After one week investigation at local K3 holder's shack with Elecraft engineering support by e-mail, my K3's trouble is found as serious.
Following video clips are the comparison of bad K3 and normal K3 submitted to Elecraft engineer for their reference.
my K3 upper, my friends' K3 lower.
receiving strong signal out of filter bandwidth.
First upper then lower and repeat.
Same setup with receiving strong signal within thefilter bandwidth.
Swiching bad K3 and normal K3 vice versa.
I appreciate Elecraft enginner's patient support everyday.
But unfortunately, no improvement obserbed.
I deceided to send my K3 to the factory.