Since the SV3800 has 3 different types of digital inputs, you must make sure you have selected the proper type of input. To check or change this selection, press the COUNTER MODE, COUNTER RESET, and PAUSE buttons simultaneously. This is described on page 28 of the manual. The screen will show

0-
IEC-C

You can push the Skip >> button to select between:
IEC-C (coaxial (RCA) S/PDIF)
IEC-O (optical S/PDIF)
AES (AES/EBU)

Press COUNTER RESET to exit.

You may want to eject the tape and/or power the unit off and back on after changing this for the change to take effect.

Make sure you also have Digital IN selected and the proper sample rate.