[time-nuts] Re: 5061A HV Supply, et al. {External}

[paul swed]

Jim
Is the unit an option 004 tube? If so they ran the CS oven hotter for a
better signal to noise. That consumed the Cs faster. In my small
experience 004 tubes do seem depleted. Non-004s may have life in them and
the unit I call Frankenstein uses a DC oven and runs at a higher temp.
Boiling off the fumes. It still locks quite well even though I don't see
any beam current. Sort of can by doing unnatural things.
Regards
Paul
WB8TSL


On Tue, Nov 15, 2022 at 8:57 PM Jim Muehlberg via time-nuts <
time-nuts at lists.febo.com> wrote:

> Ed,
>
> Thanks for all the info.  Indeed, the old HP manuals are really good and
> informative.  Your comment led me to the A18 schematic - I assumed it
> would just be a block in the schematic, but alas, HP does it right and
> shows the schematic of a "Non Repairable" hermetically sealed unit!
>
> I did rip into the supply already, but I've found no smoking gun.  All
> components are in tolerance. (though I understand it may fail under high
> voltage)  It opened up easily with some careful propane torch
> application.  No damage!
>
> The 10M input to the DMM is too low, according to one poster.  I built a
> 182MOhm divider and still no luck.
>
> I don't know much about Ion pumping, but there's a few guys here at NRAO
> that have had experience.  Back in the 70's, Green Bank had some dewars
> with ion pumps, but all that now has been superseded by turbo pumps.
>
> This little project is just a fun distraction from my real job, though
> it would be nice to have this in the lab, it's totally not necessary for
> local oscillator development.  It may help our grad student if we can
> get a mating pair of standards.  I guess the joke about have one or two
> clocks may apply here.
>
> I've been offered an A18 for a good price, which, given the labor I've
> invested so far, is a distinct possibility. (Thanks Corby!)
>
> Anyway, I can't stand to see this stuff tossed out.  It is a high
> precision tube and it has a 90's date code.  I wish knew more, but
> that's it.  They should have had a running time meter on these things!
>
> (coming soon - free 5061 parts!!)
>
> Thanks,
>
> Jim
>
> On 11/15/2022 5:14 PM, ed breya via time-nuts wrote:
> > Jim, your A18 is likely just fine, so don't rip into it without
> > knowing more about what's going on. I knew nothing about the 5061A
> > except that it's a Cs standard. I got curious about all this
> > discussion, so found the manual and did a quick look-through. It's
> > pretty fascinating and educational.
> >
> > Others have already given suggestions that cover most of the pieces.
> > Here's a quick summary as I see it:
> >
> > 1. A Cs tube that's been inactive for a very long time needs to be
> > ion-pumped down to proper vacuum level, as indicated by the pump
> > current. The system protects the hot-wire ionizer and electron
> > multiplier by disabling their activation until the vacuum level is right.
> >
> > 2. The built in 3,500 V ion pump supply only provides enough current
> > to run this function for "normal" conditions like continuous or
> > "often-enough" (every 6 months) operation. When the pump current is
> > too high, the voltage drops off, but it still pumps, just at a lower
> > rate. The fact that it's drawing (excessive) current on the meter
> > shows that it must be pumping, unless the current is taking another
> > path due to some failure. If there is true pump current, it should
> > still eventually get it down far enough, if the tube is viable.
> >
> > 3. The procedure in the manual shows the use of an external HV supply
> > to relatively quickly get the vacuum into the right range. The
> > recommended maximum current is 5 mA! I found it kind of surprising to
> > be this high, but that's the deal. It further says that if the current
> > remains 5 mA or more after a minute, the tube is likely shot. So, to
> > get an assessment and recovery in a reasonable time, you need an
> > external HV supply.
> >
> > 4. I'd recommend looking for a commercial small HV PS in the 1 to 5 mA
> > range. A 1 mA one will be much easier and cheaper to find, and should
> > still get the job done, just slower. I strongly advise against rolling
> > your own with big iron or big power HV stuff like microwave oven guts,
> > unless you're thoroughly familiar with how to safely handle and work
> > with HV. This can be fatal if you screw up badly enough.
> >
> > A small lab type HV PS can give you a good zap, but you'll live to
> > tell about it, unless you really really really screw up. Again,
> > smaller is safer, so a 1 mA or so seems good. Small HV PSs are common,
> > often used inside equipment, for all sorts of applications, or NIM
> > racks and such. Preferably you'd want the right polarity (many -
> > especially smaller ones - are reversible either way), and variable
> > from zero to at least your needed voltage, and with V and I readouts.
> >
> > If you want to make one yourself, and learn about HV with reasonable
> > safety, I'd recommend starting with some small resonant-Royer
> > converter circuits - these are everywhere, in the form of CCFL tube
> > drivers for LCD backlights. You should be able to find lots of DIY
> > examples and parts to inspire.
> >
> > 5. Back to the measurements, as has been suggested, you simply must
> > use a high enough dropping resistance for probing the HV, without
> > overloading the output. In this case though, you don't really need to
> > worry about it, since you have a probe built in. The 200 megs of R4
> > drops the output voltage for the regulator circuit. In conjunction
> > with the resistors in the bottom of the divider, it appears the
> > feedback signal is normally +4 V. It's complicated a bit by the
> > current sensing, but close enough to be a good proxy for the output.
> > The trick is to measure the feedback with a non-loading voltmeter - or
> > at least one with very high input resistance like 10 megs or more. 10
> > megs would give a few percent error, tops, in both measurement error
> > and possible upset to the circuit operation. Many lab grade DMMs like
> > HP or Fluke have a native range over +/-10 V, so can easily look at
> > the 4V signal without loading.
> >
> > So, if you properly monitor the feedback in the as-is condition, it
> > will indicate the very low actual output, divided by about 875 times
> > (3500/4). Now, if you safely unhook the tube load per the manual, the
> > feedback should come up to around the proper 4V setpoint, and the
> > output voltage should be right. If not, then there's more trouble.
> >
> > Anyway, even if there's still a problem, I wouldn't worry about the
> > A18 yet, and first do the external HV recovery. If it seems to work
> > (low enough ultimate current), then hook it back to normal and see
> > what happens. If the current stays very high, then the tube is shot,
> > and A18's condition is moot.
> >
> > BTW, you may want to study up on ion pumps in general, to get some
> > idea of what's going on. I see here that there's a huge possible range
> > - well over 100 to 1 - of ion current involved, so I'm wondering what
> > ions are being pumped here, exactly. I assume it's mostly Cs ions that
> > are loose in the beam-line, where they don't belong. At first I
> > thought the whole deal was some sort of gettering function, to trap
> > out any bad contamination ions like in most vacuum tubes, but to take
> > up to 5 mA initially, I'd say it would have be a pretty crappy vacuum
> > to begin with. So, I think the ions are the good Cs ones, that just
> > need to be relocated. The manual probably explains it, but I haven't
> > seen it yet.
> >
> > Also speaking of ion pumps, long ago I acquired a small one for
> > developing high vacuum. These kinds of pumps need the same sort of
> > deal as the tiny one in the Cs tube, just scaled up many times. The
> > main thing is some form of current limiting (ballast), forming a
> > variable voltage range depending on the current. I forget the exact
> > numbers, but mine needed up to 5 kV at maybe 15-25 mA maximum (high
> > vacuum, medium pumping rate - the current gradually drops as particles
> > are cleared), and maybe 150 mA from 1 kV to dead short. This is where
> > big iron is ideal. I used a microwave oven transformer and voltage
> > multiplier, and a big ballast choke and resonator capacitor on the
> > primary. After much fine tuning of the basic circuits and parts, I got
> > it to nicely match the desired I-V curve of the commercial unit
> > normally used.
> >
> > Ed
> >
> >
> >
> >
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> --
>
> Jim Muehlberg
> Senior Engineer
>
> National Radio Astronomy Observatory
> ngVLA Local Oscillator Lead
>
> 1180 Boxwood Estates Rd B-111
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> P 434.296.0270
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