Death Cap on SS amps

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by TweeDLX, Jan 27, 2012.

  1. TweeDLX

    TweeDLX Member

    Messages:
    3,778
    Joined:
    Mar 24, 2008
    Location:
    Port Angeles, Washington
    The harp player in our band has an old Hohner 12R solid state amp (it actually sounds pretty good). Anyway, I changed out the old two prong cord for a 3-prong. While I was in there, I noticed (what I took to be) a death cap attached to the fuse and removed it. The amp now has a slight noticeable hum. I know NOTHING about SS stuff! I'm thinking this may be an old cap issue, but would appreciate any insight into this I can get.
     
    Last edited: Jan 28, 2012
  2. Blue Strat

    Blue Strat Member

    Messages:
    30,036
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Death caps are not death caps unless the amp is ungrounded (ungrounded cord). There's no danger if the amp chassis is grounded.

    Are you sure that removing that cap is what created the hum? If so, get a new cap and put it in there.
     
  3. TweeDLX

    TweeDLX Member

    Messages:
    3,778
    Joined:
    Mar 24, 2008
    Location:
    Port Angeles, Washington
    Hi Mike,

    I really don't know what caused the hum, as I'd never paid much attention to that amp (other than to bitch about how loud it was). I first noticed the hum when I turned it on after I changed the cord and clipped the cap. I'll try replacing the cap and see what happens.
     
  4. TweeDLX

    TweeDLX Member

    Messages:
    3,778
    Joined:
    Mar 24, 2008
    Location:
    Port Angeles, Washington
    I replaced the cap tonight with no change in hum. Did some grounding changes with no change either.
    The only other thing I can think of is that I wired it wrong, but I don't think so...Looks like this:
    [​IMG]
     
    Last edited: Jan 28, 2012
  5. Blue Strat

    Blue Strat Member

    Messages:
    30,036
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    That should be right. Evidently the hum is coming from something else.

    Maybe a cap job is in order. If you plug a guitar in does the hum change at all?
     
  6. teemuk

    teemuk Member

    Messages:
    2,878
    Joined:
    Apr 21, 2009
    While performing your mods, did you by a chance move some of the mains carrying wires closer to input sections of the amp? Could be radiated hum.

    The mains side circuitry of old solid-state amps is practically identical to mains side circuitry of tube amps so I don't really know what else you should know about it. Unless we go to SMPS or solid-state-controlled soft start circuits (which you won't be finding from amps that are old enough to have "death caps") the mains side stuff doesn't include any active devices at all (neither tubes or semiconductors) so let's just forget them, right?
     
  7. TweeDLX

    TweeDLX Member

    Messages:
    3,778
    Joined:
    Mar 24, 2008
    Location:
    Port Angeles, Washington
    Well...er...um...I haven't tried that yet. :) Do it this morning. I did notice that the hum changed tonally (the volume remained constant) as I rotated the volume pot.
    They're at opposite ends of the amp, and if anything is closer, it wouldn't be by much (as in less than 1/2").
     
  8. TweeDLX

    TweeDLX Member

    Messages:
    3,778
    Joined:
    Mar 24, 2008
    Location:
    Port Angeles, Washington
    No change at all. While looking at the board, I did notice a diode that looks toasted. Another strange thing (at least to me) was that when measuring voltages on these two 1000uf/35V caps, I got a reading of about 22VDC at the positive on one and a reading in mV at the other end. The SECOND cap had a mV reading on the positive end, and a -22VDC on the negative end. Is that normal?
     
  9. teemuk

    teemuk Member

    Messages:
    2,878
    Joined:
    Apr 21, 2009
    It's a bipolar power supply (symmetric negative and positive voltage power supply rails + common ground) and is perfectly normal for solid-state amplifiers.
     
  10. TweeDLX

    TweeDLX Member

    Messages:
    3,778
    Joined:
    Mar 24, 2008
    Location:
    Port Angeles, Washington
    Here's a pic of the caps and diodes.
    [​IMG]
     
  11. TweeDLX

    TweeDLX Member

    Messages:
    3,778
    Joined:
    Mar 24, 2008
    Location:
    Port Angeles, Washington
    The 3 prong arrangement:
    [​IMG]
     
  12. TweeDLX

    TweeDLX Member

    Messages:
    3,778
    Joined:
    Mar 24, 2008
    Location:
    Port Angeles, Washington
    I can't tell what diodes those are. One has an "1N" the other two have "40" then another upside down "40". The one is too burnt to tell. Any ideas on what to use for a replacement? I'm also assuming the orientation of the burnt diode is similar to the one next to it. Correct?
     
  13. teemuk

    teemuk Member

    Messages:
    2,878
    Joined:
    Apr 21, 2009
    I'd try clipping off the death cap entirely and placing the Earth wire into some other point in chassis but the common ground terminal. Besides, for safety reasons alone that wire should preferably have bolted on connection to chassis (not soldered) and preferably it should have more slack than the live and neutral wires so if the power coord ever gets yanked loose the earth wire is the last one to come off. What the hell is that electric tape thing doing in mains wiring...?

    Doesn't matter. Any silicon diode with proper current and power dissipation rating will suffice. Most likely they are 1N4007 but as said using exactly same replacement parts isn't crucial.
     
  14. TweeDLX

    TweeDLX Member

    Messages:
    3,778
    Joined:
    Mar 24, 2008
    Location:
    Port Angeles, Washington
    I twisted and soldered those wires together, covered them with a heat shrink, and applied tape over that. :)
    I originally had exactly the arrangement you described here. I moved the AC ground wire to see if it changed anything. I wound up removing the clip on the AC ground and soldering it to the current point because the nut & bolt are rusted together, and I couldn't get them off.
    By the way, I really appreciate your help with this!
     

Share This Page