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

[Julien Goodwin]

On 4/2/22 5:15 pm, Julien Goodwin wrote:
> 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.

Or, following the FRK-L schematic[1] it is indeed a 20 ohm 2W wirewound 
(R14). All the other unused holes on the PCB lack obvious signs of 
component legs, so that might be it.

I shall see what I can scrounge up at the local electronics shop 
tomorrow[2] to see if I can bodge enough to test, hopefully without 
needing to remove the board. Glad I had new tips for my tiny precision 
iron show up today, might just be possible.

1: The FRK-L seems closer in some way than the original, although in 
others the original is. This is labelled an FRK-H "made in West Germany" 
which certainly dates it.

2: Tens of thousands of surface mount resistors here, but almost no 
through hole.

> 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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