One simple way to check out a tape loop is to take one set of IC's and connect them to the tape in and tape out creating a simple loop. Then play a CD or whatever and hit the tape monitor switch. If it's working you will not hear anything - the music will keep on playing. If it goes quiet then most probably the switch is dead. Just a WAG.
Tape Loop Testing On A Preamp?
I have an old NAD 1155 preamp whose tape out loops appear to have ceased outputting anything. It seems strange to me that both tape loops would crap out at exactly the same time (common circuit?), but I am trying to find a solution, and/or a test for the tape loops that proves they are either dead, or alive.