[time-nuts] cesium clocks..

wje wje at quackers.net
Fri Jun 27 07:32:04 EDT 2008

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 
old! ('82).

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!

Bill Ezell
They said 'Windows or better'
so I used Linux.

More information about the time-nuts mailing list