I just started up my K3 serial number 1206, factory upgraded in 2015 to “K3S” after it was in storage for about 15 months while we were building a new home. (During storage, it was packed in moisture-proof tub; never subject to rough handling.)
Prior to storage, the radio was used daily, with no problems.
Now upon first turning on radio, all bands had a low level background noise, but no signals heard.
After a few minutes, the noise level “instantly” came up, like a relay had operated (although I didn’t hear a relay click) and signals could be heard. it seemed like radio was functioning properly. After checking various modes (receive only), I turned off the radio.
After about an hour, I turned the radio back on; it was receiving as expected, then all signals dropped (again, like a relay or pin diode activated (but again, no ‘click’ noise) and the radio reverted to the low level background noise with no signals as previously described.
After radio was on for a while, there still was no signals heard, but there was an occasional “static sound” that registered on the S meter; then the static stops and the S meter shows nothing.
After waiting for awhile, noise level came back up with still no signals.
Since I have not gotten the receiver to consistently operate, I did not try transmitting.
The Elecraft service department resolved the problem with my K3.
About 5 or 6 years ago, I had a very strange startup problem that was resolved by replacing the original 'tinned' header pins between a couple of boards.
This new problem occurred after the radio was in storage for about 18 months, and ended up being caused by more of the original tinned pins. (When Elecraft solved my first problem, they did not need to go through the entire radio and replace header pins at that time.) I'm pretty sure the 18 months of downtime likely created some sort of minor corrosion/aging on pin contacts.
Once Elecraft replaced the header pins with the new gold-plated type, problem was solved.