On 8 Aug 2011 00:12:54 -0700 Winfield Hill
wrote in Message id: :

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. A pair of left-right pushbuttons sets
which decade you're adjusting with the knob, and over or underflows
operate on the next higher decade. There's a little indent near
the edge of the knob so you can spin it with your fingertip. You
can also use up-down pushbuttons or directly punch in numbers, but
the spinning knob is very convenient. Typically such encoders have
16 or 32 ticks per revolution. A very nice design feature.

The optical shaft encoders were originally made by HP, but now of
course they're made by Avago. Sometimes they call these a digital
potentiometer or a "Panel Mount Optical Rotary Encoder". For
example, the HRPG-AD16 product line, about $34 each at Mouser.

http://www.mouser.com/Search/Refine....word=HRPG-AD16

http://www.avagotech.com/pages/en/mo.../hrpg-ad1616c/

Since they use an optical 2-bit quadrature grey-code (as opposed
to using a pair of switch contacts), they're supposed to be
extremely reliable.

But recently I've encountered two instruments with idenical
tuning-knob failures, one in an elegant 33250A 80MHz model
that's on my bench, and the other in one of our standard
33120A 15MHz instruments.

In both cases the knob seems 'stuck" and not to work as you
turn it, except every now and then you'll get one or two
increments or decrements. Totally useless.

I'm wondering if anyone else has encountered this failure?

I have not seen a failure on the 33XXX series of generators, but the part
# is 0960-0892. Looking at the picture on the find-a-part website shows it
as a mechanical encoder.

http://www.home.agilent.com/myagilen...te=u4p3ex5iq_4

Which is obsolete and replaced by 0960-2545, also a mechanical one. It
looks like its the same as used on the 65XX and 66XX power supplies. I do
know that the older 65XX and 66XX do use the (obsolete) optical ones. It
is possible to use the mechanical ones to fix the obsolete optical ones
with a bit of hacking, but the mechanical ones need more turns to change
the output of the supplies for a given amount.

Are you sure it's an optical encoder you're looking for?