Amp now working but loud Hum.

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

  1. petehall347

    petehall347 the brandy coffee man Subscriber

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    stabilised power supply +b3 so maybe filter cap is there .
    do you not have a proper schematic ?
     
  2. petehall347

    petehall347 the brandy coffee man Subscriber

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    i am a little confused here as d901 looks like it sends negative to the yellow wire .
    schematic might clear things up a bit .
     
  3. stoN_Cold

    stoN_Cold Super Member

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    Yes, negative voltage -3.8V to pin 5 HA-12002.
     
  4. petehall347

    petehall347 the brandy coffee man Subscriber

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    speaker protect chip . shouldn't be a hum problem
     
  5. stoN_Cold

    stoN_Cold Super Member

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    I don't believe.
     
  6. grabread

    grabread Active Member

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    Found a better schematic on electro tayna so hopefully that will help. Perhaps diode is leaking under load?
     
  7. grabread

    grabread Active Member

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    Well, getting nowhere with this one, I have added caps to the main caps and still no difference, tried shorting inputs and still no difference. The yellow wire with ac on it I think was a red herring and not the cause. The frequency at the speaker terminal is about 110 hz if I have got my sums correct. mains hum? I have noticed that there are some resistors and small caps missing from the amp board. It looks as though the solder is as it was from the factory and I have assumed they were never there on this model. Parts missing are R759 L&R R758 L&R C725 L &R.
     
  8. gadget73

    gadget73 junk junkie Subscriber

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    I'd expect 100hz there, but yeah, its main power supply ripple. Output of a bridge rectifier has ripple at 2x mains frequency, and I believe you guys have 50 hz mains.

    Check the various power supply rails, pretty sure you will find the noise there someplace. If it was 50hz I'd say you may have a ground loop or an open ground, 100 is power supply though.

    If the parts were never there, they don't need to be. It wouldn't have left the factory humming.
     
  9. grabread

    grabread Active Member

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    D901 chip pin 8 should be 38v and it is for about 5 or so seconds then reduces to 0.8v and then hum starts (through headphones). Doesn't matter if speaker relays are on or off. Does this sound normal behaviour ?

    Clutching at straws now but who knows?
     
    Last edited: Jun 6, 2017
  10. petehall347

    petehall347 the brandy coffee man Subscriber

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    that is when the relay clicks in . so normal operation ... with the chip .
    relay that is wired to it is the mic input .check it out make sure nothing is broke off inside .
    just figuring this schematic out now
     
    Last edited: Jun 6, 2017
  11. petehall347

    petehall347 the brandy coffee man Subscriber

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    this is a bit odd as mic jack seems to connect to the speaker outs .. mic jack looks like headphone jack to me . someone marked this schematic wrongly .. so ry703 must be headphones relay not mic relay .
     
  12. petehall347

    petehall347 the brandy coffee man Subscriber

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    ry701 ry702 arent speaker protection they are just used for switching .
     
  13. grabread

    grabread Active Member

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    Thanks Pete, there is no mic jack so must be head phones? Was wondering if the voltage should disappear like that, 38v down to 0.8v?
     
  14. petehall347

    petehall347 the brandy coffee man Subscriber

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    yes voltage goes down when relay pulls in .. the ic provides the ground for the relay coil when its happy and ready .
     
  15. grabread

    grabread Active Member

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    Thanks Pete.
     
  16. Binkman

    Binkman Addicted Member

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    imo ps. looks like voltage regs are either over driven or bump sub ballast caps to 100v rated caps with same or close to uf caps. filters out the ps to rails and or look at rail sub caps from rails to the driver circuits... it really is a balance act. Elna simics etal rated in the pre boards output can handle most feedback loops for steadier recovery but good rail service standard caps provide that, too. more smoother.
     
  17. grabread

    grabread Active Member

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    Tried a pair of new 10,000uf filter caps today but the hum is still there. Not entirely sure what Binkman is suggesting in the post above, any more ideas anyone?

    Many thanks.
     
  18. Binkman

    Binkman Addicted Member

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    Just saying it might be a voltage regulator chip ic.. going open. you would get 'hum' perhaps buzz. i.e. sub circuit service lower voltage supply.... but I don't have a schematic either. the chip might even measure good in circuit but without a scope it's hard to trace.

    If the the transformer is center tapped.. that could be suspect and could produce a 'hum' through the speakers.
     
  19. grabread

    grabread Active Member

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    Well the situation just got worse, It now has a dead short and blowing fuses. If i put it on the DBT it lights up brightly but nothing is burning up. Seems a dead short perhaps bridge rectifier or transformer?
    Any tests I can do to confirm?

    Many thanks.
     
  20. petehall347

    petehall347 the brandy coffee man Subscriber

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    pull the plug from the wall and look for the short with your meter . dont forget to discharge main filter caps after pulling the plug
     

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