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:
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" wrote in message
...
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:
HAM: DJ9PZ / AB2QP

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.
[...]

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:
HAM: DJ9PZ / AB2QP

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.

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