[Elecraft] K3 (S) Weird Main RX issue

[Udo Langenohl - DK5YA]

Gentleman,

I have a problem with the main receiver on one of my K3s. The main 
receiver is between 6 and 8dB worse than the second receiver. The stupid 
thing about it: it is not always like that, but the poor sensitivity 
comes sometimes creeping or sometimes suddenly. It seems to have 
something to do with the transmit / receive switching. I checked this 
very carefully with an HP8664A signal generator and a spectrum analyzer. 
Sometimes both receivers are exactly the same, then the main receiver 
slowly gets worse and worse (within a minute the amplification drops by 
6-8dB) sometimes it happens immediately after switching from RX to TX 
and back. Then it happens that after switching from RX to TX and back 
again, both receivers are the same again. It can also happen that the 
main receiver is initially 8dB worse than the second receiver, but 
without RX / TX switching the missing 6-8dB are suddenly there again - 
in the middle of receiving mode, without switching. So it's intermittent 
and it drives me crazy not to find out what's going on. Is that another 
bad pin diode somewhere in the main receiver and if so: which one? Maybe 
someone has an idea? Help is very much appreciated.

This is a K3 #56xx with all features and all upgrades so its more or 
less a K3S rather than an K3.

73 Udo, DK5YA