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Repairing an old radiation detector.

Discussion in 'Troubleshooting and Repair' started by Lupus_Vulpes, Jul 14, 2019.

  1. Lupus_Vulpes

    Lupus_Vulpes

    6
    2
    Jul 14, 2019
    Hey there, im new here so first of all, hi everybody! :)
    First of all, i dont know much about circuits or electronics, only the basics of the basics so i need your help...

    So, i got an old Yugoslav military radiation detector. The unit was produced in 03/1986 and when i put the batteries in it works, but i can see that it is struggling to give out accurate readings.

    [​IMG]

    In the BAT bracket (battery powered 2 x 1,5V D) :
    -KN - Battery control
    -cGy/h - upper scale for use with the ionization chamber
    -μGy/h - lower scale for use with the Geiger Mühler tube

    -ISKLJ. - off position

    In the AKU. bracket (powered from 12/24V car battery through an adapter)
    -cGy/h - upper scale for use with the ionization chamber
    -μGy/h - lower scale for use with the Geiger Mühler tube

    The problem is while running on 1.5V batteries the battery control works fine (KN) and i tested the Geiger - Mühler tube on a radioactive source in the μGy/h position and it gives out accurate readings, the problem is the ionization chamber. when i try to calibrate with the KAL marking on the scale, the needle doesn't drop to 0 but stays at 0.5 (not 0.05), and i have also seen that when i leave the device turned on for about 10 minutes and i set it back to the cGy/h position, i get slight variations on the needle (sometimes it shows 0.5 sometimes 0.4)

    here you can see that the device is working fine on the mikro-gray area (as seen in the video i tested it on a radiation source), but while operating in the high-range area (cGy/h) with the ionization chamber, when the device is calibrated as per instructions, the needle won't drop below 0,4-0,5 cGy/h - and it should drop to zero.



    Now, there are 2 calibration blocks inside the device with 7 or 8 potentiometers that are used to factory-calibrate the device. But i don't know which one is used for what... I have a schematic of the device but sadly it is not the original one. It's from a Czechoslovak copy of the device called Intenzimetr IT-65.

    The layout of the circuits is the same, but the Czechs used their own domestically produced elements inside. This device also uses old capacitors which probably need replacement, but if any element inside the device is changed, it will need to be re calibrated.

    Schematic of the IT-65, the Czechoslovak copy of the DR-M3. The Yugoslav version has different circuits on the calibration panels and the power supply panel , so this schematic can only be used to see the layout of the fixed inner circuits of the chassis:

    [​IMG]


    Here i also have some pics of the insides.

    [​IMG]

    [​IMG]

    This is the power supply block:

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    The circuit elements on the uGy/h callibration block:

    [​IMG]

    Underside of the ionization chamber:

    [​IMG]

    [​IMG]

    What is your opinion on what could be causing the problem, that the needle doesn't drop to zero, but stays at 0,4-0,5 when the device is calibrated?
     
  2. Martaine2005

    Martaine2005

    2,331
    618
    May 12, 2015
    Well, for starters, those caps are nearly 40 years old.

    Martin
     
    davenn likes this.
  3. Lupus_Vulpes

    Lupus_Vulpes

    6
    2
    Jul 14, 2019
    Do you think it would be wise to replace all the capacitors and then re-calibrate the device?
     
  4. Martaine2005

    Martaine2005

    2,331
    618
    May 12, 2015
    That would be my first initial observation.
    Are you looking at restoring or just repairing?.

    Martin
     
  5. Lupus_Vulpes

    Lupus_Vulpes

    6
    2
    Jul 14, 2019
    Well, getting it into functional state, after the repairs im taking it to an accredited laboratory to get it properly calibrated and tested, both the ionization chamber and the geiger-Mühler tube. the problem is that the company that made those doesn't exist anymore for 20 years, so there is no possibility of getting any original schematics of the device. my only luck is that the laboratory where i am taking it to calibration already worked with those devices so they know how to do it. But they don't repair them, i have to do that myself or find someone who can. The circuits aren't that complicated but it's hard to find out whats wrong when you have no source material or anything... it's mostly guessing...
     
  6. Martaine2005

    Martaine2005

    2,331
    618
    May 12, 2015
    Ok, lets assume it was functional before. 40 years later, somebody powers it up. Obviously calibration wont be the same. The main reasons would be the caps.

    Martin
     
  7. Bluejets

    Bluejets

    3,432
    697
    Oct 5, 2014
    One would hope it was never used in or around Chernobyl.
     
    Martaine2005 likes this.
  8. Lupus_Vulpes

    Lupus_Vulpes

    6
    2
    Jul 14, 2019
    Well, it will be a painful process to
    nah, it wasn't... This device was stored and never used since it was produced. before i opened it, it still had the factory wax seal on one of the screws to avoid someone tempering with the device. I will have to take the device completely apart and test/exchange all the capacitors...

    Lets presume the capacitors are exchanged and the problem persists, what would be your second guess on what's causing the problem? because it's funny that the device works fine on one range setting and being faulty on other
     
  9. Bluejets

    Bluejets

    3,432
    697
    Oct 5, 2014
    I'd tend to deal with that problem if it comes up rather than pre-empting.
     
  10. Martaine2005

    Martaine2005

    2,331
    618
    May 12, 2015
    I agree.
    However, if you are going to re-cap, check all resistors too. They are probably fine but some might have drifted way out of tolerance.

    Martin
     
  11. Lupus_Vulpes

    Lupus_Vulpes

    6
    2
    Jul 14, 2019
    I talked to an expert about the device, and he told me that the parts are most probably working fine, but the problem is that over the years they have drifted way out of their indicated values, and that it would be very hard to isolate and fix the problem, because the only solution that would work 100% would be to exchange ALL the parts on the circuit board. Other than that, it's just guess work... He also said that the capacitors are most likely fine because the frequency of the current that goes through them is not that high, so they don't heat up and that the electrolytic compound inside them didn't dry out. The parts simply drifted out of their values over the years...
     
  12. Martaine2005

    Martaine2005

    2,331
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    May 12, 2015
    I would urge you to get a second 'expert' opinion....:cool:

    Well they are clearly NOT working fine....Did you read post #10?

    Electrolytic caps do dry out and or leak. Some go dead short.
    Some restorers wont turn on old equipment until they have first re-capped it. It can save other component failures in circuit
    Re-cap it, check resistor values and change any where necessary. Clean up and power it up.

    Martin
     
  13. Lupus_Vulpes

    Lupus_Vulpes

    6
    2
    Jul 14, 2019
    It will take some time, i am a total beginner in this, and i barely have the minimum equipment required for this repair (soldering iron, multi-meter, ESR meter). I never soldered anything before, and i will try to practice soldering a bit on an old circuit board i have lying around. In the next few weeks expect many question, mostly about part identification and where i can find replacements :D
     
    davenn and Martaine2005 like this.
  14. davenn

    davenn Moderator

    13,226
    1,734
    Sep 5, 2009

    Just the electrolytic ones
    eg ....

    photo 4 ... the 3 silver ones to the right of the 2 yellow ones and the bigger blue one at the bottom edge of the board

    Photo 8 .... the 2 silver ones
     
    Lupus_Vulpes likes this.
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