The most telling sympton was the noise. The noise level was always much lower on 20M. Sometimes it didn't register on the S-meter, but it rarely if ever got as high as S3. Still, I managed to work plenty of stations including DX on it. I might have even managed to work 20M digital at Field Day with it last year, as I think the problem has been around for that long.
It all came to head recently on JT65/JT9 where almost always the signal reports I received were much less than the ones I sent. I have a vertical antenna and the reports are S/N ratio so was it some man made noise source? The low S-meter reading was evidence against that. Occasionally the report I received would be 25 db higher than the one I sent, even though I was only running 10W (medium power on this mode). Usually they were about 10-12 db higher.
Here's a look at the transceiver as it was set at the time. The astute K3 owner may immediately see the problem, if he or she hadn't already figured it out from the description.
In the middle of this was a North American QSO party RTTY. 40 and 80M aren't working on my antenna right now (another painful story will unfold, I'm sure) so I tried to use 20M and found I couldn't hear much. Around the same time, trying to work PSK I also found a lack of signals, but a guy I chatted with on 30M said the band was crowded. Hmm.
The problem turned out to be (embarrassingly) simple. If I'd written Elecraft I'm sure they would have responded with the answer right away. The K3 has a separate receive antenna. It is selected by a switch right next to the power button just above the volume control. If you look just under the first zero in the frequency display you can see the letters "RX". I must have hit the button accidentally some time ago. The K3 has per-band settings for a lot of things and this is one of them, so in fact I had no antenna connected to my rig while receiving on 20M.
There's some leakage between the main antenna and the RX antenna path. It's not quantified in the manual but I estimate it's around 30db. It just goes to show how sensitivity isn't the most important thing in a receiver. I managed to work a lot of DX with that 30db "attenuator" in the signal path, although I'm happy to report that 20m is quite a bit more interesting these days!
I hope some day I'll be able to erect a separate receive antenna for even better DX. Unfortunately the noise level now usually hovers around S7.
Good thing I was on 40m not 20 at Field Day 2015 otherwise I might have received the "Melted Coffee Pot Award" for a second time.