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

[ed breya]

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