[time-nuts] Thoughts on Cs tube failure modes

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Nov 30 09:49:27 UTC 2008


John Miles wrote:
> I recently found a tube from a 5062C on eBay in unknown condition for not
> *too* much money, and thought it would be interesting to power it up on the
> bench.  Once I got it and saw the 19xx-prefix serial number, I wasn't too
> optimistic, since it could potentially be 25 years old or more.  Things went
> well for the first few steps of the process, but then the experiment failed
> big-time.
>
> 1) I first applied +2600V to the ion pump with nothing else connected.  Spec
> is < 10 uA.  There was a brief spike to ~100 uA, but within a few seconds,
> the current began to drop rapidly, ending up at about 1 uA after a few
> minutes.  So far, so good.
>
> 2) I then brought the Cs oven up to temperature slowly with a variable
> supply.  The 5062C runs its oven in a thermostatic loop, but it was easy
> enough to warm the oven up slowly over 10 minutes or so, watching the
> thermistor resistance to achieve the 200-ohm reading indicated on the tube
> label.  The ion pump current rose to about 2.5 uA during the Cs oven warmup
> process.
>
> 3) I then attempted to bring up the hot-wire ionizer, which takes 1 volt at
> about 1.6 amps (when hot).  Simultaneously, the 22-mA C-field current and
> 13.9-volt mass-spec supply was applied.  As with the Cs oven, I brought the
> ionizer voltage up slowly.
>
> 4) At that point the ion pump supply went into full current limiting at
> circa 300 uA.
>
> I killed the power quickly, removed the oven and hot-wire ionizer supplies,
> and tried powering the ion pump up by itself once again.  Although a DMM
> check indicated infinite resistance across the ion pump, the HV supply still
> went into current limiting.
>
> I'm guessing that the hot-wire ionizer element had enough crud on it to kill
> the vacuum when it vaporized.  The tube envelope is probably OK, because the
> ionizer wire itself didn't burn out.  Unfortunately I was watching only the
> hot-wire ionizer current during that part of the process, so I don't know if
> there was a point where I could have observed a rise in ion pump current and
> backed off in time to avoid permanent damage.
>
> The last step would have been to connect the -1900V electron multiplier
> supply, feed in a 9.192632 GHz signal from an HP 8672A which would be
> frequency-modulated with a slow sawtooth, and watch for an output signal on
> a scope with a high-Z opamp buffer.  Unless there is some kind of sequencing
> taboo that says "bring up the electron multiplier before the ionizer", I
> don't immediately see what I might have done wrong.  Anyone see any obvious
> newbie mistakes in the account above?  Or was it just a matter of expecting
> too much from a Cs tube that might have been 20 years old?
>
> For what it's worth, the electron multiplier also shorts out its (negative)
> supply now.  I don't know if that would've happened earlier, since I never
> tried to energize it during the pre-test checkout.
>
> -- john, KE5FX
>   
John

Perhaps everything is now coated in Caesium, especially the electon
multiplier input dynode insulation??

Bruce




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