I don't have any direct experience with this problem, but I just checked my 75ES owners manual, and see the following listed under the symptom of "The Caution indicator lights, and no button does function":
Cause: 1) Moisture condenses inside the unit - or - 2) Defective or damaged cassette is inserted.
Remedy: 1) Leave the unit as is, then turn on the power (see page 5). - or - 2) Remove the cassette.
Since you have tried a couple of different tapes, I doubt that it is "#2".
From page 5 of the owners manual:
"When the CAUTION indicator lights and the unit will not operate: Leave the unit turned on for about an hour".
Hope this helps, and sorry if you had the manual and have tried this already.
If leaving on for an hour doesn't work, you will probably have to call Prodigital to get some more help.
Tom.