[Elecraft] High pitched hum after installing KXBC3?

[Jessie Oberreuter]

      A little more experimentation showed that my noise always goes away 
when the rig is "muted" for transmit (whether actually transmitting, or 
faking it in cw mode with vox off).  Taking the KX3 apart tonight to try 
to deal with the situation, I discovered that there's a strong mechanical 
component to the noise -- it's quite audible even with the speaker 
disconnected.  Just as with the speaker component, the mechanical 
component goes away in when not receiving.
      Adding a 4.8uf cap across the speaker (the schematic shows a 22 Ohm 
resistor already in series) removed the noise from the speaker audio.  I 
still hear the mechanical component when I'm in front of the rig, but it's 
no longer being produced by the speaker as well.
      I've only had the radio assembled for a few days and I'm already 
absolutely in love with it!  That said, I'd sure love to get this fixed or 
find a unit that doesn't manifest the issue.  Sometimes great hearing can 
be a curse...  -kb7psg


On Mon, 10 Jun 2013, Jessie Oberreuter wrote:

>
>     I finally had the chance to put my KX3 together over the weekend! Yay! 
> I don't know if it's the same thing you're hearing, but there is a high 
> pitched whine in the audio on my unit (it transfers from the speaker to the 
> headphones when they're plugged in).  Sounds like a G/G# ... maybe something 
> in the 12.8 khz range?  I don't have access to anything right now that can 
> actually measure it.
>     The volume of the tone doesn't change with AF gain -- tho what AF Gain 
> mean in this radio is non-obvious :).  It also doesn't change with band or 
> mode, nor with a change of power supply (three different external supplies), 
> nor location, nor presence or absence of antenna.  I have the ATU and filter, 
> no battery charger.
>     I can easily hear it using the internal speaker, but I'm hoping I can 
> fix that with a judiciously chosen capacitor across the speaker leads.  I 
> essentially can't hear it on my Heil cans, so I'll be fine this Field Day, 
> but it turns up clearly on ear buds, so I'd like to fix that side as well. I 
> don't recall having heard it on anyone else's KX3, but stuff in this range is 
> pretty easy to either miss or misattribute unless you have good high 
> frequency hearing and you're by yourself in a quiet room, and it could also 
> vary from unit to unit.  Finally, I won't claim to be pitch accurate up 
> there, but until I can measure it on something, a good guess is better than 
> nothing :).
>
> -kb7psg
>
> On Fri, 2 Nov 2012, David F. Reed wrote:
>
>> I notice a high pitched hum (low level) when my KX3 is on, after
>> installing the KXBC3; has anyone else noticed this?
>> 
>> What might be causing it?  Is it possible to eliminate?
>> 
>> Other than the hum, everything seems to be normal and working fine.
>> 
>> Thanks and 73 de Dave, W5SV
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