Quick Fusor Check

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Dennis P Brown
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Quick Fusor Check

Post by Dennis P Brown »

Ran the fusor with the new pre-amp but didn't work with my new 3He tube. Suspected the 3He tube is having issues so I pulled out my very old one.

That can not currently be used with the new pre-amp due to connector issues. So I connected that old detector tube to my new Eberline counter/power supply and set it to 700 volts. It appeared to work well. A few dozen counts a minute and zero when I pulled it out of the paraffin. Did this three times and same result. Then tried just using current through the fusor and the counts tracked down as the power did and disappeared below 20 KV. Came back above 20 kV.

Tried varying the voltage to the old 3He detector (up to 1 KV) but that didn't change the count rate at all. Then after a few minutes, this resulted in complete silence by the unit. Puzzled by that. Turned off the fusor and will look at this question another day. I will wait till I get the new connectors and cable. Then I will try hooking up some other detectors, as well as the new 3He tube all using the pre-amp. Slow progress and always issues that make no sense - working in the dark with a blindfold.

Did have issues with the pre-amp circuit board shorting between contact points on their board (the HV contract wire feed points are dangerously close together to the ground points - a really bad design. Guess I should add epoxy at some point.) Apparently needed more than just water/soap cleaning to remove old solder flux - had to use 90% alcohol. That issue was really annoying to say the least.

Should have gone turnkey but then, wouldn't have learned (or learnt for the English) too much, either.
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Richard Hull
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Re: Quick Fusor Check

Post by Richard Hull »

Note: HV arcing to ground on the PC board can destroy the CR-110. Any induction into the pin#1 input, be it inductive, capacitive or RF, can snuff the input FET. I worry that their board is designed for PMT voltage input (600-800 volts) He3 voltages at 1500 volts and BF3 over 2000 volts might not have been designed into the board. Measure the distance from the bias input center pin to the nearest ground and report back.

I check your image of the board and note there are three regular, common BNC jacks. Very, very bad. The bias should have an SHV BNC plug on it!!!
The board design might not have allowed for this 5-10KV special plug as it demanded much larger separation on the board for the hot lead and and ground run.

I do hope the CR-110 is alive and well after the arcing. Attached to a PC board a common BNC should never accept 1 kv. A common bulk head mounted BNC might well handle 1500 Volts as its center terminal would take a single wire off to a point ground is no where near where the wire is attached.

This is why real charge sensitive preamps have all HV terminals and components suffering such potentials mounted on special Teflon standoffs that can easily hand 4,000 volts off the chassis ground. I show these in my floating circuit presented in your preamp post.

Image shows how Princeton Gamma Tech handles HV in its preamp.

Richard Hull
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PGT 4 HV points.jpg
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Dennis P Brown
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Re: Quick Fusor Check

Post by Dennis P Brown »

I will do the checks - this just proves that using your design would not only have been cheaper but far better! My luck with electronics - I did install a SHV for both the high voltage input (bias) and output to the detector (really a waste.) The regular connectors are for the chip output signal (which I believe is low voltage) and a test/pulse generator input(?) connector for the circuit - those shouldn't see any HV. But you are certainly correct that the circuit is not designed for anything even approaching voltages of 1 kV+.

I believe the shorting was between the built in ground and HV (empty) circuit board access points on said board (see pic.) The HV in from the SHV that excited the circuit likely jump the tiny gap* between those built in access points (for their std coaxial connector; which I didn't use) on the circuit board due to trace flux. But you are correct that the chip might then have been destroyed.

Guess I had better build a proper circuit as you outlined (and thanks for that design!) - one that can definitively handle the 3+ kV's that a BF3 tube requires (I have some 6 kV 100 mF caps) besides the 1.5 kV the 3He uses. I will likely (if the chip is still working) devote this circuit board to my photo-multiplier neutron detector system I built (and, again, assuming that is working, too!) That will require adding a lower voltage connector (a std HV) to the box.

* Not even a millimeter separation!
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Commercial Circuit Board for Chip
Commercial Circuit Board for Chip
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Richard Hull
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Re: Quick Fusor Check

Post by Richard Hull »

Can you photo the top of the board? I need to see the entire board's top view with components. I think the two little pins will ohm 0 ohms. The are the hot lead based on the BNCs I see that you received. I have seen the central lead of those bulk head common BNCs end in a forked two point end that is a single point. The two big knobs are ground and the hot lead is forked. I just wish I could be there and meter that board. I am sure they got it right. I would hope. I worry that is a simple test board and not an application board designed to handle 3.5 kv. on the bias. Do they supply a circuit diagram of that board?? Finally, are you sure it arc'd? Did you hear it snap?

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Dennis P Brown
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Re: Quick Fusor Check

Post by Dennis P Brown »

Here is the pic of the other side. The reverse side of the two access nodes that were shown in the previous picture are imaged here. I use one as a ground but they provide another access hole for a SHV wire as seen in this pic and is connected. When I first fired this up I heard a lot of clicking on the Eberline, and more as I slowly ramped up the voltage. After cleaning this area with 90% alcohol that noise went away. Hence I suspected this was the location of the "shorting". Suspect this damaged the chip, however.
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Reverse side photo
Reverse side photo
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Richard Hull
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Re: Quick Fusor Check

Post by Richard Hull »

Let's clear this up immediately. Go to one of their BNC jacks that you installed take an ohms reading from the barrel of its outer shell to each of the two little hole points on the board. If one of the two holes is at zero ohms that is ground. It you measure zero ohms to the other hole to the central pin on the jack input then you probably can't use this board at all related to bias! This means you were originally right and the two little holes are indeed ground and center lead connections. Very bad news....

There may be a save..... If your green wire from the ground of the SHV BNC is going to board ground that is OK. Now put the ohmmeter on that other little hole and see which of the R14 ends measures zero ohms. The end of R14 that is zero ohms to that little hole, you need to lift into midair straight up and connect a wire in air from your hot end of the SHV BNC to that now free end of the resistor.

This will end all proximity of that little hole to the high voltage bias, ending any arcing. This assumes the idiots have not brought the high voltage line from R14 still attached to the board near yet another ground point.

You said you have also used an SHV BNC for the 3He tube plug. If so you will need to do this also for whatever component is at zero ohms to that non ground little hole at that point. If you used a common BNC they supplied it will not work as those two little hole will arc as on the other end since there is also the HV there going to the tube.

I am now convinced this board is just a test board that might only be good for 800 volts maximum bias as designed due to their supplying only common BNC jacks with the board. PMT only use.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Rex Allers
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Re: Quick Fusor Check

Post by Rex Allers »

The PDF for the board claims it is good to 2000 V if you use HV rated connectors where needed.

Here's the schematic (extracted from a pdf) and the datasheet/guide pdf.

CR-150-R5-schematic_1.png
CR-150-R5.pdf
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Richard Hull
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Re: Quick Fusor Check

Post by Richard Hull »

Thanks for the schematic! I was wondering what all the other components were. They were all 100% power related.

On the board, in Dennis' supplied image, R14 is the 10meg and R4 and R13 are the 100meg resistors. If, indeed, Dennis' board arc'd at a good bit over 1000 volts, I would not trust it for any neutron proportional tubes that need over 1500 volts. I find it silly to have board pads that close when applying even 1kv!

In my image of the PGT preamp that I modified, I run 1500 volts, as you can see to the 3He tube using a common BNC, on my home-made end plate in my photo. (modification a must on all HPGe preamps). The PGT preamp is married to the cold finger intimately and must be separated with the head's FET custom transferred by the would-be user and a tube suitable BNC plug and end plate added. I have done this 3 times now, every time I get a surplus head. On the real head, they specify on the preamp a 3500 volt max HV input. As such, they all have an SHV BNC bias input on the opposite end of the amp like all regular preamps.

The person modifying the preamp can choose a suitable tube connector. I chose a common BNC, which can easily handle 1500 volts if new and clean with an insulated P.E. central pin (critical). Ideally, you would install an HV BNC for 1500 volts.

The BNCs supplied to Dennis with the board are not suitable for tube and bias inputs for really HV neutron tubes. Suitable BNCs must be of the bulk-head independent types with insulated wires running to insulated standoff or air-floating HV circuit components.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Rex Allers
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Re: Quick Fusor Check

Post by Rex Allers »

Here is a pic of the top of the CR-150 board that I cut out of the 'CR-150-R5.pdf'. I added a couple marks to it. The red circle is around the HV-in and ground pins that are suspect. I agree they look very close together for 1-2 kV.

The red arrow shows where Dennis has wired the input from the HV BNC, shown in his earlier pic, which looks fine. If it was my board I think I would cut and peel a section of the unused HV input trace, as shown with my yellow mark. That way the HV won't be connected to the pad near the ground pad and the cut section can be longer to better isolate the HV.

The pads for the detector input near the bottom-center don't look so close together, so probably ok.

CR-150-top.jpg
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Richard Hull
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Re: Quick Fusor Check

Post by Richard Hull »

Rex, thanks ever so much for the board view. No need to lift that resistor R14 to air if you completely rid that entire copper strip between that little hole and R14. Such thin copper foil runs or strips are easily lifted from the board using an exacto knife or single edged razor blade. Just lifting a large fraction of the run as in the area Rex marked in yellow will do.

Once removed, just run the lead wire from the SHV BNC center terminal (hot) to R14's pad directly.

Just proves pictures are worth a thousand words.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Quick Fusor Check

Post by Dennis P Brown »

Excellent idea ...all I can say is, experience is gold.
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