[time-nuts] PRS10 rubidium lamp module failure

[Stewart Cobb]

Follow-up: as several time-nuts noted, there was a lot of corrosion on
parts of the lamp module. Oddly, there was no corrosion on the rest of the
PRS10, or on the TS2500 which it came out of.  The resistor lead was
severely oxidized and partly melted. It's possible that the inductance of
the wire-wound resistor helped sustain an arc. Alternatively, the grounded
lead may have arced to the hot end of the lamp drive coil, although the
coil itself showed very little damage. Perhaps ozone from an arc
accelerated the corrosion. Dunno, it's a mystery.

The resistor is specified at 2W, but it's much smaller than a typical 2W
resistor. I removed the damaged lead and replaced it with a bit of 16 AWG
magnet wire. This gave mechanical support as well as an electrical
connection. Buttoned it all back up and applied power, and the lamp lit
almost immediately (it wasn't lit before) and it locked quickly.  Tuned it
to match a GPSDO, and it seems to be working fine.

A note on tuning: the documentation says that the "SF" tuning command has a
resolution of 1E-12. That's true, but the actual available tuning
resolution seems to be more like 1E-11.

The actual tuning is done by adjusting the C-field with a 12-bit DAC. The
"MR?" command gives the present value of that DAC.  Unfortunately, the full
range of the SF command (4000 counts) only moves the DAC about 600 counts.
You can increment the SF command several times before seeing a response.
Anyone planning to use a PRS10 inside a control loop should be aware of
this quirk.

Cheers!
--Stu