external audio input works for less then a second then shuts off

On my new HP 250 G2 Win 7 64 bit laptop an external audio input works for less then a second and shuts off.
I am inputing a transformer isolated audio output from an amateur radio which I have used on other hp laptop for years.
I am using band 3 and 4 on a minature 4 band phone plug.
On the Sound panels:
"Mic in at front panel (black)"  at start shows "Not plugged in"
When I insert the minature phone plug it shows "Ready"
When i attach the sound source it works for a second then shows "Not plugged in"
The same thing happens when I connect a microphone.
My RTTY application starts to work with the signal at the right level but the signal trace dies after a second.

How can I fix this?
Thanks
Tim