In article ,
Scott wrote:

I measure the signal coming out the back of the counter
(with another counter) at 10.000066 MHz. I adjusted the "coarse tune"
adjustment on the back of the unit and this is as close as it gets to
10.000000 MHz, so I assume there must be some other adjustment
somewhere.

Scott-

I have a different HP counter that had a similar problem when I got it.
There was a capacitor adjustment in the crystal oven, but it did not
have enough range to bring the oscillator to 10.0 MHz.

It turned out that there was an open thermal fuse in the supply line
inside the crystal oven. It was a plug-in device about the size of a
half watt resistor, maybe smaller. There were two wire-sockets that its
leads plugged into.

I managed to buy the last thermal fuse HP had in stock at the time.
Before it arrived, I had fabricated replacements using a thermal fuse
from Radio Shack, wired in parallel with a high value resistor which had
the correct lead diameter to plug into the wire sockets.

Using a rubidium oscillator purchased on E-Bay, I aligned the repaired
oscillator fairly accurately using an oscilloscope. After several
years, it seems to still be within a fraction of one Hz!

It turns out that this is a common problem. I wrote it up and sent it
to QST for their Hints & Kinks several years back. Since then, one of
the HP designers of the oscillator posted a comment that the thermal
fuse wasn't really needed, since the heating element shouldn't cause any
damage if left on continuously. Even so, I feel better with the thermal
fuse installed!

Fred
K4DII

In article ,
Fred McKenzie wrote:

It turned out that there was an open thermal fuse in the supply line
inside the crystal oven. It was a plug-in device about the size of a
half watt resistor, maybe smaller. There were two wire-sockets that its
leads plugged into.

I dug out my notes to find out the specifics. My counter is an HP 5334B
with Option 1, the high stability oscillator. The oscillator is one of
the HP 10811 series. The original thermal fuse, F1, was rated for 108
degrees C. HP revised the part to one rated for 115 degrees C, part
number 2110-0617 (10811-80003).

The Radio Shack thermal fuse was 270-1322A, rated for 128 degrees C.
There is also an NTE Electronics NTE8115, "Thermal Cut-Off", rated for
117 degrees C. Either is larger than the HP part, and is a tight fit.
I wrapped them in electrical tape to protect against short circuits.
When soldering to the high-value resistors, I clamped the fuse leads in
a pair of pliers held with rubber bands.

Of course this may not apply to your counter if it doesn't have the
crystal oven!

Fred
K4DII

Fred McKenzie wrote:
In article ,
Scott wrote:

I measure the signal coming out the back of the counter
(with another counter) at 10.000066 MHz. I adjusted the "coarse tune"
adjustment on the back of the unit and this is as close as it gets to
10.000000 MHz, so I assume there must be some other adjustment
somewhere.

Scott-

I have a different HP counter that had a similar problem when I got it.
There was a capacitor adjustment in the crystal oven, but it did not
have enough range to bring the oscillator to 10.0 MHz.

It turned out that there was an open thermal fuse in the supply line
inside the crystal oven. It was a plug-in device about the size of a
half watt resistor, maybe smaller. There were two wire-sockets that its
leads plugged into.

I managed to buy the last thermal fuse HP had in stock at the time.
Before it arrived, I had fabricated replacements using a thermal fuse
from Radio Shack, wired in parallel with a high value resistor which had
the correct lead diameter to plug into the wire sockets.

Using a rubidium oscillator purchased on E-Bay, I aligned the repaired
oscillator fairly accurately using an oscilloscope. After several
years, it seems to still be within a fraction of one Hz!

It turns out that this is a common problem. I wrote it up and sent it
to QST for their Hints & Kinks several years back. Since then, one of
the HP designers of the oscillator posted a comment that the thermal
fuse wasn't really needed, since the heating element shouldn't cause any
damage if left on continuously. Even so, I feel better with the thermal
fuse installed!

Fred
K4DII

Thanks Fred! I haven't had time to dig into it yet as I'm out playing
on 10 GHz this weekend. Fortunately, I have a second 5245L for
comparisons. I did find the capacitor on the oven as you mention, but
it did not have enough range as you noted. If the heater is working,
does the enclosure get warm? Seems to me that as I was feeling around,
it was ice cold on the outside. Maybe we're onto something here!

Scott
N0EDV

Fred McKenzie wrote:
In article ,
Fred McKenzie wrote:

It turned out that there was an open thermal fuse in the supply line
inside the crystal oven. It was a plug-in device about the size of a
half watt resistor, maybe smaller. There were two wire-sockets that its
leads plugged into.

I dug out my notes to find out the specifics. My counter is an HP 5334B
with Option 1, the high stability oscillator. The oscillator is one of
the HP 10811 series. The original thermal fuse, F1, was rated for 108
degrees C. HP revised the part to one rated for 115 degrees C, part
number 2110-0617 (10811-80003).

The Radio Shack thermal fuse was 270-1322A, rated for 128 degrees C.
There is also an NTE Electronics NTE8115, "Thermal Cut-Off", rated for
117 degrees C. Either is larger than the HP part, and is a tight fit.
I wrapped them in electrical tape to protect against short circuits.
When soldering to the high-value resistors, I clamped the fuse leads in
a pair of pliers held with rubber bands.

Of course this may not apply to your counter if it doesn't have the
crystal oven!

Fred
K4DII

Yes, mine has the oven (and the high stability Oscillator option). I've
gotten some good leads on this problem, so when I get a bit more time,
I'll rip into it! Thanks again!!

Scott
N0EDV

Fred McKenzie wrote:
In article ,
Scott wrote:


I measure the signal coming out the back of the counter
(with another counter) at 10.000066 MHz. I adjusted the "coarse tune"
adjustment on the back of the unit and this is as close as it gets to
10.000000 MHz, so I assume there must be some other adjustment
somewhere.


Scott-

I have a different HP counter that had a similar problem when I got it.
There was a capacitor adjustment in the crystal oven, but it did not
have enough range to bring the oscillator to 10.0 MHz.

It turned out that there was an open thermal fuse in the supply line
inside the crystal oven. It was a plug-in device about the size of a
half watt resistor, maybe smaller. There were two wire-sockets that its
leads plugged into.

I managed to buy the last thermal fuse HP had in stock at the time.
Before it arrived, I had fabricated replacements using a thermal fuse
from Radio Shack, wired in parallel with a high value resistor which had
the correct lead diameter to plug into the wire sockets.

Using a rubidium oscillator purchased on E-Bay, I aligned the repaired
oscillator fairly accurately using an oscilloscope. After several
years, it seems to still be within a fraction of one Hz!

It turns out that this is a common problem. I wrote it up and sent it
to QST for their Hints & Kinks several years back. Since then, one of
the HP designers of the oscillator posted a comment that the thermal
fuse wasn't really needed, since the heating element shouldn't cause any
damage if left on continuously. Even so, I feel better with the thermal
fuse installed!

Fred
K4DII

Scott
I seem to recall that there is a second adjustment inside the plug-in
area. I assume you found
both of the adjustments.

Bill K7NOM

Bill Janssen wrote:
Fred McKenzie wrote:
In article ,
Scott wrote:


I measure the signal coming out the back of the counter (with another
counter) at 10.000066 MHz. I adjusted the "coarse tune" adjustment
on the back of the unit and this is as close as it gets to 10.000000
MHz, so I assume there must be some other adjustment somewhere.


Scott-

I have a different HP counter that had a similar problem when I got
it. There was a capacitor adjustment in the crystal oven, but it did
not have enough range to bring the oscillator to 10.0 MHz.

It turned out that there was an open thermal fuse in the supply line
inside the crystal oven. It was a plug-in device about the size of a
half watt resistor, maybe smaller. There were two wire-sockets that
its leads plugged into.

I managed to buy the last thermal fuse HP had in stock at the time.
Before it arrived, I had fabricated replacements using a thermal fuse
from Radio Shack, wired in parallel with a high value resistor which
had the correct lead diameter to plug into the wire sockets.

Using a rubidium oscillator purchased on E-Bay, I aligned the repaired
oscillator fairly accurately using an oscilloscope. After several
years, it seems to still be within a fraction of one Hz!

It turns out that this is a common problem. I wrote it up and sent it
to QST for their Hints & Kinks several years back. Since then, one of
the HP designers of the oscillator posted a comment that the thermal
fuse wasn't really needed, since the heating element shouldn't cause
any damage if left on continuously. Even so, I feel better with the
thermal fuse installed!

Fred
K4DII

Scott
I seem to recall that there is a second adjustment inside the plug-in
area. I assume you found
both of the adjustments.

Bill K7NOM

I did see a fairly large air variable capacitor sitting next to the oven
assembly, but since I didn't know what it was for, I didn't adjust it.
I'll go through the manual I downloaded and see if I can get it on
frequency. Thanks for the note.

Scott
N0EDV

Scott wrote:
Bill Janssen wrote:
Fred McKenzie wrote:
In article ,
Scott wrote:


I measure the signal coming out the back of the counter (with
another counter) at 10.000066 MHz. I adjusted the "coarse tune"
adjustment on the back of the unit and this is as close as it gets
to 10.000000 MHz, so I assume there must be some other adjustment
somewhere.

Scott-

I have a different HP counter that had a similar problem when I got
it. There was a capacitor adjustment in the crystal oven, but it did
not have enough range to bring the oscillator to 10.0 MHz.

It turned out that there was an open thermal fuse in the supply line
inside the crystal oven. It was a plug-in device about the size of
a half watt resistor, maybe smaller. There were two wire-sockets
that its leads plugged into.

I managed to buy the last thermal fuse HP had in stock at the time.
Before it arrived, I had fabricated replacements using a thermal
fuse from Radio Shack, wired in parallel with a high value resistor
which had the correct lead diameter to plug into the wire sockets.

Using a rubidium oscillator purchased on E-Bay, I aligned the
repaired oscillator fairly accurately using an oscilloscope. After
several years, it seems to still be within a fraction of one Hz!

It turns out that this is a common problem. I wrote it up and sent
it to QST for their Hints & Kinks several years back. Since then,
one of the HP designers of the oscillator posted a comment that the
thermal fuse wasn't really needed, since the heating element
shouldn't cause any damage if left on continuously. Even so, I
feel better with the thermal fuse installed!

Fred
K4DII

Scott
I seem to recall that there is a second adjustment inside the plug-in
area. I assume you found
both of the adjustments.

Bill K7NOM

I did see a fairly large air variable capacitor sitting next to the
oven assembly, but since I didn't know what it was for, I didn't
adjust it. I'll go through the manual I downloaded and see if I can
get it on frequency. Thanks for the note.

Scott
N0EDV

Scott,
Those capacitors inside the plugin bay are medium and fine frequency
adjustments. If the coarse frequency adjustment inside the oven won't get
it on frequency, then the others certainly won't have enough range to get it
right.
As I and others have suggested, you really need to verify whether the oven
is being heated before you do any more adjusting. Set the coarse tuning
back to its starting position and troubleshoot the oven circuitry. I'm
convinced that's where you'll find the trouble.
Get your voltmeter out and check the oven heater connection. It will tell
you a lot.
--
David
dgminala at mediacombb dot net

On May 1, 5:56*pm, Scott wrote:
Fred McKenzie wrote:
In article ,
*Scott wrote:

I measure the signal coming out the back of the counter
(with another counter) at 10.000066 MHz. *I adjusted the "coarse tune"
adjustment on the back of the unit and this is as close as it gets to
10.000000 MHz, so I assume there must be some other adjustment
somewhere.

Scott-

I have a different HP counter that had a similar problem when I got it.
There was a capacitor adjustment in the crystal oven, but it did not
have enough range to bring the oscillator to 10.0 MHz.

It turned out that there was an open thermal fuse in the supply line
inside the crystal oven. *It was a plug-in device about the size of a
half watt resistor, maybe smaller. *There were two wire-sockets that its
leads plugged into.

I managed to buy the last thermal fuse HP had in stock at the time. *
Before it arrived, I had fabricated replacements using a thermal fuse
from Radio Shack, wired in parallel with a high value resistor which had
the correct lead diameter to plug into the wire sockets.

Using a rubidium oscillator purchased on E-Bay, I aligned the repaired
oscillator fairly accurately using an oscilloscope. *After several
years, it seems to still be within a fraction of one Hz!

It turns out that this is a common problem. *I wrote it up and sent it
to QST for their Hints & Kinks several years back. *Since then, one of
the HP designers of the oscillator posted a comment that the thermal
fuse wasn't really needed, since the heating element shouldn't cause any
damage if left on continuously. *Even so, I feel better with the thermal
fuse installed!

Fred
K4DII

Thanks Fred! *I haven't had time to dig into it yet as I'm out playing
on 10 GHz this weekend. *Fortunately, I have a second 5245L for
comparisons. *I did find the capacitor on the oven as you mention, but
it did not have enough range as you noted. *If the heater is working,
does the enclosure get warm? *Seems to me that as I was feeling around,
it was ice cold on the outside. *Maybe we're onto something here!

Scott
N0EDV

Yes, most certainly the oven should be noticeably warm, unless your
ambient temperature is already really hot. You can also monitor the
current the oven draws (perhaps with some difficulty, depending on how
the oven is wired into the circuit). You should see moderately high
current when the oven is first turned on, and the current should drop
to (very roughly) half as much as the oven reaches operating
temperature. I'd expect the oven control loop to be reasonably
damped, so the current falls from the startup value to the idling
value within just a few seconds and stabilizes without overshoot, or
with only minor overshoot. At least that's what I've seen on several
models, including the HP10811.

These days, you can get some remarkably small and low-power oven
oscillators, down to TO-5 transistor can size I believe, and possibly
even smaller. The power to run them is low milliwatts. Even
temperature-compensated oscillators have gotten remarkably good...I
have some 3x5 millimeter ones that are rated half a part per million
max deviation over -20C to +55C (and run on about 2.5 milliwatts), and
you can get better than that.

Cheers,
Tom

Dave M wrote:


Scott,
Those capacitors inside the plugin bay are medium and fine frequency
adjustments. If the coarse frequency adjustment inside the oven won't get
it on frequency, then the others certainly won't have enough range to get it
right.
As I and others have suggested, you really need to verify whether the oven
is being heated before you do any more adjusting. Set the coarse tuning
back to its starting position and troubleshoot the oven circuitry. I'm
convinced that's where you'll find the trouble.
Get your voltmeter out and check the oven heater connection. It will tell
you a lot.

Yup...I will delve into the oven circuitry when I get a chance...have a
couple other higher priority projects to clean up first...

N0EDV

K7ITM wrote:


Yes, most certainly the oven should be noticeably warm, unless your
ambient temperature is already really hot. You can also monitor the
current the oven draws (perhaps with some difficulty, depending on how
the oven is wired into the circuit). You should see moderately high
current when the oven is first turned on, and the current should drop
to (very roughly) half as much as the oven reaches operating
temperature. I'd expect the oven control loop to be reasonably
damped, so the current falls from the startup value to the idling
value within just a few seconds and stabilizes without overshoot, or
with only minor overshoot. At least that's what I've seen on several
models, including the HP10811.

These days, you can get some remarkably small and low-power oven
oscillators, down to TO-5 transistor can size I believe, and possibly
even smaller. The power to run them is low milliwatts. Even
temperature-compensated oscillators have gotten remarkably good...I
have some 3x5 millimeter ones that are rated half a part per million
max deviation over -20C to +55C (and run on about 2.5 milliwatts), and
you can get better than that.

Cheers,
Tom

I don't think it's POSSIBLE to get REALLY HOT here in WI (by some
standards, that is...to me, 75 is really hot!)