Hoping a Panasonic RF-3100L owner can help me...

I bought a Panasonic 3100L and it works realy well except when you
place the antenna switch into the INT position to use the whip
antenna, the set loses the signal completely and the digital freq
display tuns off yet the red operation light stays on. Has anyone ever
experienced this or have ideas as to what may be wrong? The antenna
switch works fine in the other two positions with external antennas
hooked up.

Many thanks!