DBSRX and bias voltage issue


I am experimenting with an old Primestar DBS dish and GNU Radio using
the USRP and the DBSRX daughterboard. In order to power the dish’s LNB,
I need to send 12 volts up the antenna line. After reading previous
posts to the GNU Radio mailing list about antenna bias and the DBSRX, I
decided against a bias tee and applied 12 volts at 300 mA to the J100
jumper on the DBSRX.

When I hooked everything up indoors, I measured 12 volts, as expected,
at the LNB end of the coax. Thinking all was well, I disconnected
everything and moved the whole setup outdoors to start experimenting.
However, after reconnecting everything, I now measured only 5 volts at
the LNB end of the coax. Working backwards, I measured 12 volts at the
J100 pins and 5 volts downstream. I recalled from previous posts that
shorting J101 would give 5 volts from the USRP power supply. To be
certain that this wasn’t happening in this case, I disconnected the 12
volt supply and measured no voltage on the line. Reconnecting the 12
volt supply to J100 restored 5 volts to the line.

I saw a reference in a previous post that, with the 5 volts in place
from jumping J101, the DBSRX could not withstand a short at the SMA
connector. According to the post, a small inductor acting as a fuse
would be fried but could be replaced. Of course my first suspicion is
that I caused this, maybe by an accidental short across the coax while
measuring the voltage or connecting to the LNB. However, the fact that
I’m still getting 5 volts as opposed to no voltage, like with a blown
fuse, makes me wonder.

I would greatly appreciate any insight or suggestions for additional
diagnostics that you might have.

Thanks in advance.

Jon Jennings