Great write up and than you for sharing Regards Paul. On Thu, Jan 16, 2020 at 7:42 PM Skip Withrow <[email protected]> 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 > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
