EICO 710 Grid Dip Meter

I recently purchased a used EICO Model 710 Grid Dip Meter (GDM).  I have checked it out and the voltages/resistance levels are per the operating manual and have verified RF power output at the frequencies as indicated on the dial.  The sensitivity control works as expected and, in short, it all looks very good except, it simply will not dip when I try to couple it to a resonant circuit.  I have build any number of test circuits, including simple loops to couple the output energy to a crystal (per the manual) while monitoring the output frequency.  No luck.  The manual suggests trying the audio output as a more sensitive measurement, as you zero beat to the resonant frequency; I hear no tones during this exercise.  I'm new to GDMs, but have watched numerous videos to get the coupling correct, and have tried it many, many ways all without success.

Any ideas?