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60 cycle hum

Discussion in 'Troubleshooting and Repair' started by ValleyJim, Feb 19, 2014.

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  1. ValleyJim


    Apr 18, 2012
    Hi all

    I have a guitar amp that is brand new out of the box and I have a hell of a hum. So I called the manufacturer and told them and they sent me another unit, did not even want the original one back. <???>

    Anyway, I wanted to see if I could fix this one. If I am not mistaken a 60 cycle hum comes from bad caps or am I wrong there? Been a while since I have been in electronics class. :)

    I pulled the back and saw about 6 caps and am thinking of replacing all of them.

    How can I test a cap to see if it is working properly?

    Any suggestions?
  2. davenn

    davenn Moderator

    Sep 5, 2009
    hey Jim

    if its brand new then its unlikely to be the caps unless its been sitting on the stock shelf for 10 - 20 + years between manufacturing and sale.
    ( OK there is a remote chance that there were a batch of bad caps??)

    More likely its an earthloop or poor earth hum .... firstly I would be checking ALL earth connections right from the input plugs to the output. checking any PSU earth, chassis earth connections.

    Assuming the replacement they sent you is an identical unit and works OK .... no hum ... you could do some comparisons of wiring etc

  3. shrtrnd


    Jan 15, 2010
    I'm with davenn.
    It's probably an earth ground problem (no connection or bad connection).
    And yeah, a few years ago there was a big problem with a large quantity of
    electrolyte the Chinese whipped-up. So bad caps (new) sometimes exist.
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    If it's Cheap and Chinese, there's also the possibility that it wasn't assembled or soldered correctly. Look for missing components (by comparing it with the other unit) or solder missing or shorting where it shouldn't.

    If it's nothing obvious, take the chassis out and post a few photos here. That will give us some idea of what we're dealing with, and we should be able to make suggestions from there.

    What service equipment do you have (or have access to)? A multimeter? An oscilloscope?
  5. Jagtech


    Feb 22, 2014
    Does it hum all of the time, or just with volume turned up? Constant hum with volume off indicated power supply faults. Bad caps will produce a 120 cycle hum. Check first for bridged solder joints, or a joint that didn't get soldered at all. A shorted bridge rectifier in the power supply section would also be suspected.
    Its really difficult to adequately test a capacitor without the proper equipment. Better to just replace it with same value and voltage rating, or simply bridge another one across it in circuit.
    If it hums only with volume turned up, you have a bad ground.
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