[time-nuts] Another hydrogen maser success story

[Skip Withrow]

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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