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

[Bob kb8tq]

Hi

Quick couple hints on basic Rb operation:

1) The cells / lamps need to be at the proper temperatures. Oddly enough
they don’t all work when at the same temperature. There often are two oven 
controllers in there to accomplish this. 

2) The synthesized “interrogation signal” needs to be pretty darn close before
it tries to lock. An local oscillator that has drifted a few tenths of a ppm in the
wrong direction may be all it takes to stop things.

3) What you normally look at out of the photo diode is an audio tone. It’s the
recovered modulation on the interrogation signal. Pretty much everything needs
to be close in order to get that tone. 

4) There is an RF exciter circuit that “lights up the bulb”. If it has stopped working,
you get no nice purple glow and everything comes to a halt. 

Yes there is a lot more to it than that. I’m just trying to point out a few areas that
probably need to be checked before you start a deep dive into this or that. 

Bob

> On Feb 1, 2022, at 4:17 AM, Julien Goodwin <time-nuts at studio442.com.au> wrote:
> 
> 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?
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