[Elecraft] K2 and 60Hz magnetic fields.

[Masleid, Michael A.]

A few weeks ago someone had requested a map showing what parts of the K2
are sensitive to 60 Hz magnetic fields.  I figured I should try to find =
out.

I have an old Radio Shack tape head demagnetizer.  (Don't apply that any =
where
near the latching relays by the way!)   Turns out that the K2 is not =
happy with the
demagnetizer any where near it.

I switched to a 12 volt step down transformer, followed by a 2500 ohm 25 =
watt
wire wound potentiometer, followed by the demagnetizer.  I set the K2 to =
40 meters,
then tuned to a short wave broadcast carrier so that I had a steady CW =
tone.

I reduced power to the demagnetizer and ran it along the bottom of the =
K2.  The area
under the VFO circuitry is bothered by the demagnetizer.  The rest of =
the K2 is not
sensitive.

I removed the bottom cover of the K2 and reduced power to the =
demagnetizer much
further.  The area under the VFO tuned circuit is bothered by the =
demagnetizer.  The
RF chokes, toroids, and shielded inductors are not sensitive.

I reduced power to the demagnetizer still further.  The K2 is most =
sensitive to a AC
magnetic field directed into the bottom of L30.  It also responds =
slightly to a magnetic
field into K14, but that may be due to magnetic coupling between K14 and =
L30.

I put the K2 back together and checked to make sure the relays still =
worked.  I don't
think it is a good idea to wave a demagnetizer around K14.  I had the =
demagnetizer
field strength reduced to about 1/40th normal, so I guess it is OK.

Now, what's going on?

Does the inductance of L30 change due to core saturation?  Does L30 =
couple 60 Hz
current into T5 and change T5's inductance?  (Easy to check, since the =
modulation would
be at 120 Hz.)  Or does L30 put out enough 60 Hz voltage to modulate the =
varacter voltage?
Hard to believe with T5 in parallel...   I did try a few thousandth =
inches of steel shielding,
but it takes more than that to block 60 Hz.

73, de Michael, AB9GV
K2 SN 3430