Amp now working but loud Hum.

Discussion in 'DIY' started by grabread, May 29, 2017.

  1. grabread

    grabread Active Member

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    I have been working on a solid state amp, it's now in working condition but there is a loud hum (AC)?
    It is always present on both channels and is not altered with the volume control.
    there is about 180mv of ac on each channel dc is only - 1.8 mv .
    I have attached a couple of photos of the scope connected to the speaker teminal . one of just the AC ? wave and the second is with a 1khz tone on top of it.

    I am thinking it is a capacitor problem any idea's?
    DSC02962.JPG DSC02964.JPG

    many thanks.
     
  2. petehall347

    petehall347 the brandy coffee man

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    try shorting the signal inputs ..
     
  3. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Sounds like a PS problem to me.
     
  4. petehall347

    petehall347 the brandy coffee man

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    i haven't heard it yet:D
     
  5. grabread

    grabread Active Member

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    Could you elaborate please Pete,
     
  6. petehall347

    petehall347 the brandy coffee man

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    just trying to eliminate ground hum .. if you short main amps in to ground it might make a difference . or might not . see what happens ,
    it is probably a fault in the power supply but easy things first .
    p.s you never said what you have done already and what amp is it ?
     
    Beau Geste likes this.
  7. grabread

    grabread Active Member

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    It's a Hitachi HA -6 . Bought as non working. When opened up it was obvious it had been worked on and given up on. 14 transistors missing and about a dozen fuse resistors needed replacing as they had gone way off spec. I have replaced all those components and had a good check around and it now produces sound.
    I hope it's not the main power supply caps as they are huge and likely to be expensive.
     
  8. petehall347

    petehall347 the brandy coffee man

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    what value are they ?
     
  9. grabread

    grabread Active Member

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    12000 uf 63v
     
  10. petehall347

    petehall347 the brandy coffee man

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  11. grabread

    grabread Active Member

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    The original caps are almost the size of coke cans, I would like something that is almost the right size if poss. But is it the main caps at fault or something else?


    upload_2017-5-30_18-51-26.png
     
  12. petehall347

    petehall347 the brandy coffee man

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    tack some extra ones to them and see if it gets better ..thats if you have some .
     
  13. grabread

    grabread Active Member

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    Thanks Pete. What value should I use?
     
  14. petehall347

    petehall347 the brandy coffee man

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    depends what you have to hand .
     
  15. grabread

    grabread Active Member

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    I have had a probe around the power supply today with the scope. the main supply looks clean but there seems to be a supplementary supply to the amp board. the yellow wire has both dc and half wave? ac on it ( around 21 v) the yellow wire seems to be supplied by a diode and a cap. Diode checks ok on the meter and cap is not shorted. I have added some more pics.
    Any Ideas?
     

    Attached Files:

  16. stoN_Cold

    stoN_Cold Super Member

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    What is the voltage on the main capacitors?
     
  17. gadget73

    gadget73 junk junkie Subscriber

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    Got a schematic? That would help ID that wire and figure out what it should have on it. Taking a guess, its probably not supposed to have a vast amount of ripple but without knowing exactly what you're measuring or what its supposed to do, its hard to say if its wrong.
     
  18. grabread

    grabread Active Member

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    It's partly stripped at the moment so not able to apply power to try. I think I am in the correct area with the yellow lead that I identified with ac on it.
    Anyone understand this schematic of the power supply and if the yellow wire attached to d901 is likely to have ac on it

    upload_2017-5-31_23-24-40.png
     
  19. petehall347

    petehall347 the brandy coffee man

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    yes it will be noisy as no filtering apparent ..where does the yellow wire go to ? p.s that looks like underside of the board .
     
  20. grabread

    grabread Active Member

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    Thanks Pete, the service manual is not that good (glad tp have it though). I'm trying to see where it goes to, if I am reading this correctly it goes to Q801 and zd801


    upload_2017-5-31_23-52-23.png
     

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