[Elecraft] K2 transmit spur

[Brian Machesney]

I have K2 #3070 with SSB, NB and 160 options plus the KPA100. It is =
truly a
great radio to use, with one exception. I hope someone on the reflector =
can
help me resolve this problem.
=20
When I called CQ on a "clear" frequency during the recent ARRL DX Phone
contest, another station came on frequency to tell me I was "plainly
audible" 3-KHz down the band - he thought I was CQing on top of him! =
Sure, I
was running the legal limit, but I've never received such a report in =
more
than thirty years as a ham and I've only heard such an artifact once in =
that
time. I considered it to be a serious problem and verified it with =
another
station after the contest while I was running 80-W barefoot; with an S7
signal, he reported that he could hear a spurious signal "about 12-dB =
down"
in his IC756PRO.
=20
Upon investigation, I determined the following. Listening at 21.207 in a
second receiver with a short wire antenna about 2-feet from the K2, I =
hear a
signal that sounds like a collection of carriers when I enter K2 "tune" =
mode
at 21.207-MHz, with output of about 1-W into a dummy load. The K2 signal
registers about S9+10-dB on the second RX. When I move the K2's transmit
frequency to 21.206, I hear a cleaner "carrier," but now there are spurs
1-KHz above and below the carrier, about 20-dB below the carrier (3-1/2
S-units) in amplitude. When I move down to 21.205, the spurs are 2-KHz =
from
the carrier, and so on, as I move down the band. Same thing happens when =
I
move up the band. The farther I move from 21.207, the weaker the spurs
become. To recap, spurs appear at a separation from the carrier equal to =
the
difference between the carrier and 21.208-MHz. Note that this also means
that there is always a signal - carrier or spur - at 21.207-MHz.
=20
I believe the problem is leakage from the KPA100's HV osc into the VFO
circuit. Note that TX freq. - TX BFO =3D 21.207 - 4.913 =3D 16.294-MHz, =
which is
where I pick up the stray radiation from the HV osc and find that it is
zero-beat with the TX VFO. The problem disappears when the KPA100 is =
removed
from the K2, even though the auxiliary 12-V, PA, ribbon cable and =
speaker
connections are still in place between the KPA100 and the K2. The =
problem
also disappears when I disable the HV osc by disconnecting 12CTL power =
via
the ribbon cable. I have checked for good ground connections, measuring =
less
than 1-ohm from the KPA100 shield to the K2 ground, etc. I even bought a
moto-tool to ensure that all of the anodizing and paint were removed =
from
their respective surfaces and installed ground jumpers between the =
KPA100
antenna jack and the metal shield and between the shield and the speaker
frame. With a 100-MHz analog scope on the 50-mV/div range, there is no
perceptible ripple on the aux 12-V line between the K2 and the KPA100. =
The
HV osc is producing 146-V to the TR switch.
=20
To "probe" further into the condition, I connected a =BD-inch diameter =
loop of
insulated hookup wire to one end of a 6-inch piece of RG174 and pulled =
back
about =BC-inch of shield from the far end. With the KPA100 sitting "dead =
bug"
on a cardboard box next to the K2, I can reproduce the condition by =
putting
the loop between the HV osc and the metal shield, then bringing the =
exposed
end into the "volume" of the open K2 cabinet over the area of the VFO. =
The
spurs are strongest when the probe is brought close to C68, K15, =
terminal 2
of T5, and C73, in that order. Interestingly (to me, at least), TX mixer =
U10
does not seem to be a sensitive area. I re-heated all of the joints in =
the
VFO area to ensure good connections; no change observed.
=20
Any ideas where I should check next?
=20
Brian
=20


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