[time-nuts] Efratom FRK with "inoperative lamp"

[Julien Goodwin]

I picked up an Efratom FRT a while ago, and it finally arrived the other 
day, sadly it doesn't lock.

The oscillator inside is an older Efratom FRK, and it's also clear 
someone had been inside already, although it does appear to be complete.

The FRK manual does include a key description I wish I'd noticed a few 
days ago before full disassembly:

"Terminal pin 7 provides an indication of proper operation of the 
rubidium lamp. For nominal operation the signal is 6 to 12V. An 
inoperative lamp is indicated by an signal of approximately 3V. The 
internal resistance of this circuit is approximately 6K."

... indeed I'm seeing 3v on that pin.

Based on the circuit this appears to simply be the output of the first 
stage op-amp from the photo-cell, so is this effectively an indication 
that the (electrical part of the) lamp is dead? Certainly it wasn't 
anywhere near the temperature it should have been when I removed the 
bulb after running for over an hour.

Just for completeness, the unit does draw up to ~0.8A when started up 
cold with 24V, dropping down to about ~0.5A after a while. The sealed 
sub-section of board A4 seems be where all the heat is, and I'm assuming 
that's an ovenised part of the crystal oscillator.

I also popped the rubidium bulb out and confirmed there's no obvious 
issues there.

I do have another FRK here, but as it's an FRK-LN, and it's also my 
working house standard I'm somewhat loath to open it up lest I end up 
with two dead units and zero working.

Any suggestions?