[Question] How to test an LNB

[popayetwo]

I have an elderly DsTV decoder that I use to receive only the Free-To-Air channels, of which I only watc Botswana TV and France24.
Recently, my signal dropped to zero and I suspected the decoder was faulty so I took it to a friends house and plugged it in there.
It worked fine.
So that leaves the co-ax cable and the LNB (Low Noise Block) mounted on the dish.
I measured the voltage coming out of the co-ax at the LNB and noted that it changes, depending on whether Vertical or Horizaontal
polarisation is chosen, between 13V DC and 17v DC.
But still no incoming signal so I suspect the LNB itself is faulty.
Is there any way, apart from swapping the LNB, to test it? Can I plug a satelite tester into the LNB and what sort of signal can I expect?
Would the drop test work? Are there any active components inside that I could test?
Thanks for any help.

[AndyD]

The 13 and 17VDC you're measuring is the phantom power coming from the decoder. If you tested the decoder on a known good installation then I wouldn't worry about the different voltages for horizontal and vertical orientation so long as the voltage doesn't exceed 20VDC it should be fine.

The fact you're getting the DC voltage at the LNB indicate there's a good chance the cabling is in tact so I would just try a replacement LNB. You can only test the LNB operation using a sat finder or sat tester and unless you have one handy it will be cheaper to just buy a replacement.

[popayetwo]

It WAS the co-ax cable - the piece between the wall socket and the dish. Here's the funny part. When I measured the voltage at the output of the decoder (aerial) I got a certain voltage
but when I measured it at the dish end I got a voltage drop of several volts. This over a 7 metre run. I put a short on at one end and measured the resistance of the core and braid and
found it to be several thousand ohms. Why? Beats me. So I replaced the co-ax and BINGO!

[AndyD]

Well thanks for the follow up and I'm glad to hear you've got TV again