[time-nuts] Another hydrogen maser success story

[paul swed]

Great write up and than you for sharing
Regards
Paul.

On Thu, Jan 16, 2020 at 7:42 PM Skip Withrow <skip.withrow at gmail.com> wrote:

> Congratulations to Magnus on his new toy.
>
> I would like to relate my recent hydrogen maser experience.  Hope you
> enjoy the story.
>
> Probably about a year ago I was introduced to Dave Howe at NIST.  Last
> fall at a hamfest we were talking and I mentioned that I was recently
> retired and wondered if it might be possible to 'play' up at NIST. He
> mentioned that there was a hydrogen maser that had experienced a power
> outage and did not come back up.  It had been taken out of the time
> scale and was sitting in what was essentially a back store room.  He
> asked if I would like to try and bring it back up?  Duh, of course I
> said yes.
>
> So began what has been quite a journey over the last six months.  I
> have gone up to Boulder for a day every week or two over that time.
> First, was getting the maser from its storage location to a lab where
> it could be left and I could work on it.  The unit is a Sigma Tau
> model NIST-2001 s/n-1 manufactured about 1990!  It resembles Corby
> Dawson's maser kit, and is the precursor to the current Symmetricom
> (which bought Sigma Tau) MHM2010.
>
> The unit had been sitting unpowered for quite some time.  Turns out
> both of the 24V supplies had failed at some point, so they were
> missing along with the power cords and fuse holders.  It had a cart
> with it for powering the unit from the External DC and a kludge to
> charge the batteries.  Next all the covers were removed, and the
> vac-ion pumps and hydrogen valved off.  At this point power was
> applied and the DC fuse blew.
>
> Turns out one of the DC/DC converters in the power section was toast.
> That assembly was dismantled and a new DC/DC converter was fitted for
> the unobtanium blown part.  No more Ext DC fuse blowing.  Currents on
> the vac-ion pumps are high (as expected) but after a week they came
> down to reasonable levels.  Next the valves were opened up to the
> maser vacuum system and the vac-ion currents take off again (as
> expected), but after another week looked good so no vacuum leaks.
>
> At this point all the heater fuses were plugged in to get the cavity
> up to temperature (another week).  Now all is ready to try and light
> the source bulb.  The Pirani guage and source heaters were powered up
> and the hydrogen turned on.  But all attempts to light the source bulb
> with the RF generator fail.  The RF generator was dropped from the
> unit and after several weeks of trying lots of different things was
> finally rebuilt.  After installation the source bulb still did not
> light.
>
> We resorted to a cattle prod and a piece of aluminum tape up to the
> source bulb to get anything to glow.  The palladium leak heater was
> operated open-loop and the hydrogen pressure was monitored with the
> vac-ion currents.  Things seemed to get better with higher H2
> pressures.  So, the RF generator was adjusted some and reassemble and
> YES, there was light.  In fact, the hydrogen pressure could be lowered
> all the way down to its normal level and the source bulb was still
> glowing.
>
> But alas, there was still no IF signal meaning that it was still not
> oscillating.
>
> So today the top of the maser was disassembled to get to the 1.240MHz
> circuits and the vacuum chamber RF connection.  There are about a
> dozen different layers of electronics, magnetic shielding, and
> insulation to get through.  A -100dBm signal was stuffed into the
> receiver front end and there was IF signal indication - good news, the
> RF chain is good / bad news, the cavity is not oscillating.  So all of
> the top was re-assembled (I meant to get a picture of all the goodies
> on top of the vacuum chamber, but spaced it).
>
> Where to you go from here?  I figured more excited atoms into the
> cavity could not be a bad thing and figured I would try cranking up
> the source RF generator power.  I had installed a trimmer cap that
> would help me control the operating point of the oscillator during the
> rebuild.  When tweaking the cap all of a sudden, BAM, lots of light.
> When the maser started masing if was obvious!  IF signal was
> immediately at the right level.  After a few minutes the green LOCK
> light was even solid!
>
> Along the way both of the 24V power supplies were replaced, along with
> the fuses and power cords.  Batteries are now back on line on the
> internal supplies.  Next steps are to get the covers back on and let
> it settle for a while.  We will check the ADEV against the (local)
> NIST time scale (don't have the issue of a good source as Magnus
> does).  The goal, I believe, is to get it moved to Colorado
> University.
>
> I would like to thank Dave Howe for the opportunity and all the
> insight, help, and suggestions along the way.  It has been an
> invaluable educational experience.  I have attached one picture of the
> bottom of the unit.  There is a mirror laying in the bottom reflecting
> the bottom of the RF generator where you can see the nice purple light
> of the source bulb through the hole.  The other picture is the front
> of the unit (with the two new Lambda power supplies) showing the IF
> level reading and the green lock light.
>
> Sorry for the long post.
>
> Regards,
> Skip Withrow
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