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, 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.

73, Fred, K4DII

"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 "

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.


Leland-

I paid $225 plus shipping for the used rubidium-controlled oscillator. I had
to build a 24 volt DC power supply for it. I have read various articles about
lamp life. It seems that E-Bay sellers will claim that they were designed for
several years of continuous use. The trouble is that you don't know how many
years a second hand unit has already been used! They are most likely being
sold after having been replaced as preventive maintenance. I only run mine
when I need it for calibrating an oscillator. I would expect it to be cheaper
to buy another used rubidium oscillator than to repair the one I have. If it
does go bad, I will look into one of the HP GPS-synchronized oscillators that
are available on E-Bay.

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.

The oven won't draw current but the oscillator will. An Ohm meter check
between the oven power lead and ground should be relatively low, maybe a few
hundred Ohms?

You DO NOT want to disassemble the oven! All you need to do is remove the end
plate. As I recall, there is a flexible cable near the card-edge connector,
that hides the thermal fuse. The original thermal fuse looked like a tiny
electrolytic capacitor, and plugged into tiny pin-sockets on a circuit board.
The new version looks more like a ceramic quarter or half watt resistor.

If you are interested, I wrote up my experience and E-Mailed it to ARRL's Hints
and Kinks. It was published on Page 68 of the November, 2002, issue of QST.

Please do not take this as a condemnation of the 10811 oscillator. I am very
impressed with how stable it has been over a long period of time.

73, Fred, K4DII

"Fred McKenzie" wrote in message
.
...
Please do not take this as a condemnation of the
10811 oscillator. I am very
impressed with how stable it has been over a
long period of time.

Thanks again for the info.


--
Leland C. Scott
KC8LDO

Saw this on a Tee-shirt:

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

"Leland C. Scott" wrote in message
...
Thanks again for the info.

Exactly the same thing happened to me. It's fortunate I found a
service manual for my frequency counter on eBay or I might never
have diagnosed the problem. The manual states that the normal
operating temperature for the oscillator is 80-84C. I bought new
thermal fuses from Digi-Key. Investigating the properties of these
devices I found that the exact temperature at which they open isn't
a simple thing to determine. To be safe but avoid the fuse opening
again under normal conditions, I selected a Panasonic "N124" type
rated at 130C. The Digi-Key catalog number is P10922-ND. This
turned out to be a bit larger than the original part. I suggest
you try the smaller "F130" part, catalog number P10913-ND. Just
to be safe, of course, verify these part numbers before ordering.

Taking apart the oscillator is a bit nerve-wracking if you don't
know what to expect, but the fuse replacement isn't at all hard
to do.

Have fun.

-Henry

Exactly the same thing happened to me. It's fortunate I found a
service manual for my frequency counter on eBay or I might never
have diagnosed the problem. The manual states that the normal
operating temperature for the oscillator is 80-84C.

Henry-

I had the service manual for my counter, but it did not have sufficient
information about the 10811-series OCXO, for the HP/Agilent parts department to
identify the thermal fuse. I had to obtain a separate manual for slightly
different oscillators in that series.

The temperature of the oven is chosen to match each specific crystal. The
thermal fuse should be sufficiently high that it won't open in normal
operation, but will if there is a problem. The original part for my oscillator
was rated at 108 degrees C. The revised part is rated at 115 degrees C. It is
HP/Agilent part number 10811-80008. Last year the price was $10.50 plus tax.

I also found a 117 Degree C "Thermal Cut-Off", NTE Electronics NTE8115. Radio
Shack has a #270-1322A thermal fuse rated at 128 degrees C. Both of these are
larger than the original part, and their leads are too fat to fit in the
original pin sockets. I solved that by finding a high value resistor with the
correct lead diameter, and soldering it in parallel with the thermal fuse. It
was necessary to use a heavy pair of pliers as a heat sink to avoid melting the
fuse element while soldering! The larger thermal fuse with its parallel
resistor is a tight fit. I used some fiberglass tape to insulate it, to keep
from shorting to nearby components.

I eventually replaced the substitute with the correct part, and it has been
working for about a year now with less than a tenth of one Hertz drift at 10
MHz, or better than ten parts per billion compared to the Rubidium controlled
oscillator.

73, Fred, K4DII

Exactly the same thing happened to me. It's fortunate I found a
service manual for my frequency counter on eBay or I might never
have diagnosed the problem. The manual states that the normal
operating temperature for the oscillator is 80-84C.

Henry-

I had the service manual for my counter, but it did not have sufficient
information about the 10811-series OCXO, for the HP/Agilent parts department to
identify the thermal fuse. I had to obtain a separate manual for slightly
different oscillators in that series.

The temperature of the oven is chosen to match each specific crystal. The
thermal fuse should be sufficiently high that it won't open in normal
operation, but will if there is a problem. The original part for my oscillator
was rated at 108 degrees C. The revised part is rated at 115 degrees C. It is
HP/Agilent part number 10811-80008. Last year the price was $10.50 plus tax.

I also found a 117 Degree C "Thermal Cut-Off", NTE Electronics NTE8115. Radio
Shack has a #270-1322A thermal fuse rated at 128 degrees C. Both of these are
larger than the original part, and their leads are too fat to fit in the
original pin sockets. I solved that by finding a high value resistor with the
correct lead diameter, and soldering it in parallel with the thermal fuse. It
was necessary to use a heavy pair of pliers as a heat sink to avoid melting the
fuse element while soldering! The larger thermal fuse with its parallel
resistor is a tight fit. I used some fiberglass tape to insulate it, to keep
from shorting to nearby components.

I eventually replaced the substitute with the correct part, and it has been
working for about a year now with less than a tenth of one Hertz drift at 10
MHz, or better than ten parts per billion compared to the Rubidium controlled
oscillator.

73, Fred, K4DII

"Leland C. Scott" wrote in message ...
"Fred McKenzie" wrote in message
.
..
Please do not take this as a condemnation of the
10811 oscillator. I am very
impressed with how stable it has been over a
long period of time.

Leland,

Your email return address is a bit too munged (;-) to be useful, so I
have to post...drop me an email and let me know what characteristics
you are looking for in an OCXO and maybe I can help you out.

Cheers,
Tom

"Leland C. Scott" wrote in message
...
Thanks again for the info.

Exactly the same thing happened to me. It's fortunate I found a
service manual for my frequency counter on eBay or I might never
have diagnosed the problem. The manual states that the normal
operating temperature for the oscillator is 80-84C. I bought new
thermal fuses from Digi-Key. Investigating the properties of these
devices I found that the exact temperature at which they open isn't
a simple thing to determine. To be safe but avoid the fuse opening
again under normal conditions, I selected a Panasonic "N124" type
rated at 130C. The Digi-Key catalog number is P10922-ND. This
turned out to be a bit larger than the original part. I suggest
you try the smaller "F130" part, catalog number P10913-ND. Just
to be safe, of course, verify these part numbers before ordering.

Taking apart the oscillator is a bit nerve-wracking if you don't
know what to expect, but the fuse replacement isn't at all hard
to do.

Have fun.

-Henry

"Leland C. Scott" wrote in message ...
"Fred McKenzie" wrote in message
.
..
Please do not take this as a condemnation of the
10811 oscillator. I am very
impressed with how stable it has been over a
long period of time.

Leland,

Your email return address is a bit too munged (;-) to be useful, so I
have to post...drop me an email and let me know what characteristics
you are looking for in an OCXO and maybe I can help you out.

Cheers,
Tom