Re: Powering an Old, old geiger counter...
Karmaslap, Wed Dec 14 2016, 07:37AM

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.

Re: Powering an Old, old geiger counter...
johnf, Wed Dec 14 2016, 06:23PM

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

Re: Powering an Old, old geiger counter...
klugesmith, Wed Dec 14 2016, 08:38PM

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.

Re: Powering an Old, old geiger counter...
macona, Fri Dec 16 2016, 07:55AM

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.

Re: Powering an Old, old geiger counter...
Proud Mary, Sat Dec 17 2016, 12:41AM

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 .0047μF 1600V (or possibly 1800V - difficult to see) and C5 .0047μF 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.

Re: Powering an Old, old geiger counter...
Inducktion, Sat Dec 17 2016, 08:15AM

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 (

Re: Powering an Old, old geiger counter...
Proud Mary, Sat Dec 17 2016, 10:32AM

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. :)

Re: Powering an Old, old geiger counter...
Inducktion, Sat Dec 17 2016, 08:11PM

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!

Re: Powering an Old, old geiger counter...
Proud Mary, Sat Dec 17 2016, 10:46PM

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.

Re: Powering an Old, old geiger counter...
Inducktion, Sat Dec 17 2016, 11:59PM

Even ceramics? A lot of the capacitors inside of it are ceramic ones (and one bumblebee?), but there was only one electrolytic and I already replaced it with a modern one.


Edit

So I replaced the bumblebee capacitor, after realizing that they too, are ones you need to replace. Value of it was WAY off.

That being said, I got excited! The counter was working perfectly, everything was great, and I started to put it all together so I could finally be done with it and move on to my next project...


But then suddenly, it stopped working right... No real indicator as to what happened, either.


Ugh.

I played around with it for a few hours, checking connections, checking resistors and capacitors and they are ALL within spec, to my knowledge. Then suddenly it starts working right, again, at least, sort of.

Wuh?

It works, but it seems like the tube is really "insensitive". Before when it was working correctly it would pick up a decent amount of background radiation, and give a healthy click every 1-3 seconds.

Now, it sometimes takes as long as 6-7 seconds to register any background radiation. Additionally, when I attempt to give the counter the proper 90 volts the one rail requires, the neon bulb gets dimmer and the meter starts to register less radiation... But the voltage itself doesn't sag?

Any ideas? :\

Re: Powering an Old, old geiger counter...
Proud Mary, Sun Dec 18 2016, 10:44AM

Intermittent faults are the worst cos you can't detect 'em when they ain't faulting.

You've got a source, right? OK - place your source so you're getting a steady deflection on the meter, and tap each component in turn a few times with a non-conductive rod or pencil not forgetting the meter switch and the valves/tubes.
Jiggle VT2 and VT3 in their holders if you can and consider the possibility of dry solder joints (which look OK but aren't) to see if you can flush the fault out.

In other circumstances I would have suggested changing VT2 and VT3, but I know perhaps that isn't always that easy, so I have left the possibility till here.

I see on page 13 of the 'Operation and Maintenance Manual, Precision Geiger Counters, Model 107B Professional' (1955) that if VT1 is changed, it may be necessary to add to, or remove, a 22 megohm resistor from the string collectively designated as R3 on the circuit diagram in order to re-set the high voltage to 900V. It says that it is essential to use an electrostatic voltmeter for this task so as not to pull the voltage down during measurement.

Re: Powering an Old, old geiger counter...
Inducktion, Sun Dec 18 2016, 08:19PM

So... I came back and retested it again this morning, and the counter was back to working as it should... Correct sensitivity, meter was reading right, everything seemed to be working correct.

Then after about 10-15 minutes of being on, the geiger counter goes into this weird "insensitive" mode where the neon barely lights and flashes whenever the geiger tube is held close to the check source...

Sounds like a heat related issue, but even with a fan blowing on it it doesn't seem to fix it.

Maybe one of the tubes is leaky somehow? I can buy new ones on eBay for relatively cheap. $14 for two of them shipped to me.

Or go slightly cheaper and get 2 1AJ4 tubes for around $8.

Re: Powering an Old, old geiger counter...
Proud Mary, Sun Dec 18 2016, 10:07PM

OK - this is what I suggest.

Leave the set alone overnight as you have done before.

Refer to the fact box at the bottom of the circuit diagram "Typical Voltages To Ground"

Put your meter in voltage mode and clip its positive probe onto pin 2 (the pentode anode) of VT1 valve type 1U5. Clip the negative meter lead to the chassis.

Now switch on your GM counter. If it is working you should see a voltage around 95V - anywhere between 90 and 100 will do.

If the counter is working as before leave the meter connected until the circuit goes into failure mode. Did the voltage on the anode change when the set stopped working?

Re: Powering an Old, old geiger counter...
Inducktion, Sun Dec 18 2016, 11:53PM

well, this was before I read what you said Stella, but I actually checked what you suggested anyway;

So. I poked around, a lot more.

Somehow the meter kicked back into its normal mode of operation... I'm positive it's something to do with the amplifier section of the counter. Voltages (both the 90 volt and the 120 volt rail) stay rock solid, no matter what mode of operation the counter is in.

Upon shorting the 90 volt rail to pin 2 of VT2, the meter resumes normal operation for some time. After a while it would normally go back into the weird insensitive mode, but... hasn't this time around? I don't really know, it's not making sense to me.


Instead, now I'm dealing with some oscillation. It seems to be roughly 1 second interval, with the interval changing a bit when adjusting the calibration knob. I can stop the oscillations completely if I turn the calibration knob a bit more, but then the meter becomes relatively insensitive and doesn't register much background radiation. Nor does it correctly measure the radiation from my check source.

When it IS in the oscillation mode, the meter is EXTREMELY sensitive, and works how I think it should. But, oscillations. So... how do kill oscillation? And these oscillations happen even if the geiger tube is disconnected, so there's that too.

Re: Powering an Old, old geiger counter...
Proud Mary, Mon Dec 19 2016, 09:26AM

What is oscillating, can you tell? Keep in mind that VT2 is a multivibrator which turns the GM pulses into pulses of equal width and amplitude to be integrated by VT3. .

Re: Powering an Old, old geiger counter...
Inducktion, Tue Dec 20 2016, 02:41AM

Alright, so.
Turns out there wasn't enough bias voltage on the input of VT2. Added in another 1 M ohm resistor to the 1.5 M ohm resistor, and the oscillations cease to be, and it's still as sensitive.

Now I'm just getting ready and putting it all back together nicely. :) Thank you so much for your help Stella. Your troubleshooting helped me keep going at it and it's paid off finally!

Re: Powering an Old, old geiger counter...
Proud Mary, Tue Dec 20 2016, 06:47PM

Oh I am pleased! I must say that the guts are in really beautiful condition for something so old - not a spider in sight.

The user manual goes right back to the early Cold War era when the US Govt wanted as many people as possible to go out prospecting to find new sources of uranium in the Lower 48 - a bit frightening really.