[time-nuts] (Datum) FTS-4040A cesium, and STEL-1173 NCO chip

[Tom Knox]

Hi Stu;
Interesting, I have a problem Datum 4040A as well. I purchased it several years ago and when it fail to work correctly I put it aside.  I wonder if mine has the same problem, I find your assessment that the STEL-1173 is problematic interesting. I have found over the years that certain models of equipment have a weak link and that most of those models that fail will have a common poorly designed or selected part. 
Thomas Knox



> Date: Sun, 28 Apr 2013 22:51:37 -0700
> From: stewart.cobb at gmail.com
> To: time-nuts at febo.com
> Subject: [time-nuts] (Datum) FTS-4040A cesium, and STEL-1173 NCO chip
> 
> Guys,
> 
> I'm working on a sick FTS-4040A cesium frequency reference, which is
> basically a box and power supply wrapped around the FTS-5045 cesium beam
> module.  (Datum bought FTS, then Symmetricom bought Datum, but this 1995
> device is too old to appear on the Symmetricom website.
> 
> This was one of the first "digital" cesiums.  It uses a microcontroller to
> drive a frequency synthesizer to sample many different points in the cesium
> tube's frequency response.  Part of the synthesizer is a 12.6+ MHz DDS,
> built from a Stanford Telecom STEL-1173 digital numerically controlled
> oscillator (NCO) chip and an Analog Devices fast D/A chip.
> In my unit, the STEL chip seems to be broken.  All the inputs seem to be
> correct, but the output bits to the D/A are all continuously low, so there
> is no 12.6 MHz signal and thus no lock.  The unit also throws a fault code
> "11", which translates as "12.6 MHz signal power low", confirming this
> diagnosis.
> 
> Web searching gives the impression that these STEL-1173 chips are known to
> be fragile.  There was a reference on time-nuts in 2006 to repairing a
> cesium by replacing the STEL chip, In 2009, a Steve Swift who read the 2006
> thread offered to sell some STEL chips to the thread originator, but the
> deal (if there was one) was conducted off-list and I can't find valid
> contact information for Steve Swift.
> 
> So it's time to consult the hive mind:
> 
> 1) Are these STEL-1173 NCO chips known to fail early?  Are there ways to
> "baby" them so they don't fail early?
> 
> 2) Does anyone know a source for replacement STEL-1173 chips?
> 
> 3) Is there any detailed documentation (schematics, procedures, test
> points, etc) available for the FTS-5045 cesium module?  (I have the 4040A
> operators manual, but it doesn't show much detail.
> 
> 4) Is there a way to tell whether the cesium beam tube itself is good,
> without the frequency synth working?  All other telemetry points (ion pump,
> electron multiplier, etc) seem to settle to nominal values after warmup, so
> I have some hope that the tube is still good.
> 
> 5) Any other suggestions or hints for repairing this unit?
> 
> Cheers!
> --Stu
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