[Elecraft] Noisy K3 Receiver

[Richard Ferch]

I believe there may be a hardware variation component to this. At least, 
my main RX and my sub-RX sound like two different radios (S/N #1595).

In summary: the main RX sounds near-perfect. On the other hand, the 
sub-RX has audio hiss above the DSP bandpass at high AF gain settings, 
and what appear to be digital artifacts within the bandpass that are 
noticeable at high AF gain settings and narrow bandwidths.

In detail: On the main RX, with no antenna, RF gain set to zero and NR, 
NB and AGC all off, I can turn the AF gain control all the way to the 
top and still hear nothing - the receiver is completely quiet regardless 
of the ATT, PRE, filter and mode settings. As I move the RF gain control 
up, I start hearing receiver noise within the filter bandpass, as expected.

With AGC on, I can hear some in-bandpass receiver noise even with the RF 
gain control at zero, and the within-bandpass noise level is higher than 
without AGC up to the point where the S-meter goes to zero, after which 
levels are the same with and without AGC.

So far so good, no surprises other than the excellent performance.

Now, with all settings in the sub-RX identical to the main RX, I tried 
the same experiment and got quite different results. With the RF gain at 
zero, as the AF gain control is increased beyond about 12 o'clock I 
start hearing high-pitched hiss (outside the filter bandpass). Even with 
RF gain at around 1 o'clock, with AF gain all the way up the hiss is 
louder than the in-band noise.

With the sub AF gain all the way up and RF gain down, I also hear other 
noises, a bit like a poorly-controlled audio oscillator with lots of 
harmonics and other burbling noises; the dominant frequency appears to 
be the same as the filter Fc. I can hear this in the internal speaker as 
well as in my headphones. This junk is most evident at narrow bandwidth 
settings. The junk decreases when the AF gain is backed off, and is 
drowned out by normal band noise with normal RF gain settings, although 
I can imagine that it might still contribute to aural fatigue. Again, 
none of this is audible at all on the main receiver, only the sub-RX.

I can only conclude from this, plus the reports on this reflector, that 
not all K3 receivers are the same. Certainly the two receivers in my K3 
sound different.

73,
Rich VE3KI