[Elecraft] K3: LINE OUT weirdness

[Erik Basilier]

Earlier I reported a problem where the LINE OUT level setting had no effect
on the actual output level, but the front panel volume control did.

Well, I downloaded and installed latest firmware, which was different from
what I downloaded just a few days ago. That fixed both symptoms, except:

 

*         While the front panel volume can now be reduced to nothing while
the line output level is good and controlled by the menu setting, if I
increase the front panel volume above a certain level, it starts to cause
increased levels in the line output, as evidenced by the connected laptop's
waterfall.

*         The increased noise is not uniform across the audio spectrum, but
appears amplitude modulated with a sine wave so that about two noise maxima
are found per 500 Hz. Further increase in speaker volume results in stronger
"banding" of the noise.

*         With external speakers plugged in, the same happens, but
disconnecting one speaker leads to a marked reduction of the banding.

*         When no speaker is connected (uncorded plug in speaker jack),
there is no banding.

 

Note that I have disabled the built-in microphone in the connected computer,
and the speaker volume - related waterfall noise increase is not diminished
when I muffle the radio speaker by putting a book over it; nor do I see any
reaction when I whistle into the laptop's microphone.

 

My best guess at this time is that the added noise is caused either
inductively by speaker current affecting the radio's isolation transformers,
or the speaker amp current draw causing a variation in supply voltage, maybe
only in a local part of the radio. I have not yet checked the power supply
for hum etc. There might also be some grounding deficiency, as I have not
done any bonding between the boxes.

 

The waterfall shows a couple of constant audio tones of moderated strength,
located at about 1185 Hz and about 2370 Hz (if I can trust the calibration
within MMVARI). Years ago I did the hardware mod's relating to the speaker
amp, but I haven't checked for recent mod's.

 

Any comments or suggestions are welcome!

 

Thanks and 73,

Erik K7TV