[time-nuts] cesium clocks..
Dave 'SqueezeBox' Carlson
dgcarlson at sbcglobal.net
Fri Jun 27 18:36:08 EDT 2008
Careful how you toss around the 'stupid' when discussing oscillator design.
Some folks still have a close relationship to those old 00105-6xxx
----- Original Message -----
From: "wje" <wje at quackers.net>
To: <time-nuts at febo.com>
Sent: Friday, June 27, 2008 4:32 AM
Subject: [time-nuts] cesium clocks..
Having just gone through the process of finding, acquiring, and fixing
an HP 5061A, here are a few pointers for qualifying one for purchase.
The biggest problem you're likely to face is a beam tube that's at
end-of-life. If there is an electronics failure (like mine, read on),
these clocks are really fairly simple to troubleshoot and fix; almost
everything is discrete components; the main circuitry is mostly analog.
If the seller says the unit locks and goes into continuous operation
mode, the quickest check you can ask them for is to report the beam
current. It should be above 15. However, note that this is a relative
measurement. There is a meter adjustment control that sets the meter
sensitivity, and it's possible it's just not properly set. So, a low
beam current isn't an absolute failure indication. With low beam
current, if it locks, then you can still have a working clock. The
primary impact of low current is more noise in the signal, which leads
to greater short-term frequency variation. Even with that, it's still
going to be in the 10e-10 or 10e-11 range. If you average, over time the
accuracy will be about as good as a newer tube.
If it goes into continuous operation for a while but then loses lock,
you're taking your chances. The tube could be so depleted that it can't
maintain lock, or the clock could just not be adjusted properly, or you
could have an electronics failure. A quick check is to ask the seller
for the ion pump current reading. If it's not less than 10, then the
problem could just be that the unit has been sitting around for too long
without the ion pump having been run. This is curable just by having it
run for a few days, or in extreme cases, using an external 3500v 5 ma
supply to run the pump more energetically than the clock itself can.
In any case, if you're feeling ambitious and can get a good price ($500
or less?), give it a shot.
Finally, if it won't lock at all, then either the tube is gone, there is
an electronics problem, the clock is way out of alignment, or the ion
pump hasn't run in a long time. Have the seller report the ion pump
current. If it's over 10, then you might want to take your chances if
you get a good price. If it's less than 10, buy it if you like a
challenge and can get a good deal.
I got mine for $300. It wouldn't lock. The ion pump current was high, so
I decided to give it a shot. I almost got lucky. After running for a few
days, the pump current went to zero, which is good. But, the clock would
only lock for a second or so, then lose lock. After a bit of testing, I
found that the crystal oven had fried itself and some wiring inside the
can. (The design is really stupid; can't imagine why it was packaged the
way it was) Anyway, I rebuilt the oven, fired it up, and now have a
nicely-working clock that locks, stays locked, gives a nice 20 reading
on beam current, and has a high-output tube. BTW, the tube is 25 years
Finally, if you do need to troubleshoot and align the clock, you can
easily get by with a good ac/dc DVM (10 meg or higher impedance) a 100
Mhz scope, and a reasonably good counter, one that can reliably read 12
Mhz to 1ppm).
If anyone wants any more tips or info, feel free to ask, and good luck!
They said 'Windows or better'
so I used Linux.
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