[time-nuts] cesium clocks..
wje at quackers.net
Fri Jun 27 19:40:56 EDT 2008
Yes, but my comment is rather specific... my oscillator failed because
the heater pass transistor shorted.
This sent the oven heater into full-on. The overtemp sensor is far
removed from the heater. There's a ribbon cable between the driver board
and the temp bridge sensor board that runs directly over the heater, not
outside the oven insulation. Net result, transistor shorts, cable fries
before overtemp fuse opens. There's no reason the cable should be inside
the oven insulation.
My rebuild fix - (after removing all the carbonized foam insulation)
refoam the oven, replacing the cable and transistor, and moving it
outside the foam!
While I respect in general the brilliance of HP engineers, my classmate
was one, this isn't one of the more intelligent decisions.
They said 'Windows or better'
so I used Linux.
Dave 'SqueezeBox' Carlson wrote:
> 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
> 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!
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