[Elecraft] K3 sidetone puzzling malfunction

[Carl Clawson]

After years of using the same* station configuration, for the last couple
weeks I have a weird thing that I haven't been able to explain or isolate a
cause for.

Using one of my headsets, the sidetone decreases in volume by at least
about 10 dB during a transmission. Usually it happens quickly but sometimes
gradually. After a short rest it usually resumes its normal volume. It
happens regardless of monitor volume setting. The tone produced when you
adjust the volume is not affected. RX audio is not affected.

It's not 100% due to RFI because it happens even if I have transmit
disabled. It happens regardless of whether I use the rear panel or front
panel jack and regardless of CW/SSB/digital. It does seem to be more likely
to happen when I'm transmitting, though, which is why I say "not 100%" RFI.
Let's just say that the behavior seems to be aggravated by RF or the
fluctuating DC voltage that comes with it. I've checked my bonding and
connectors for tightness and everything seems OK.

The offending headset is a Koss SB/49 that I've used for more than three
years. It doesn't happen with any of my other headsets. One distinguishing
factor is that this set has a high impedance, 280 ohms DC. The other sets
are in the range of 40-120 ohms.

73, Carl WS7L

* Well not QUITE the same. Last summer I did several of the K3S upgrades
including the KIO3B.