On Monday, August 8, 2011 3:12:54 AM UTC-4, Winfield Hill wrote:
We have about 10 Agilent 33120A frequency-generator / synthesizers
in our lab, and a few other similar synthesizers. They feature a
convenient digital frequency and amplitude programming knob that's
based on optical encoders.
....
Since they use an optical 2-bit quadrature grey-code ...

But recently I've encountered two instruments with identical
tuning-knob failures...

In both cases the knob seems 'stuck"

There's two optointerruptors out-of-phase with a toothed wheel (or
somesuch, like in the old mice). Your symptom is of one blocked
or dim lamp.