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pioneer SX-750 240hz hum

Discussion in 'Pioneer Audio' started by rocknroller, Dec 7, 2018.

  1. rocknroller

    rocknroller Active Member

    Messages:
    395
    have this audible hum when nothing connected, irrespective of input selection. When music is playing it's masked but definitely present when not. Both channels. With an audio tester, I confirmed this same hum on pins 16/18 of the power supply board. All DC output voltages from board match schematic, though the AC one is a little higher (8.xvac) Some of the larger caps related to the right hand circuit had more metal top showing then i would expect, though they tested fine on a in circuit esr meter. I replaced them all anyway (everything 100uf and higher on right side of diagram, see below) no improvement. This includes the 330ufs that people hadve changed that fixed hum - but not for me.

    Should something else in that circuit be looked at/measured, or are the large 15000uf filter capacitors suspect? Hate to change them unless absolutely sure, given the cost.


    pioneer sx-750 power supply.jpg
     
    Last edited: Dec 7, 2018

     

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  2. tsd71

    tsd71 RIP Tom Petty

    Messages:
    2,571
    Location:
    New Jersey
  3. Watthour

    Watthour Electron Rancher - JS3600

    240 Hz = 4 ½ wave pulses from a quad bridge rectifier. That should be an indication of the potential problem areas.

    What connects to Pins 16 & 18 on the PS board?

    I bet a nickel earlier today that the major source of the hum in an SX-750 was C17 and C18. I haven't lost any nickels yet today.
     
  4. rocknroller

    rocknroller Active Member

    Messages:
    395
    16 & 18 come from (one of) the bridge rectifier on the power supply 100 board. C17/18 were the first ones I replaced before i even posted. I had seen other posts mentioned those failures but it didn't resolve mine. i keep going down the board unitl everything on the right hand side of that schematic that was 100uf or higher was replaced. No change. I think you owe me a nickel :) But you can keep it if we can resolve this!
     
  5. Watthour

    Watthour Electron Rancher - JS3600

    Looks like I'm running up a tab.

    Do you have an oscilloscope available? If not, an analog voltmeter? They can be used to check the power at 16/18, and the power at ±40V supply (15KµF caps) for ripple, or for AC voltage. Much more than a few volts AC could be creating noise.

    It might also be beneficial to check the grounding at the PS board and transformer secondary.
     
  6. rocknroller

    rocknroller Active Member

    Messages:
    395
    Yes both, though I'm a novice with the oscilloscope usage so would need some guidance on what settings to use to check for ac ripple (1x or 10xprobe AC coupling or DC coupling etc. I am getting 40/-40 at the pins (+/- 0.5v)
    That said, something I did made it go away! I was trying to get a voltage reading on the right hand side 15Kuf caps, i pushed the red/brown wire bundle pair down so I could get on the red led of the cap, and the hum stopped! No idea why. I don't have a before pic, but that bundle was laying against the right hand side fuse
    previously. I've tried to make the hum come back by jiggling those wires around (tempting fate I know but I can't stand not knowing WHY something is doing something wrong.) but i haven't been able to make it reoccur (yet)

    IMG_0257[1].JPG
     

     

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  7. Watthour

    Watthour Electron Rancher - JS3600

    You would normally isolate the scope chassis, use AC trigger and signal coupling, and set the vertical for somewhere around 10V/D to begin.

    But since disturbing the connections at the big filter caps "solved" the problem, I'd concentrate on the connections there and in the surrounding area. Perhaps a bit of flux and solder on the wire wrap connections to the big caps would help, and possibly at the wraps on the AF board as well, where those wires feed in.
     
  8. Watthour

    Watthour Electron Rancher - JS3600

    And dare I suggest that you may be experiencing the beginning phase of a power switch problem?
     
  9. Watthour

    Watthour Electron Rancher - JS3600

  10. rocknroller

    rocknroller Active Member

    Messages:
    395
    What do you mean by "isolate the scope chassis"?
     
  11. Watthour

    Watthour Electron Rancher - JS3600

    Do not connect the grounding prong on the scope's power cord to ground. I would NOT advocate cutting off the grounding prong. Instead, acquire an old-school grounding adapter, and do not connect or use the ground lug. I keep several around specifically for scopes.

    [​IMG]
     

     

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  12. Watthour

    Watthour Electron Rancher - JS3600

    You must understand the ramifications of this. Isolating the scope chassis can allow the scope to reach the potential of whatever the grounding clip on the scope test lead/probe may be connected to. If the item you are working on is a CRT horizontal oscillator, or a PWM servodrive amplifier, the scope chassis could reach a couple KV, so you need to institute protective measures. I always presumed that's the reason the scope controls and faces are usually plastic.
     
  13. rocknroller

    rocknroller Active Member

    Messages:
    395
    I have these, Thanks
     
  14. rocknroller

    rocknroller Active Member

    Messages:
    395
    Luckily I only plan to use this on consumer level audio equipment (receiver, tape deck etc) As I understand it, these devices rarely see more then 80 volts on the rail
     

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