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Powering an Old, old geiger counter...

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Inducktion

Wed Dec 14 2016, 02:01AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Hey all.

So my latest project, right now, is trying to get this 1940's vintage geiger counter working again.

^ is the schematic for the counter. I'd prefer to try and keep as much of the original circuitry...original. That being said though, I'm trying to make the geiger counter powered off of a single 18650 cell, and have designed and made some buck and boost converters for the job. HV batteries are hard to come by, after all.

However I completely ... missed the fact that the circuit uses some weird grounding stuff, and as such I feel a little lost right now.

Is there any number of different connections I could make, so that it would have a single ground instead of the multiple ones? Buck and boost converters aren't isolated, and because of that they all share a common ground, something I can't utilize with the way the circuit is right now.

Any idea, thoughts or input?

Thank you guys!

Karmaslap

Wed Dec 14 2016, 07:37AM

Registered Member #58215 Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 65

I'm afraid I can't help you, but it looks like a cool project. Real mess working with everything already hooked up.

A clearer picture of the schematic might be useful for anyone who can help. Post updates.

johnf

Wed Dec 14 2016, 06:23PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

I would use a standard buck or boost convertor depends on filament voltage used i presume 2.5V but you use a transformer instead of the inductor so your primary is the needed inductor for the chip chosen and the secondary would be your HT battery equivalent after rectification of course. There are some converor chips that are specifically designed for use with transformers. I did one a year ago for an ultrasonic driver + - 150 volt DC rail from a 12 v gel battery.
When I get to work i'll look up the chip used and post

klugesmith

Wed Dec 14 2016, 08:38PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Do you mainly want to use the Geiger counter from time to time, or make another DIY switching power converter?

The original 90 volt battery stack could be replaced with ten 9V batteries. If total battery volume is comparable then the service lifetime should be similar.

Looks like the GM tube doesn't need a 300 volt bias. So this is no excuse to stack 100 lithium coin cells in a plastic tube.

macona

Fri Dec 16 2016, 07:55AM

Registered Member #3272 Joined: Mon Oct 04 2010, 11:40PM
Location: Beaverton, OR
Posts: 101

If you want to keep it intact I would say just find another geiger, otherwise you could replace the guts with more modern stuff. Youre probably not going to get it to run off a single 18650, you need to keep the sections isolated.

Proud Mary

Sat Dec 17 2016, 12:41AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The 1U5 diode-pentode filament needs 1.4V/50mA and the two 1AF4 filaments need 1.4V/25mA each, so you're into 100mA from your low tension batteries to get the set warmed up.

Two separate batteries are used to heat the filaments of VT1 and the pair VT2 and VT3 as a simple solution to keeping them electrically separate, which was always a problem with directly heated (filament) valves. With the design as it it is, you will have to ensure that the filament supplies to VT1 and VT2/VT3 are also completely independent of eachother, or the filaments will burn out in an instant.

VT1 and associated neons form an amplified Pearson-Anson oscillator producing a high voltage sawtooth across the choke in the anode line. The values of C1 .0047Î¼F 1600V (or possibly 1800V - difficult to see) and C5 .0047Î¼F 1600V, suggest that this voltage would have been of the order of 1 kV.

GM tubes of that vintage used organic quench agents such as ethanol or butane which typically required 900V - 1100V to get them on the GM plateau.

VT2 and VT3 form an RC time constants-type rate meter whose scale was chosen by manual selection of a capacitor by means of the switch.

Inducktion

Sat Dec 17 2016, 08:15AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Do you guys think it would be considered bad to replace the guts with more modern electronics? It seems to be pretty vintage/unique so I'm not sure if it would be a "waste" so to speak to remove them.

I mean otherwise, I just said screw it and slapped two D cell batteries in it. 0.1 amps from a D cell means that they should last quite a long time anyway, so I'm not too concerned about that.

I also loosened the constraints on the batteries; I'm gonna use two 18650 batteries instead to make boosting a little simpler, but I'm still having issues figuring out how to get the 120 volt source, for the oscillator circuit, and the 120 volt source for the rest of it.

I actually put together a small ZVS driver, with a regulation circuit (apparently ZVS drivers actually make pretty darn good SMPS, it draws basically no current with no load, boosting up to 90 volts from a single li-ion)

It's pretty similar to this circuit;

Just minus the LED indicator bits, and using a non-center tapped transformer with two inductors.

Problem is that the boost converters I made, seem to draw a large amount of current, even when under no load. They use an MCP1650 (

Proud Mary

Sat Dec 17 2016, 10:32AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Have you checked to make sure the valve filaments are still intact? If not, pull the valves and test across pins 1 and 7 on all of them with an ohm meter to make sure they are not open circuit.

Then take 1.5V from a battery and apply it across pins 1 and 7 of each valve. Does the filament light up?

I suggest for testing purposes you substitute two 9V batteries in series with three 1.5V AA or AAA batteries for B1, and ten 9V batteries in series for B2-B3.

Then you will know if it works and is worth spending any more time on.

If you apply the correct power sources in the right places, and the filaments and neons are glowing when seen in a dark room, but the set is still not working, get back to us here for fault finding part two. :)

Inducktion

Sat Dec 17 2016, 08:11PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Oh, I actually did get it working! I guess I didn't say so, but I troubleshooted it for a few hours, trying to figure out why there wasn't any HV developing on the geiger's output... neons were oscillating, tube was receiving the signal, but I wasn't getting anything out from the 10 M ohm resistor!

So... turns out there was actually an intermediate short, between the lines on the geiger tube's leads. Desoldered, restripped, and re-tested again, and bam, HV.

I'm also using a 6993 tube, instead of the original one because the original tube had a case of the scrunchies.

Put the tube on, and I had to mess around with the calibration knob to get it to work, but finally, I was getting clicks. Held the check source closer, and counting went up significantly.

That being said though I can't seem to calibrate it properly. The counter came with what it said was 1.3 mR/H of radium, in the check source but when I try to get it to read the proper amount (I have to switch it over to the x10 setting) the counter will not work anymore on the x1 setting. It seems to almost oscillate on an RC time scale, when I adjust the calibration knob it gets faster or slower, at least until a certain point when it stops oscillating and resumes normal operation.

But, anyway, I didnt consider it "working' quite yet because like I said the boost converters I had made were drawing excessive current, even with no load. No load draw was around 300 mA, which i deemed ridiculous considering the ZVS driver was drawing none when under no load.

As such I went back to the drawing board for powering the thing!

Proud Mary

Sat Dec 17 2016, 10:46PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The working voltage range of Anton's 6993 was 860V - 920V with a preferred voltage of 890V (i.e. the centre of the GM plateau).

Now to what is called 'own background' in Russian data sheets. This is the tube striking in response to radionuclides in its own construction, such as radiolead and radiobismuth in the solder. To estimate 'own background' the tube is encased in a lead block lined with aluminium, to shield it from most external radiation, while the aluminium shields the tube against natural beta activity in the lead block. Anyway, to pass Anton's quality control, 'own background' must not have exceeded 100 c.p.m. - a figure which you must deduct from any readings you take with 6993, though probably most tubes had an 'own background' perhaps half that figure.

Trouble with the rate meter: elderly capacitors quite often leak, old resistors can wander in value, and switch contacts can corrode and become coated with gunge. You might do best to change all the resistors and capacitors in the timing circuit as a matter of course.

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