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Hiwatt custom 20 tube amp hum problem

Discussion in 'Repairing Electronics' started by frank57, Nov 8, 2009.

  1. frank57

    frank57 New Member

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    Took this amp in for repair.
    I've been told it has 60 hz hum and static issues.
    The rectifier diodes are good but they seem to think it might have to do with the power supply.
    I can't find a schematic anywhere.
    Any ideas?
     
  2. frank57

    frank57 New Member

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    More info:
    The noise increases with the master volume being turned up.
    The tubes are good.
    The gain low and master up: increase in noise and some buzz.
    Gain high and master low: putting master up increases hum and noise.
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Valve amps are dead easy to mend, you don't really need a circuit, they are pretty similar and crude.
     
  4. dave

    Dave New Member

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  5. frank57

    frank57 New Member

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    This amp is a pcb board valve amp.
    I was told without the schematic the cost is very high.
     
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Many modern repair agents don't understand valve amps, but they are dead easy as they are so simple.

    A friend of mine bought a fairly new (only a couple of years old) Fender 60W combo off a guy he knew for £5. The original owner had taken it to three different repair shops, and been told it wasn't worth repairing.

    I told my mate it couldn't be much, and even told him EXACTLY what was wrong with it before he brought it me. I was dead right, it wanted anode load resistors, as valve amps always have.
     
  7. frank57

    frank57 New Member

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    So what do you think is causing the problem?
    They seem to think it has something to do with the power supply.
    The diodes check good.
     
  8. mneary

    mneary New Member

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    The electrolytic cap that filters the B+ for the preamp stage is probably dried out. The preamp stage probably has a B+ power that's behind a second RC filter.
     
  9. bountyhunter

    bountyhunter Well-Known Member

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    +1 Anytime you have an old piece of equipment, suspect the electrolytic caps. They have very short life spans compared to most electronics. If you want to keep and use the piece, replace all the power supply filter caps.
     
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    If you had your location filled in, perhaps someone near you could have a look for you?.
     
  11. frank57

    frank57 New Member

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    This amp actually is from probably 2004.
    I'm in Verdun near Montreal Canada.
     
  12. frank57

    frank57 New Member

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    Here's a board drawing.
    How do I make a schematic from this?
    Anybody notice any problems?

    [MOD-EDIT: IMAGE REMOVED DUE TO ADULT CONTENT.]
     
    Last edited by a moderator: Dec 7, 2009
  13. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    Well I see that when ever I click on the picture I am routed to a 'dirty girls' site! :eek:

    Thats a problem sort of. :D

    Yep, I keep trying and they keep coming back! :D:D:D
     
  14. frank57

    frank57 New Member

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    Crap! I'll try to find another place to post it.
     
  15. frank57

    frank57 New Member

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    Thanks Nigel!
    Here it is. I drew it as best I could.
     

    Attached Files:

    Last edited: Dec 8, 2009
  16. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Why not just post it here? - click on 'Go Advanced', then on 'Manage Attachments'.
     
  17. frank57

    frank57 New Member

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    Thanks for the tip! I redid my post.
     
  18. mel8030

    mel8030 Member

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    is the hum 60 or 120 hertz? Is the power supply a full wave rectifier type? If so, it seems the hum should have a 120 hertz fundamental if the filter caps are bad.
    Might have some heater to cathode leakage that many basic tube tester don't catch. If the preamp tubes have the same number as later tubes, you can try interchanging them to see if the hum level changes.
     
    Last edited: Dec 12, 2009
  19. frank57

    frank57 New Member

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    I was told it's 60 hz hum with some high end static.
    The hum increases as the master volume is turned up.
    Static shows up more with distortion. Not there at low volume but turn the gain up more static.
    I'm waiting for it to be looked at but it's a month already.
    The tubes were replaced and the hum I think is coming from the first 2 tubes.
    The wires on the outside also were not tied down to the chassis.
    Not sure about the power supply. Can you tell anything from the Layout?
    One suggestion was to ground the input jack straight to the ground bolt.
    But I think the jack might be defective. There still appears to be signal with nothing plugged in. The hum is still there.
     
  20. frank57

    frank57 New Member

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    Okay, the tubes and caps and resistors are all good.
    Tube 4 has a bit too high voltage 350 instead of 285.
    The hum seems to be coming from the power supply.
    Anybody got any ideas?
    My tech was looking at the output transformer,it's kind of small,
    but it looks like it's the right part.
     
  21. HiTech

    HiTech Well-Known Member

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    They want you .... go to them!
     

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