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