[Elecraft] K3 "hum" problem - not the usual?

[Brian Machesney]

After enjoying my K3 for about nine months, I was recently alerted to
audible 60-Hz hum on my transmitted audio signal. After reading the many
"hum related" posts on the reflector, I began investigating. Step 1 was to
remove all connections to the rig except the DC power and the coax to the
external PA (I placed a direct short on the keying line input at the PA).
Without a mic connected, the indication of the problem is that the PA
registers output power with no audio input. The problem only occurs in an
SSB mode when the nearby PA is both on and enabled (i.e., not on "standby")
and when the K3 is both within about 8-in of the PA and parallel or at right
angles to it; rotating the rig about 15 degrees around the z-axis eliminated
the hum. All in the usual vein, so far.

There are several aspects of this problem that I find hard to understand:

- With all conditions apparently constant, the problem is at least somewhat
intermittent and will build up to full output over about a 3 second period
when it does occur
- When "hum" is being produced, the K3 output power meter reads zero while
the external PA is putting out something near full power! (Ordinarily, it
takes 20 watts of output power from the K3 to drive the external to this
power level)
- Lowering the output power setting on the K3 reduces the amount of "hum
power" registering on the PA output power meter; there is a marked reduction
in "hum power" when I pass the K3's internal PA power threshold and the "hum
power" goes to zero when the K3 power output is set to zero
- There is hysteresis in the output power registered by the external PA vs.
the output power setting on the K3; i.e., starting with K3 output power set
to zero, the K3 output power setting which produces the onset of "hum power"
as shown by the external PA is a larger value than the K3 output power
setting at which the external PA "hum power" decreases / disappears.

I am "space challenged" inasmuch as I have an SO2R setup, so moving the K3
will be difficult. Any thoughts on what's going on?

-- 
73 -- Brian -- K1LI