IBG 58 T Radiation indicator

Description: Czechoslovak army radiation indicator.
Manufacturer: TESLA or Czechoslovak army
Made in year(s): late 50s-early 60s; 1962 was my model
Country of origin: Czechoslovakia
Status: Not available

The Indikátor beta-gama, tranzistorový, IBG-58-T (“Beta-gamma indicator, transistorized, IBG-58-T”) is an army radioactivity indicator used in the former Czechoslovak People’s Army (ČSLA). This was a primitive Geiger counter with just optical and acoustic indication, i.e. with no metering device equipped. It has been designed to be worn on the neck of a soldier, with a pushbutton momentarily turning the machine on, indicating and checking for unsafe radiation intensity levels (0.1-1 R/h). I have got it back in 2009 for a price of 10 EUR including shipping; it was one of my first items that I’ve bought through an auction.

IBG-58-T SchematicSchematic (click to enlarge)

As you can see, the circuit is pretty straightforward. The inverter runs from a single 1.5 volt D cell and there’s a single germanium NPN switching a transformer (flyback topology), again very similar to my 380V circuit tester. The small iron-cored transformer’s secondary gave circa 130V~ at a few hundred Hz, 150-300Hz if I remember correctly. This went through a voltage multiplier, which gave 400 volts on the output. Just a note, the two diodes in the schematic were actually 500 volt 2 mA selenium rectifiers; something considered new at that time.
Before going to the Geiger tube, a special glow lamp, called a corona stabiliser at that time, was used to limit the maximum voltage to approx. 440 volts. The Geiger-Müller tube (a Soviet STS-5, the predecessor of SBM-20) went through a series of calibration resistors, which changed the amplitude of the final signal. This signal charged the capacitor across the second glow lamp, which was used for indication purposes; when it was charged with enough voltage for the second glow lamp to strike momentarily, a loud pulse was heard in the headphones. And that’s it…

IBG-58-T DisssembledIBG-58-T disassembled, after some minor hacks

In fact, this meter did not indicate every pulse from the Geiger-Müller tube. The few-megohm calibration resistors were pre-set so that the final glow lamp would blink randomly at radiation levels above 0.1 up to 0.5 R/h. Above 0.5-1 R/h, the blinking should change into a permanent shine, and the occasional audible “ticks” to static.

Accessories included were the meter itself, an instruction manual/log book, some neck straps, a battery compartment “screwdriver” and last but not least – 4000 ohm high impedance headphones for acoustic indication, dating back to either WWII or early 1950s…

IBG-58-T headphonesLooks like Hitler must have worn these.

This device sure is rare nowadays, since it has been completely phased out of service in the mid 60s. But I did not want a cool army-green paperweight, so I did some hacks to convert it to a normal Geiger counter, which would indicate every single pulse with proper piezo-speaker and LED indication. It was a success (check the video), but the exterior did not look very well. Finally, I have got a hold on better army Geigers and in the end I have even made my own Geiger counter, too.

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