[Elecraft] K1 with a strange XFIL problem...

[Bruce Beford]

OK, folks.
Time to put on your thinking caps...
I just purchased a used K1. Has NB, ATU and 4 band board, set for
7/10.1/14/21 MHz. All appears to be operating normally, except...
When cycling through the crystal filter bandwidths from FL1-FL3, once FL3 is
reached, all buttons become unresponsive. The VFO readout still works, but
the MCU will not respond to any key presses. The filter does actually switch
to the narrowest setting, based on the audio response. Cycling the power
returns the rig to normal operation (on FL1), and all works fine again until
trying to use FL3.

About the only other anomaly I notice is what seems to be a louder pop in
the speaker when switching in RIT or XIT than I remember from my last K1. I
have not yet begun any troubleshooting (except for a full reset). Just
thought I would poll the collective wisdom here before digging in. Actually,
I'll be getting ready for Lobstercon over the next couple of days, so I
don't know how much time I'll be able to commit until after the weekend.

I'd appreciate any thoughts or ideas on this one.
Thanks es 72/73,
Bruce, N1RX