Ilya said:smodak, if you only have one STB, then it's likely just a single wire coming from the LNB to the diplexer. Disconnect it on either end. Cover the open connector of the diplexer with some rubber or a watter-resistant tape to prevent watter from getting into it. If the cable on the roof is long enough, it is better to bypass the diplexer completely.
P.S. I would still check the power supply and all of your connections first...
Thanks Ilya. Just to double check
1. The coax from the roof diplexer is connected to the radioshack diplexer down below (say, D).
2. The 'sat output' of D (marked with DC pass) is connected to the PS 9370 terminal marked with "to Pre-amp"
3. The other end (marked with "To Antenna") of PS 9370 is not connected to anything.
4. The 'antenna out' of D is connected to the HTPC.
I am not missing anything, right?