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

[Stewart Cobb]

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