[Discuss-gnuradio] DBSRX and bias voltage issue

[jonj]

Hello!

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