Broken DP-5A Geiger counter

[Davee]

Hello!

I'm in need of some help with this piece of vintage electronics... as well as some translation issues.

I am by no means an electrician, but I am very interested in learning more about electronics. Fixing this piece of equipment could be an interesting project! Please go easy on me. ;)


(click the images to go to the full gallery)






Built in '72, this DP-5A Soviet Russian Geiger counter was working fine up until a few weeks ago, when I accidentally left it on (at the voltage calibration setting) and the batteries were drained. After replacing the batteries it was still dead silent when turned on - only the background lights worked. When I opened it up, I found this broken part in the probe, marked "47 MC". Of course, this is in Russian. A Cyrillic "C" is essentially pronounced like an "S". It appears to be a resistor (more below).




I'm using the three 1,5V cells from a stripped-apart European 4,5V battery to power the counter instead of the original soviet-design 1,5V cells that are no longer easily found (for obvious reasons). For all I know the soviet cells may have had some built-in feature to prevent this damage, but It still seems strange that it would break simply due to drained battery cells, so there may be other things going on... However, this resistor is the only obvious damage I could see, opening it up for the first time, and it's the only thing I know is wrong with it right now.


Here's the second resistor of the same type, seemingly intact:




Looking at the documentation, the broken resistor seems to be marked number 7 and the intact one number 8:




Here's the list of parts with number 7 and 8 listed as "Pe?????? ???-0,125-47 ??? +/- 10%" (sorry, Cyrillic script dosn't seem to be supported by these forums):




Loosely translated it seems to mean "Resistor KIM (or KEM)-0,125-47M Ohm ± 10%". If this was a normal resistor I should be able to find some replacements in my collection... But I'm not sure it is. All the other resistors listed - the red ones that actually look like your average, if mostly unmarked, resistor - have another prefix: "???" (MLT).
And what is actually the specifications of these two? "-0,125-47M Ohm" it says. Is the first one some sort of amperage or voltage limit? Would the second value then be the resistance? My multimeter isn't helping either - it is not detecting any connection at all between the two ends of the seemingly intact resistor most of the time and the few times it does it jumps around the 20M Ohm range. I figure this could be due to coating on the wire and solder points... Then again, reading on the leads on the sides works just fine. Maby this second one is broken too? Must I remove it before accurately measuring?

And that is the issue.

TL;DR: What are those two resistors and what could I replace them with? Please share any ideas/info you have!


External pictures and general info on the counter can be found here (the B variant is very similar to the A): http://boginjr.com/electronics/old/dp-5b/

Russian alphabet: http://en.wikipedia.org/wiki/Russian_alphabet#Alphabet

I figured I may as well document the rest of it for people interested in it. You can find a bunch more pictures of both the unit and the documents related to it in the album here: http://imgur.com/a/4tYBY


Why did I post here of all places? Well, most of the forums I found regarding vintage electronics were either very inactive, completely focused on radio or audio or didn't take help requests. If you have any tips of forums that may be of help; do share! :)


Thanks for reading. I will of course keep you updated if any progress is made!

[N2IXK]

My guess would be .125 (1/8) Watt / 47 Megohms.  High value resistors like that are common in high voltage high impedance circuitry like this. Shouldn't be too hard to find a suitable replacement. Wattage rating of the replacement can be higher than 1/8W, and probably not a bad idea to do so. Look for a 47M resistor of about the same physical size to prevent problems with high voltage arcing over a smaller resistor.

BTW, you are aware that Geiger tubes operate on very high voltages, right?

[Davee]

Thanks for the quick reply!
And yes, I am aware of the high voltage. Won't be poking around all too much - and definitely not when turned on or with batteries attached!

[fluxcapacitor]

They might have some stock here if you want the original parts.

http://zapadpribor.com/en/info/buy/

http://zapadpribor.com/en/dp-5a/

[rob77]

My guess would be .125 (1/8) Watt / 47 Megohms.  High value resistors like that are common in high voltage high impedance circuitry like this. Shouldn't be too hard to find a suitable replacement. Wattage rating of the replacement can be higher than 1/8W, and probably not a bad idea to do so. Look for a 47M resistor of about the same physical size to prevent problems with high voltage arcing over a smaller resistor.

BTW, you are aware that Geiger tubes operate on very high voltages, right?

agree - definitelly 0.125W 47Mohm

[rob77]

btw... it's beautiful piece of vintage gear    it looks just great. i hope you'll repair that beauty

[Davee]

Thanks for the help, everyone. Any ideas as to what caused it to fail in the first place?

[M4trix]

I was going to ask how do you test a Geiger counter at home but then I stumbled on this video. 

[rob77]

I was going to ask how do you test a Geiger counter at home but then I stumbled on this video. 

actually the old clocks with digits with phosphor coating are exciting the Geiger tube a little bit.

[Stonent]

Looks like an SBM20 and possibly SI3BG (CN3BG) tube. Fortunately for you, those tubes are very common and very cheap on ebay if they ever need to be replaced.

Another source of radiation (though small) is potassium chloride salt, sold as "lite salt" at grocery stores. Just very slightly more noticable than background radiation.

Post a video of it once you get it working again, I'd like to see how well it works.  I've got 2 CDV-700 counters, but only one works.  The other has a bad transformer that makes high pitched noises and no tube.

[Stonent]

I was going to ask how do you test a Geiger counter at home but then I stumbled on this video. 

Bionerd23 and Antiprotons are my two favorite radiation related youtube channels.

Though if I were trying to capture some Technetium from my urine, I'd look for a way to denature the proteins out and separate anything like urea and ammonia.

[Davee]

Looks like an SBM20 and possibly SI3BG (CN3BG) tube. Fortunately for you, those tubes are very common and very cheap on ebay if they ever need to be replaced.

Another source of radiation (though small) is potassium chloride salt, sold as "lite salt" at grocery stores. Just very slightly more noticable than background radiation.

Post a video of it once you get it working again, I'd like to see how well it works.  I've got 2 CDV-700 counters, but only one works.  The other has a bad transformer that makes high pitched noises and no tube.

This unit does indeed supposedly take the common SMB-20 tube. The currently fitted one says "CTC-5", in Cyrillic, though.

As for test sources; the faux-leather case comes with a fairly strong source under a cover - apparently a bit of Strontium-10. It easily goes up to 25 Milli-Röntgen/Hour when the probe is held in position just above.

I also ordered some uranium glass beads (glows green in UV light!) and Fiestaware plate shards which is a brand of ceramic plates from 1930-40'ies America coated with uranium-rich paint. Yummy. All of this from the US, with the package literally having the word "Uranium" on it. While these do not give off a lot of radiation and are only really unhealthy if you'd try to eat them or grind them down and breathe the dust I hope I haven't been blacklisted from entering the US, as I'd like to visit again some day.

But other than the Strontium-10 test source, the strongest source I currently have comes from a common fire detector. They often contain a bit of radioactive material, something to do with the smoke detection. This one has Americium-241 supposed to give off 10 Micro-Curies or 37kBq. Very short-range, but it nearly goes off-scale on the lowest setting (so around .5 mR/h) when held as close as possible at 4-5 mm.

I did also take it for a spin in a local gravel pit a while ago. Eventually found a few boulders that gave off noticeable readings over background at about 50% increase in counts per minute. Probably just granite rock, but interesting non the less, seeing as how it's on the same ridge as (and near) an in-use water-source.


I'll probably make a video of it sometime if I get it working again. Almost definitely if I manage to put together a little add-on for it in the form of a vintage-style (heck, even steampunk-style) portable vacuum tube amplifier with speaker that attaches to the headphone jack on the side. Because those characteristic ticks should be heard by everyone! A blinking light and perhaps a counts-per-minute reader would also be useful.
Steel sheets for the case, copper rivets, salvaged Volkswagen horn, bunch of old amp tubes and sockets, amplifier circuit kit, battery packs - I already have the parts needed. I realize using modern transistors and semi-conductors would be much "better", but that'd sort of defeat the point.

In the mean time, here's a guy with a demo-video of a DP-5B (very similar):

I was going to ask how do you test a Geiger counter at home but then I stumbled on this video. 

Bionerd23 and Antiprotons are my two favorite radiation related youtube channels.

Though if I were trying to capture some Technetium from my urine, I'd look for a way to denature the proteins out and separate anything like urea and ammonia.

Those are in fact my two favourite RADIAC-related channels too! Very helpful when I was doing research/learning more about the counters and how they work.

[amyk]

I think it's accidental physical damage and not electrical, as there does not appear to be any burning/arcing. The body is glass which is typical for these high-value resistors, so they're more fragile than usual.

[Davee]

Replacements have been ordered, together with some other high-ohm resistors that the unit uses - just in case - as they were difficult to find in local stores/websites. Will update once they're in!

[txescientist]

I have DP-5B and it is great. If you have any more trouble with it, let me know. Also, did you checked high voltage? Electron tubes in probe have plasma glow which intensity changes when Geiger tube detects radiation. Another good test source is dust collected from CRT screens. Regards

[N2IXK]

Why would CRT screen dust be radioactive above normal background levels? 

[SeanB]

Lathanide oxides and actintide oxides that are slightly radioactive above background naturally. These are used to generate the colours. Older tubes used zinc oxides and Beryllium oxides, but only gave yellow and white with low efficiency. You need a rare earth oxide to get better efficiency.

[txescientist]

Simply because the radon gas is often attached to the dust, but when you collect the dust with a wipe, it starts to decay: http://www.imagesco.com/articles/geiger/radon01.html
It gives more cps than the K40 in the salt...

[N2IXK]

Lathanide oxides and actintide oxides that are slightly radioactive above background naturally. These are used to generate the colours. Older tubes used zinc oxides and Beryllium oxides, but only gave yellow and white with low efficiency. You need a rare earth oxide to get better efficiency.

But those compounds aren't "dust", they are the phosphors.

By "dust collected from a CRT screen", it sounded like you meant the airborne cruft that gets attracted and stuck to the screen by the static charge.

[Stonent]

Concentrating the dust may mean that stuff that already was within the normal background radiation range gets closer.

Antiprotons on youtube has a video where he wipes a wet car after a rain and shows the rag is much more radioactive than normal.

[Davee]

The broken 47MOhm resistor has been replaced, but no luck. The counter is still dead-silent.

Note sure what else may be wrong. Will do a more thorough examination soon. Again; any suggestions, questions, ideas or such are very welcome!

[Stonent]

The broken 47MOhm resistor has been replaced, but no luck. The counter is still dead-silent.

Note sure what else may be wrong. Will do a more thorough examination soon. Again; any suggestions, questions, ideas or such are very welcome!

If you have a neon bulb you should be able to tell if there's any voltage making it to the tube.

[daqq]

Another commonly available test source are thorium welding rods. Not extremely radioactive, but well above the background (in reasonable quantities).