I looked at the extension device.. but shouldnt it have BNC type connection for Cable input instead of RCA ?
Sorta like the 500 Card interface... ??
Your thoughts...
Are you sure you’re using the right connectors on the pvr-500 card?

You said you have digital cable. You should be connecting the Svideo-out & the audio-out from your cable box into the Svideo-in & RCA stereo-in of the pvr-500 card.
You shouldn’t be using the F-connector (coax) at all.
The extension card that
SpHeRe31459 linked has a second pair of Svideo/composite-input jacks for your second digital STB.
Also, how are you changing channels? Are you using the IR blaster that is part of the MCE remote control package?
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By the way, if you also have analog out of that cable of yours, you could use the coax connector on the pvr-500 along with the Svideo port that is connected to your digital STB.