[time-nuts] Re: HP 5061A - bad cesium beam tube?

[Chris Hastreiter]

Matt,

If you're seeing a second harmonic reading of 15 it's very likely that
there is still *some* cesium left in your tube.  No telling how long it
would last though.  If you haven't yet I'd recommend downloading a copy of
the service manual as there is a lot of good information there for
adjustment.

You should be able to get it to lock with a beam current of only 2, but it
may be difficult to see the peaks if you follow the normal startup
procedure.  Do you have another reference you can use to get the oscillator
close to on-frequency and try closing the loop?

Depending on the measurement equipment you have available I'd suggest
checking the following:


   1. Measure the electron multiplier voltage (-2500V supply A19).
   Depending on the age of your unit this could be adjustable on the A15
   assembly either with a potentiometer or by changing out a resistor.  The
   "Beam I Meter" adjustment on the front panel only changes the meter
   indication, but by changing the electron multiplier voltage you can try to
   increase the beam current (should be measured at the output of the tube
   where it plugs into A7-J1, there is a procedure in the manual)
      1. If the tube is running low on cesium (likely) you can improve the
      SNR for a while by boosting this voltage
   2. Perform the loop gain adjustment procedure on A7.  By increasing the
   gain of the AC amplifier you can boost the amount of 2nd harmonic signal
   available to help with lock


If you can get it to lock with the above then there are further adjustments
you can make.  Mine is in a similar boat and I just keep it in CS Off most
of the time and power up occasionally for a test.

Chris

On Sat, Feb 25, 2023 at 10:25 AM Matt Huszagh via time-nuts <
time-nuts at lists.febo.com> wrote:

> I received a 5061A that I think probably has a dead tube, but I'd like
> to be sure before I give up hope.
>
> I expect the unit has been off for quite a few years, so I first powered
> it on with CS OFF. The ion pump reading pretty quickly dropped below 25
> and stayed there. I left it like that for about 12h before switching it
> to LOOP OPEN. The ionizer, cesium oven, etc cycled on and off in that
> state for roughly another 12h before the ion pump reading was low and
> stable enough for these circuits to remain on (below a reading of about
> 25). Once it got to that point the ion pump dropped down to about 20
> over a few minutes, where it stayed. I then coarse-tuned the OCXO to
> match the frequency of my HP 5061A. Unfortunately, the beam current
> indication was only about 2. I waited another 12h. The ion pump reading
> had decreased to about 18 and the beam current was still around 2.
>
> Here are all the indicator readings, and the specified normal range in
> the manual:
>
> | indicator    | value | normal |
> |--------------+-------+--------|
> | battery      |    NA |     NA |
> | supply       |    40 |  35-45 |
> | ion pump I   |    18 |   0-15 |
> | osc oven     |    39 |  20-45 |
> | cs oven      |    14 |   5-35 |
> | 5 MHz        |    37 |  35-45 |
> | mult         |    42 |  35-45 |
> | beam I       |     2 |  15-30 |
> | control      |    -2 |        |
> | 2nd harmonic |    15 |  20-45 |
> | 1 MHz        |    37 |  35-45 |
> | 100 kHz      |    37 |  25-45 |
>
> I measured the +3500V supply as +3.2kV with it still powering the
> beam. That's within the >+3kV spec.
>
> The ion pump reading is still above the 15 spec. Should I leave it on
> for a couple days to see if that comes down? The manual does say this
> could take a few days to be in line. The extremely low beam current
> reading concerns me, however. I haven't touched the BEAM I METER pot
> yet, so I don't know how that's set.
>
> Anything else I should do?
>
> Thanks
> Matt
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