[Elecraft] silent subreceiver on 14 MHz, need help

[István Szabó]

Just observed that the subreceiver of my K3 does not work on 14 MHz 
only. When I tune the sub it looks like that the VFO changes its 
frekvency as I hear "spurr" while tuning. No signal heard coming from 
antenna input. This phenomena is on 20m only the rest of the bands are 
fine. No difference between modes. Where shall I start the diagnosis?

73'
István Szabó
HA4ZD