[Elecraft] K3 - bad CW note

[Curt Knight]

I have K3 SN 469, a kit K3-100 with transverter interface, tuner,
several filters, and a recently added subreceiver.  It's updated with
the latest beta firmware.

This K3 works well at home, but I am having some trouble using it at
the KH6LC station.  Those of you who operated the Cal QSO Party and
worked KH6LC heard my K3, and it is bad signal reports from this
operation (Thanks AF6EV!) that prompt this message.  The bottom line
is that when the K3 is connected to the antenna at KH6LC there is a
significant modulation of the output amplitude, synchronous with the
power line, and independent of power level.  It seems to be a buzz
(square wave) rather than a hum (sine wave).  Replacing the antenna
connection with a dummy load removes the buzz.  Turning the TX AGC off
has no effect.  The K3 is grounded to the station ground bus via the
rear panel ground screw. There is some evidence that simply connecting
a long coaxial cable (from the radio room to the garage at KH6LC)
terminated in a dummy load can introduce the buzz.

So what, you say, there's a ground loop and when you find it you'll be
fine.  Well, Ken N6KB and I have spent a fair mount of effort on just
that and so far we have come up empty handed. In the interest of
brevity I won't list all the things we tried. The reason why this
might be interesting to K3 owners is that no other radio that we have
tried seems to be affected by whatever condition is causing the
problem.

So I wonder if there is an assembly error that might be a contributing
factor.  What I'm looking for is some internal part that should be
grounded but isn't, or vice versa.  Before I dig into it I'm hoping
that those of you with more K3 experience than I have can suggest a
place to look.  In the meanwhile, we are going to try for symptomatic
relief by using isolation transformers, etc.

Curt AH6RE