Hi Stu, Got your private email, but it slipped through the cracks of time.
On 04/29/2013 07:51 AM, Stewart Cobb wrote:
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.
The output drivers of the STEL-1173 seems to fade, so they are obviously not designed with sufficient margin.
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.
I have some STEL-1173 chips, but not of the same packaging as the one I got from another source which actually solved the issue. I do not know which is the correct one for the FTS-4040A caesium, and I have not cross-checked.
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?
It seems to be a common problem, their output degrades over time until bit after bit get stuck with interesting results to the loop until it completely looses lock.
2) Does anyone know a source for replacement STEL-1173 chips?
They are not easy to find.
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.
I have not seen any.
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.
I had about the same issue. Everything else looked good, so it was not until someone experienced helped me to check the STEL-1173 outputs.
5) Any other suggestions or hints for repairing this unit?
There are folks that knows them better than me. I has an experienced hand to assist me.
Cheers, Magnus _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
