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

[paul swed]

Julien
I don't think I would read a lot into the failed R just replace it. They
are in a nasty hot environment and simply can age and die. Get at leat 2
resistors just incase there really is a shorted transistor or something
else going on.
Regards
Paul

On Fri, Feb 4, 2022 at 3:50 AM Julien Goodwin <time-nuts at studio442.com.au>
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.
>
> 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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