>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 ... You could try powering it up and checking its telemetry with the Monitor3 program available on the Symmetricom website. If it's like mine, after it's fully warmed up, the "DAC gain" and "numeric gain" will be maxed out, and there will be a bunch of alarm codes. One of them will be alarm 11. The Monitor3 program (which was designed for a newer model of cesium) will not translate that code, but the 4040A manual says it means "12.6 MHz signal power low". If you get that error, it's worth probing the lines between the STEL chip and the DAC. Mine were all stuck low. The STEL chip, its DAC, and related filters and amplifiers are all just under the top cover of the unit, on the far right of the PCB as seen from the front. Units with serial numbers lower than 2000 apparently used a different frequency synth scheme that does not include an STEL chip. Cheers! --Stu