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