I have found the selector switch on my 7800 a little finicky too.
Mine usually gives me the same problem on phono 2 or aux., I assume because I don't use them very often.
I make sure the volume is off and cycle it through the different selections a few times and it clears it up.
Now......the power switch, that's another story.
I have to push it in out sometimes 20 times to get it to function, and it just dies sometimes.
Does anyone have one of these for the 7800 they're willing to part with (pun intended)?
Does anyone know if the power switch (looks similar) from a Sony V-4 will work in the 7800 sd as there is one available for about $40 at a local Hock shop?
Seems a shame though to part out a working receiver........