there are 2 issues:
1) can you watch/listen to all your sources going thru the receiver when receiver is turned on? if yes -
2) how to setup Standby or pass-thru so you can watch/listen when receiver is turned off
my assumption is #1 is yes but if that's not the case, then that's a different problem and troubleshooting could involve the input assignments, associated an HDMI input with a named device input. you don't have to assign them, can leave them unassigned as HDMI 1,2,3...
and any incompatibilites with cable box etc will show up here - if you can't get video to your TV going thru the receiver but it works OK going directly to the TV. if you can't get video/audio from the Comcast box when the receiver is turned on, then it may not work in Standby either, but it's also possible it might. pass-thru does imply exactly that. but the input assignment has to be correct. IOW, if you want cable HDMI to work when you select CBL/SAT on the receiver display, you manually have to assign whatever HDMI port it's connected to, to the CBL/SAT named input, so it will be piggy-backed on top of the analog AV connections labeled CBL/SAT on the rear panel. also, make sure SIGNAL SELECT is set to Auto or HDMI . with audio inputs, the selection priority is always HDMI >> Digital >> Analog, so literally you can have both HDMI and digital or analog connected to the same source and choose which one to use manually. but Auto will always pick HDMI 1st. SIGNAL SEL is a button on the remote.
Standby pass-thru isn't setup as an automatic feature, you have to set it up and only for one device at a time. this entails going into the Setup Menu >> select System Setup >> select HDMI Setup >> Standby is default turned off, so scroll down to it, use right/left buttons to move it off OFF and select the source you want to have passed -thru when receiver is off >> then exit. from how the SC-68 works, there is not a way to have all HDMI sources passed thru when receiver is off. if you want the family to be able to watch Comcast, then you have to select that HDMI input or if its assigned to CBL, then select CBL for the Standby device. you can't also have a Apple TV in Standby mode at the same time. in the SC-68, you only get 1 choice for Standby at any one time, without going back into the setup menus to change it to the other source.
good luck today.