"Fred McKenzie" wrote in message
.
...
I understand that HP has used three different
OCXOs, E1938A, HP-10811, and the HP-10544. I
haven't found any info on the first one.

Leland-

My HP counter has a variation of the 10811
oscillator. To calibrate it, I
bought a 10 MHz rubidium-controlled oscillator
on E-Bay,

What did that cost you? Also how long are the
lamps good for anyway? I imagine they are
expensive to replace.

and it is impressive
how little drift the 10811 has over a long
period of time. I use an
oscilloscope to compare the two 10 MHz signals,
and can count the number of
seconds required for the sine wave to drift one
division, where ten divisions
equal one cycle.

I also bought the counter on E-Bay. The seller
insisted it had been checked
out and worked fine. What I found was that the
10 MHz oscillator could not be
adjusted onto frequency, and it never quite
stopped drifting. After an
investigation that included purchasing a
10811A/B manual, I found that the
10811 had an open thermal fuse. Further, the
HP/Agilent parts department was
not able to determine the correct replacement
part until I had the manual in
hand, and could tell them the original part
number!

It seems that the 10811 (and possibly the 10544)
oscillator series was
originally designed with too low a temperature
rating on the thermal fuse. At
some point a new higher value was chosen. If
you happen to encounter the
problem, there is a good chance that if it
doesn't have a burned smell, all it
needs is the new part. Mine has been working
flawlessly for over a year now
with the newer thermal fuse.

I assume if the thermal fuse is shot the
oscillator won't draw any oven current? I did find
a page that shows how to disassemble the 10811. It
doesn't look all that inviting. Seems the author
of that page had most of his problems dealing with
the sticky tape they use internally. He had by the
way also reported a drifting problem. That's why
he took it apart. But he did say he couldn't find
anything wrong with the unit either.

Thanks for the heads up about a known problem with
the 10811.


--
Leland C. Scott
KC8LDO

Saw this on a Tee-shirt:

"I am a
bomb
technician
if you see me
running
try to keep up "