Sunday, April 21, 2013

DRSB-01 Geiger counter failure

My DRSB-01 ( ДРСБ-01 ) geiger counter has gone mad. Far from any source it gives up to some counts per second, when the normal rate for it in my home is around 20 counts per minute.

Once opened was easy to spot the failure: The high voltage applied to the tube was around 800 volts. The normal working voltage of the SBM-20 tube is around 400 volts, so 800 volts produces self-triggering, and therefore, the extra counts.

Surprisingly, the high voltage filter capacitor is rated only at 400 volts, but it is still ok.

The SBM-20 tube has a glass end, so with this overvoltage, is easy to see how the entire tube lights like a giant neon bulb with each count.

Looking at DRSB-01's PCB, you will find a resistor labeled R2, mounted over two small posts. This resistor controls the base current of the oscillator who produces the high voltage. I suspect this resistor is mounted in this way because it is selected at factory for each unit to produce about 400 volts at the high voltage power supply. In my unit it was a 6800 ohms resistor.

I removed the 6k8 ohms resistor and replaced it with a 22k variable resistor. In this way I am able to adjust the high voltage output returning to its original 400 volts value, but I found a problem doing this.

The high voltage output depends strongly from the DC power supply value. For example, If you use a pair of fresh batteries (who can produce up to 3.2 volts) and adjust R2 to get 400 volts, once the batteries are drained just a bit (for example down to 2.9 or 2.8 volts) the high voltage will not be enough to make the tube work and you will get no counts at all, producing an very poor battery life.

The best way to solve this is to use the plateau region of the tube. The SBM-20 tube has a working voltage range about 350 to 475 volts. In this range, the tube sensitivity is almost constant, so maybe you have guessed it: The trick is to adjust R2 to get around 475-500 volts, or even 525 volts with a pair of very fresh batteries (3.2 volts). In this way the battery life is maximized having a constant sensitivity during the whole battery life time.


  1. Do you have schematics for this geiger-counter?

    1. They are very easy to find in Internet. Just google a bit or: