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

[Julien Goodwin]

Well that was unexpectedly easy (to diagnose).

Thanks all for the replies, much appreciated, you have once again 
confirmed my appreciation for this group.

I sat down this evening to work through your various suggestions, and, 
by chance, picked up the unit without my lab power supply (and more 
importantly, its noisy fans) being on, where I heard a little rattle I 
must have missed before.

Removed the bottom panel, which I hadn't done yet, and out dropped the 
remnants of a Vishay RW80U 20-ohm resistor. Based on the circuit this 
pretty much has to be R2 on the lamp board (following the December 73 
circuit I have, which has that at 10 ohms), which is directly in-circuit 
for the lamp, so would explain exactly what I've seen.

The lamp board in general seems to have a few holes missing components, 
so I'm going to have to get it out which looks like a project in itself.

If anyone has any pointers to decent guides on optimum process on 
further disassembly, otherwise I guess it's copious photos, notes, and 
hoping I don't stuff up anything too much.

I'd also appreciate suggestions for replacement foam to use in the lamp 
cavity as the original stuff has long since degraded.

Paul, Peter Robert,
Thanks for reinforcing what I'd suspected with the lamp.

Matthias,
I hadn't bothered with an actual temperature meter, luckily on the FRT 
it's actually easy to confirm lamp temperature in use, and it certainly 
wasn't anywhere near 100c. I think what heat there was likely leaked 
through from the crystal oven, which was getting pretty hot.

Bob,
Fortunately the onboard crystal does seem likely to still be close 
enough, based on what I was seeing on a counter, so I feel like I have a 
hope of recovering the unit if I do manage to get the lamp going again.

On 1/2/22 8:17 pm, Julien Goodwin 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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