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