED's, cleaned everything re-veneered the cabinet in cherry wood with at dark cherry stain and high gloss varnish. Very happy with it and glad I kept at it until done.
    Jibe01, Ranchman and mbz like this.
  6. Ranchman

    Ranchman AK Subscriber Subscriber

    Northern Colorado
    That looks fantastic. Glad you tracked the problem down.


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  7. Awesomeaudio

    Awesomeaudio AK Subscriber Subscriber

    Sydney, Australia
    Glad to hear you got it sorted.
    I feel your frustration and thankfully I haven't experienced what you have.
    Your shared information would be valuable to others should this issue arise for them.
    It looks beautiful in Cherry wood too.
    Nice work :thumbsup:
    DJH33 likes this.
  8. rocknroller

    rocknroller Active Member

    Which were bad? The 330uf caps on the power supply board or the 15000ugf caps?

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