FH40T

This is a very interesting device. I bought it for the high sensitivity 0.5mr/h which is 10x better than the SV500, the later model. The sensitivity of the FH40T is build into it by very clever electronics, which amazed my while working on it. Not to forget this was all done with Germanium transistors, and the thing has a battery life of 100 hours. The size is relatively small. I think. You can use it as a hand tool, or you can connect an external sensor to it. So the kind with a cable on it.

You can say when you have it on good condition, as mine here, it is as good any new Geiger Counter.

It can measure:

  1. High Gamma radiation. For this you need to insert the 50r/h Counter tube and turn the main know to the left. You get one scale of 50r/h
  2. Low gamma radiation, which is what you probably want to do with it. For that insert the 1r/h counter tubes and turn the main knob to the right. You have three 1r/h, 25mr/h 0.5mr/h
  3. Beta Radition. That means particles. Rotate the knob furthet to the right. Now the grid mechanically opens with a clever mechanism, operated by the main knob also. You can count now: 12500 particles/min or 320 particles/min.

To get it in good condition, requires normally following actions:


CORONA TUBE GLOWING

  1. Check if the Corona tube inside, has EXACTLY 500V, measured with a good, high impedance voltmeter. I bought two FHT40, and both had only 380V on it. Now the tester still works at 380V, but sensitivity is far too low. At 380V the corona tube is not glowing. If not 500V, you need to exchange the recitfier diodes (the white thing on the picture below with SST on it) and exchange the two HV capacitors. This means taking it completely apart. You cannot access it from any other way.
  2. Get the main knob going again. Forget a quick spray of oil. You need to take it completely apart The main knob also controls a mechanical grid, which closes or opens the slots in the front so beta particles can come inside, or not. You need to lubricate this also with vaseline, but make sure you don't smear the delicate (10micron) foil which keeps dust out, and let pass beta particles. Lubricate the on-off contact, which is on the roll
  3. Re-glue the glass window

This is NOT how to take out the assembly, you need to remove it completely with the switch on the bottem right.

 

How to take it apart

This is absolutely not self explaning and makes you crazy if you try. Here is what you need to do:

  1. Make pictures of the wiring
  2. Remove the three screws that hold the inside assembly. You can remove it in one piece.
  3. Before you remove it, you need to solder off the wires.
  4. Set the battery knob such that the slot gets visible. In the position you can take it out
  5. Set the main knob to 320imp/minute scale.
  6. Now you can remove the inside assembly. The roll with the scales stays in
  7. Do not rotate the main knob or the switch assembly, but first make pictures of this, and note their exact position. Do not remove the roll.
  8. To remove the main knob you set the scale in the middle between battery test and 50r/h. ONLY IN THIS POSITION you can take out the knob
  9. When the knon is off, take the whole mess apart for cleaning and lubricating.

I do not recommend this, unless you are a skilled person. On the other hand you cannot use a FH40T "as is" they seem to have allways problems with the main know rotate too heavy, and the 500Volts being too low.

Interesting is the internal circuit of the pulse counter. The raw impulse of the Geiger tube, is changed into a trigger pulse first. The next circuit is a mono flop, build of one transistor and a Germanium transformer. It works amazingly nice, and is the heart of the electronics. The handbook says, this is done to make sure long and short pulses get counted the same way. Then after the monoflop comes a rectifier with capacitor, which works as integrator. So bacically adding all pulses up. I have not understood each detail of the circuit. The transformer on the right and in the center, it is the same core. So a quite complex "one shot" oscillator, powered by the O460, and triggered by the OC140. The Diode D2 is the freewheel diode, which togther with the magnetics energy of the core determine the impulse time. The winding 5-6 is the feed forward winding.