https://www.febo.com/pipermail/time-nuts/2017-April/thread.html

I spoke to Corby on the phone a few days ago about our first HP5061B
that locks fine but has ion current on the order of 76 μA.  It has not
gone down significantly in two months of pump operation.  He mentioned
that the electrodes in the pump are made from titanium and that sharp
whiskers can form on the electrodes from metal migration.  They
apparently cause corona and keep the ion current high indefinitely.
According to Corby, the vacuum in the tube may be fine and the leakage
current make it appear otherwise.

We performed an experiment using our second HP5061B that we suspect of
being out of cesium.  When we first got it, we had about 10 μA of ion
current and within a day it went down to nearly zero.  Today we
jumpered R12 on the cesium oven board to raise the oven temperature.
We previously had checked all waveforms for normal operation on the
board.  This includes measuring cold resistance of the cesium heater
at 2.6 Ω and hot wire ionizer of 0.1 Ω.  Power to each was close to
the nominal 2.6 Watts and 4 Watts respectively.

We bumped oven heater voltage up to 11 Volts with the short on R12.
This could have put up to 48 Watts into the oven heater unless its
resistance went up significantly.  After a couple of minutes the oven
150° C  overtemp circuit shut down the switching regulator.  We saw no
increase of beam current even though normal oven temperature is 85°
according to the tube data plate.  We let the tube cool down and
repeated the experiment several times.  We had turned the beam current
adjust all the way up to -2,880 V.  On our good instrument we can get
20 μA beam current with only -1,700 V or so out of the -2,500 V
supply.  We therefore concluded that the beam tube was hopeless and
decided on the risky experiment.

We removed the +3,500 V supply on the suspected bad tube from the ion
pump and connected the -2,500 V supply to the pump.  We left the
-2,500 supply on the electron multiplier as well.  We saw no drop in
its -2,880 Voltage.  We would easily have seen 200 M Ω worth of
leakage on the ion pump.  Therefore the ion pump will work with either
polarity of voltage.  We have  decided to take the risk of reversing
the diodes on the +3,500 V supply on our good instrument and watch the
ion current.  We hope that the reverse polarity will burn out the
whiskers or other leakage caused by long application of positive
voltage.  We have devised a test that will show up to 1,000 M Ω of any
resistive leakage on the tube before we apply reverse voltage to it.

πθ°μΩω±√·Γλ
WB0KVV

https://www.febo.com/pipermail/time-nuts/2017-April/thread.html

We did not attempt to achieve lock because we saw absolutely no beam
current with the beam adjust all the way up.  We were running the oven
above 150° C which is more than 65° above normal oven temperatures.
Cesium pressure should have been about 30 times normal.

A photo of the A11 Cesium controller board is posted at
http://gonascent.com/papers/hp/hp5061/photos/cesiheat.jpg taken by
KB7APQ.  Note that there are no electrolytics but tantulums installed
instead.  They must have been very costly 30 years ago.  This also
prevents hard to find problems with high ESR electrolytics with age.
We have found open 47 μFd electrolytics inside two of our four HV
power supplies.  That is a place for tantalums if there ever was one.

We are working on our HP5061B's from 500 miles apart so photographs of
boards and waveforms help greatly.  The cesium oven servo is
underdamped.  It overshoots and rings down at a 10 second time
interval during temperature steps.  You can see the R12 that we
shorted out to get 150° oven temperatures for troubleshooting our
suspected bad tube.

I think that a website should be started with the collected wisdom of
this list.  I will host it if someone will maintain it. Corby's photo
graphs of dismantled beam tube parts should be on a nice web page.
If anyone has a high ion current 05061-6077 beam tube that they can
part with please contact me.  One of our beam tubes is completely out
of cesium.

πθ°μΩω±√·Γλ
WB0KVV

---------- Forwarded message ----------
From: paul swed <paulswedb @ gmail.com>
Date: Thu, Apr 6, 2017 at 5:09 PM
Subject: Re: [time-nuts] HP5061B High Ion Current/Tubes Out of Cesium
To: Discussion of precise time and frequency measurement time-nuts@febo.com

Donald running higher temperatures on a a normal tube may indeed give you a
bit more life. Thats exactly how Frankenstein works. Its a hand me down
tube that normally showed as dead. Believe me it doesn't even move the
current meter and its working. My firm belief is that option 004 tubes do
not have anything left to give.

But yet with the higher temp (10-15 higher as I recall) it locks all on its
own after a good warmup period. Serious fumes.
Its been operating this way for some 4 years now. I don't run it all of the
time and I actually recently found what was wrong that always gave it a
slight offset.

So a very good conversation running here with everyone sharing really good
insights and pictures of detail I had only read about and generally without
any pictures. The entire thread should be gathered up, cleaned up, and
presented as the dummys guide to the care and feeding of old 5061s.

Regards
Paul.