Vox AC30 Intermittent Buzzing Noise

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by MrKite89, Jun 24, 2019.

  1. MrKite89

    MrKite89 Member

    Messages:
    1,404
    Joined:
    Nov 15, 2014
    Location:
    Milano, Italy
    The buzzing is present also with the back panel removed, which yes, has an aluminium foil applied to it...

    I'll proceed with surgical testing and cleaning of the input jacks as suggested... :)
     
  2. MrKite89

    MrKite89 Member

    Messages:
    1,404
    Joined:
    Nov 15, 2014
    Location:
    Milano, Italy
    It probably was the grounding washer. :)

    The amp will be under observation for the next days but seems OK now, after cleaning washer and chassis with light sandpaper and applying Deoxit the amp stayed quiet for a couple of hours of playing. :aok

    Fingers crossed...

    Thanks a lot HotBluePlates and pdf64! :)
     
    HotBluePlates likes this.
  3. MrKite89

    MrKite89 Member

    Messages:
    1,404
    Joined:
    Nov 15, 2014
    Location:
    Milano, Italy
    The noise is back. :(

    I will check the jacks and all the connections again...in the meanwhile, new ideas?
     
  4. HotBluePlates

    HotBluePlates Member

    Messages:
    5,759
    Joined:
    Mar 27, 2017
    But the jumper still kills the noise? You may want to try installing that and waiting with the amp on for a couple hours to see if the noise fooled you before by being quiet for a little bit, then returning.
     
    MrKite89 likes this.
  5. MrKite89

    MrKite89 Member

    Messages:
    1,404
    Joined:
    Nov 15, 2014
    Location:
    Milano, Italy
    I'll check tomorrow and report back... :)

    Maybe I can also try to solder a temporary jumper wire from the jacks to a "solid" ground in the circuit to see if the noise disappears...
     
  6. MrKite89

    MrKite89 Member

    Messages:
    1,404
    Joined:
    Nov 15, 2014
    Location:
    Milano, Italy
    With a jumper wire with alligators between ground (C18 in this case) and the meeting point of R27 and R31 the higher frequencies portion of the "noise floor" of the amp gets eliminated, so I don't know for sure if the noise is there but I can't hear it (becuse it is a high freq buzz). It seems to go away.

    The jacks are still connected to ground even without the grounding washer, with a 3-conductor cable which carries signal and ground: if I loose the top mounting "screws" of the jacks there's no difference in noise, and intentionally producing false contacts (by moving the jack up and down againts the chassis) doesn't alter the buzz at all.

    At this point I'm pretty sure that the jacks are fine, and so the connections: also my DMM with "beep" function for contacts produces a solid "beeeep" in every grounding point in the jacks area...

    Is there something else which can produce this noise?
     
  7. ldizzle

    ldizzle Supporting Member

    Messages:
    5,083
    Joined:
    Jan 13, 2009
    Location:
    Tejas
    Is the facility it's plugged into actually grounded?
     
  8. HotBluePlates

    HotBluePlates Member

    Messages:
    5,759
    Joined:
    Mar 27, 2017
    The capacitors that connect across the jacks to the ground washers are there to eliminate high frequency noise. I don't know why any of them would be bad, but I'd guess they've gone (or are going) open-circuit. Get yourself a couple 10pF and 100pF caps, and try bridging the existing caps. If the noise stops for good, the cap(s) is/are bad, and need replacing.

    "Continuity" function.

    Be careful with it: some meters will give the beep for continuity when the resistance between the probes is as high as 50-100Ω. That's too high for the noise suppression caps to do their job (some implementations of a Ground Lift switch use a cap in series with a ~50Ω resistor).

    Instead, use your meter's resistance function. Look for a reading below 1Ω (your meter will probably show at least 0.2Ω for a dead-short).

    EDIT: Just saw @Kyle B said the same thing here.
     
    Last edited: Jun 30, 2019
  9. MrKite89

    MrKite89 Member

    Messages:
    1,404
    Joined:
    Nov 15, 2014
    Location:
    Milano, Italy
    I think so, but the problem is on ONE channel only, so I would exclude a power issue...
     
    ldizzle likes this.
  10. MrKite89

    MrKite89 Member

    Messages:
    1,404
    Joined:
    Nov 15, 2014
    Location:
    Milano, Italy
    Thanks. Sorry for my bad English! ;)

    Yeah, my meter doesn't go below 50 ohms...

    The amp stayed quiet since our last conversation, with any tube: maybe disassembling and reassembling the jacks a couple of times did the trick...I can't even rule out an external interference, maybe the Top Boost input stage is more sensitive to high frequency noise...I really don't know and I can't do anything at the moment, except enjoying the beautiful tone of this amp, so I would just like to thank you and let's hope for the best! :)

    Cheers,

    MrKite
     
  11. HotBluePlates

    HotBluePlates Member

    Messages:
    5,759
    Joined:
    Mar 27, 2017
    You can't measure good grounding at/above 50Ω. If we look towards laboratory gear, there are milli-ohmmeters for this particular task (though most DMMs can manage an indication below 1Ω).
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice