[time-nuts] Schematic / advice on repair of Datum PRS-50 cesium - ionizer issue?

[Glenn Little WB4UIV]

As a tungsten filament ages, the resistance goes up.
We saw this as a problem when I worked with tower light monitoring.
As a tungsten filament lamp ages, it draws less current and produces 
less light and heat.
This is one reason that the US FAA regulations require that a tower 
light must be replaced when it has been in service for 75% of its rated 
life or when it fails, which ever occurs first.
I do not know of anyone that replaces tower lights unless they fail.

If the heater in question is tungsten, and it has a lot of hours on it, 
this may be the source of your problem.
If this is the case, unless the filament is replaceable, you might be 
able to get the temperature up by raising the filament voltage.
This is what we do in the broadcast industry when a transmitter tube 
starts to produce less power due to lack of electrons being emitted by 
the cathode.

If the heater is not tungsten, disregard the above.

I wish you well with your repairs.

Glenn
WB4UIV
Retired TV/FM chief engineer

On 12/19/2018 2:16 PM, Warren Kumari wrote:
> Hi there all,
>
> I have 2 Datum PRS-50 "Cesium Beam Primary Reference Source" units.
> One of them works perfectly (although, annoyingly the system time/date
> cannot be sent to anything later than 2017 because of firmware version :-))
>
> The other, well, not so much - I'd like to get both units working, and then
> sell them -- I've also got an FTS 4060, and my wife keep reminding me that
> I really don't need 3 cesium devices :-)
>
> The units are managed through TL1 (which I always find unfriendly), but
> dumping the alarms gives:
> DATUM 08-12-15 14:21:49 M 1 COMPLD "*C,21,08,18,00,00";
>
> This decodes to:
> 21 - ionizer supply error (>+-0.16 V of memorized nominal value)
> 08 - oscillator control voltage (>90% <95%)
> 18 - primary loop gain near limit (1 or 128)
>
>
> Dumping the variables (RTRV-VARS) gives:
> DATUM 08-12-15 14:22:37
> M  1 COMPLD
> "2,R+Z,11,21,01,08,18,00,+000,+000000,+25.83,01.0,-510,-0011,-068,-0025,-4780,0081,-0039,0131,+0332,5.00,-0006,-4123,+5.08,+24.2,+14.9,-15.1,c,05.2,14.6,0.49,18.6,08.3,002,+279";
>
> Most of these values look within their expected ranges, other than the 5th
> to last one (0.49) which is "ionizer supply - voltage (V) across the
> ionizer filament", which  should be more like 1.6V, and the 2nd to last
> (002) which is "IP HV supply - current (μA) being supplied by the ion pump
> high voltage supply (+3500 V)" which the example shows as 025.
>
> I've measured the voltage going to the HV supply, and it aligns with the
> numbers the system is reporting (8.4V (3rd to last variable) and 25.7V
> (11th variable (+25.83)). I'm not comfortable measuring the output of the
> HV supply, but I'm guessing the current it is reporting (002 uA) is not
> unreasonable if the ionizer filament isn't getting sufficient voltage.
>
> Is this a known issue? This unit sat (unpowered) in storage for a while,
> but I would expect the the voltage over the ionizer filament to be "normal"
> if it lost vacuum (unless it completely lost it, and the filament oxidized
> and burnt off?!). I've had it sitting and powered for a while and there is
> no change on any of the values.
>
> Does anyone have a schematic for the PRS-10, or the Datum 5045A package it
> uses? I'd like the try measure the resistance of the ionizer filament /
> measure the voltage myself and possibly trace the power section for this...
>
>
> W
>

-- 
-----------------------------------------------------------------------
Glenn Little                ARRL Technical Specialist   QCWA  LM 28417
Amateur Callsign:  WB4UIV            wb4uiv at arrl.net    AMSAT LM 2178
QTH:  Goose Creek, SC USA (EM92xx)  USSVI LM   NRA LM   SBE ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"