HP 10544 OCXO

  • Thread starter Arnd Gronenberg
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Arnd Gronenberg

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Hi,

the HP 10544A OCXO driving my HP5328A counter failed due to a blown
thermo fuse. The underlying problem was a broken Thermistor lead
(unfortunately within the sealed hole) which caused the owen to run away
(and blow the fuse). I already searched, but didn't find answers to the
following questions. In order to find a suitable replacement thermistor,
I would like to know:
- At what temperature does the OCXO nominally operate?
- What are the characteristics of the original thermistor used by
HP? (my copy of the 10544 schematic lists the thermistor as
"9.93K @80C 0637-0122", but it is difficult to distinguish "6" and
"8")
- Which temperature specification is required for the thermo fuse?

Thanks in advance!

73 de Arnd

--
Arnd Gronenberg
EMail: arnd@gronenberg.com
HAM: DJ9PZ / AB2QP
 
Arnd,
There is some really good info on the HP oscillator at
http://www.realhamradio.com/GPS_Frequency_Standard.htm, which describes uses
for the HP Z3801A GPS Receiver/Time Standard. The page at
http://www.realhamradio.com/z3801a-turning-point.htm gives you some really
great info on setting the crystal turning point temperature for the best
stability. This will help you to understand your questions about the
thermistor also. The oven isn't set to a standard temp, but to the best
temp that puts the crystal in the middle of its curve. The thermistor is
spec'ed at 100K @ 25C and 9.93K @ 80C. You'll have to shop around for a
suitable replacement, keeping in mind the physical size of the thermistor.
Mouser, Digikey, Allied, etc. are good sources of those. After replacement,
you'll have to go through the turning point calibration as described in the
above article. The thermistor is part of the oven mass assembly, and isn't
separately orderable from Agilent.

The thermal fuse isn't described in the documentation, just given a part
number. I don't have the manual for the 10544A, but I do have one for the
10811A/B oscillator, which is very similar.

--
Dave M
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!
"Arnd Gronenberg" <arnd@gronenberg.com> wrote in message
news:crsdhn$imb$05$1@news.t-online.com...
Hi,

the HP 10544A OCXO driving my HP5328A counter failed due to a blown thermo
fuse. The underlying problem was a broken Thermistor lead (unfortunately
within the sealed hole) which caused the owen to run away (and blow the
fuse). I already searched, but didn't find answers to the following
questions. In order to find a suitable replacement thermistor, I would
like to know:
- At what temperature does the OCXO nominally operate?
- What are the characteristics of the original thermistor used by
HP? (my copy of the 10544 schematic lists the thermistor as
"9.93K @80C 0637-0122", but it is difficult to distinguish "6" and
"8")
- Which temperature specification is required for the thermo fuse?

Thanks in advance!

73 de Arnd

--
Arnd Gronenberg
EMail: arnd@gronenberg.com
HAM: DJ9PZ / AB2QP
Click to expand...
 
<< "Arnd Gronenberg" <arnd@gronenberg.com> wrote in message
news:crsdhn$imb$05$1@news.t-online.com...
Hi,

the HP 10544A OCXO driving my HP5328A counter failed due to a blown thermo
fuse. The underlying problem was a broken Thermistor lead (unfortunately
within the sealed hole) which caused the owen to run away (and blow the
fuse). I already searched, but didn't find answers to the following
questions. In order to find a suitable replacement thermistor, I would
like to know:
- At what temperature does the OCXO nominally operate?
- What are the characteristics of the original thermistor used by
HP? (my copy of the 10544 schematic lists the thermistor as
"9.93K @80C 0637-0122", but it is difficult to distinguish "6" and
"8")
- Which temperature specification is required for the thermo fuse?
Click to expand...
Arnd-

Are you sure the thermistor is bad? Apparently HP designed this oscillator, as
well as the 10811 series, with too low a temperature thermal fuse. It is a
very common failure mode. They subsequently replaced the fuse with a higher
temperature part.

A few months ago, someone in one of these newsgroups claimed to have been
involved in the design of these oscillators, and believed the thermal fuse
should be replaced with a piece of wire. He claimed that even if the oven's
heater were turned full-on for a long period of time, the resulting temperature
would not cause damage. If you choose to replace it, be sure to get the newer
part.

In the bridge circuit (of the 10811) there is a resistor hand-picked to match
the crystal, that determines temperature. If you are sure the thermistor is
bad, then Dave M's suggestion is a good one.

Fred
 
Arnd Gronenberg wrote:

The information regarding the temperature calibration is very helpful. I
found a replacement thermistor, which seems to be suitable: 83K @ 25C,
10.24K @ 80C, reaction time 0.7s, size 1.5mm diameter.

As the HP 10544A uses an AT cut crystal I assume that I have to
calibrate for a frequency minimum (according to the above mentioned
website). As the HP 10544A contains a 10 turn variable resistor instead
of a hand-picked resistor, calibration should be quite straight forward.
Click to expand...
If your oven works at 80 degC, it contains an SC cut crystal and not an AT.
AT crystals have their optimum temperature at abt. 28 degC, they are close
to useless at 80 degC.

73, Gerhard dk4xp
 
While we are talking HP 10544 stuff...

I bought one on eBay recently. It seems to keep time ok, but the output
is not a sinewave. Looks like a sine wave flattened near zero volts.

That means something is broken, no? Any ideas if it is worth trying to
fix?
 
On Wed, 12 Jan 2005 08:30:13 -0800 "Rick Karlquist N6RK"
<richard@karlquist.removethis.com> wrote:

For NON-oven applications, you want to operate at the inflection point.
Click to expand...
Why? It seems like this would just minimize your frequency stability.

Yes, I understand "inflection point." You explained it well, but
operating where the second derivitive is zero also automatically puts
us where the first derivitive is at its max (pos or neg) so the slope
is the steepest here.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------
 
DaveM wrote:
Arnd,
There is some really good info on the HP oscillator at
http://www.realhamradio.com/GPS_Frequency_Standard.htm, which describes uses
for the HP Z3801A GPS Receiver/Time Standard. The page at
http://www.realhamradio.com/z3801a-turning-point.htm gives you some really
great info on setting the crystal turning point temperature for the best
stability. This will help you to understand your questions about the
thermistor also. The oven isn't set to a standard temp, but to the best
temp that puts the crystal in the middle of its curve. The thermistor is
spec'ed at 100K @ 25C and 9.93K @ 80C. You'll have to shop around for a
suitable replacement, keeping in mind the physical size of the thermistor.
[...]
Click to expand...
Hi Dave,

thanks a lot for your reply! And also to Fred for his remarks.

The information regarding the temperature calibration is very helpful. I
found a replacement thermistor, which seems to be suitable: 83K @ 25C,
10.24K @ 80C, reaction time 0.7s, size 1.5mm diameter.

As the HP 10544A uses an AT cut crystal I assume that I have to
calibrate for a frequency minimum (according to the above mentioned
website). As the HP 10544A contains a 10 turn variable resistor instead
of a hand-picked resistor, calibration should be quite straight forward.

Thanks again, best regards, 73,
Arnd

--
Arnd Gronenberg
EMail: arnd@gronenberg.com
HAM: DJ9PZ / AB2QP
 
Gerhard Hoffmann wrote:

Arnd Gronenberg wrote:
[...]
As the HP 10544A uses an AT cut crystal...
[...]

If your oven works at 80 degC, it contains an SC cut crystal and not an AT.
AT crystals have their optimum temperature at abt. 28 degC, they are close
to useless at 80 degC.

73, Gerhard dk4xp
Click to expand...
Hmm... On 1998/03/18 in rec.radio.amateur.homebrew (Message
<6epe5u$jnh$1@hpscit.sc.hp.com>) Richard Karlquist wrote:

[...]
The big difference between the 10544 and 10811 is the change from
AT cut to SC cut crystal.
[...]
Rick Karlquist N6RK
HP Santa Clara Division
rkarlqu@scd.hp.com
Click to expand...
That's why I assumed that the 10544 contains an AT cut crystal. I would
be interested to know which temperature calibration procedure is
applicable to the 10544...

73, Arnd DJ9PZ / AB2QP

--
Arnd Gronenberg, DJ9PZ / AB2QP
 
"Gerhard Hoffmann" <dk4xp@arcor.de> wrote in message
news:41e44e8e$0$23137$9b4e6d93@newsread2.arcor-online.net...
If your oven works at 80 degC, it contains an SC cut crystal and not an
AT.
AT crystals have their optimum temperature at abt. 28 degC, they are close
to useless at 80 degC.

73, Gerhard dk4xp
Click to expand...

This is totally wrong.

In the first place, the 10544 definitely uses an AT cut
crystal, not an SC cut. (The 10811 uses an SC cut). I worked at the HP
Santa Clara Division where these oscillators were made for 19 years.

Secondly, 28 degrees is the *inflection point* for an AT cut, not the
*turnover* temperature.
For NON-oven applications, you want to operate at the inflection point.
For oven applications, you want to operate at a turnover temperature.
An AT cut can certainly be made with a turnover around 80 degrees if
cut at the proper angle.

Definitions:

Turnover: first derivative of frequency w.r.t. temperature = zero
Inflection: second derivative of frequency w.r.t. temperature = zero

Rick Karlquist N6RK
 
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