Expanded: I have access to two Slingbox PRO-HD's. One works and one doesn't (the problem one). Two power supplies work with the working unit and not with the non-working unit. So, it's not the power supply. Ktec model #KSAFF0500400W1US. V-Infinity model #DSA-20PFE-05 FUS.
The Slingbox PRO-HD that does work (same model # SB300-xxx) is HEAVIER, lists as 4.0A MAX and is assembled in Indonesia. The non-working unit is 3.0A MAX and assembled in China. I figured there might be a recall based on the two different types.
I apologize you are having an issue with getting the Slingbox Pro HD to power up. I highly recommend that you give our Technical Support Team a call. Below I have provided the contact information:
Sling Media Moderation Team
How did you make out? Did you contact support? Were you still under warranty? I have the exact same problem and have the same unit (3A, assembled in China, etc). I even just bought a new power supply from Sling and that didn't fix the problem either (waste of $20). We have two of these units at work that don't have any problems, I wonder if they are the other kind (4A, assembled in Indonesia). Really disappointed - purchased in 2012 and used it maybe 10 times. My Slingbox Classic is still going strong (after a power supply replacement).