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.

About the unit:
Its a DR-M3, it measures ionizing radiation in ranges from
0.5 - 5000 μGy/h (mikrogray - alpha/beta/gamma) - using the Geiger Mühler tube
0.05 - 500 cGy/h (centigray - gamma only) - using the ionization chamber inside the device

It was last calibrated in 1986 when it was produced.
It can be powered with two 1,5V D batteries, or from a 12 or 24V car battery using an adapter.

On the front plate it has a switch with 6 positions, 2 buttons and 1 potentiometer

SVETLO - backlight button to illuminate the scale
KAL - callibration button in the upper right corner of the picture. It is used when the device is operating in the "cGy/h" range to calibrate the ionization chamber, while the button is pressed, the needle on the scale needs to be aligned with the KAL designation on the scale using the KAL. potentiometer in the right lower corner of the image.

The positions of the switch are for selecting the mode of operation and they are:

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)

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

So, here is the question:
-who would be so kind to look at the schematic and tell me which elements would be wise to change and
-which potentiometers on the calibration blocks are used to calibrate which scale

I will open the device and send you some pics of the insides tomorrow.
Thank you very much.

Schematic of the IT-65, the Czechoslovak copy of the DR-M3

 

Ok, i made a video demonstrating my problem...
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.

Here i also have some pics of the insides.

This is the power supply block:

The circuit elements on the uGy/h callibration block:

Underside of the ionization chamber:

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?

 

I have decided to re-cap the board, i am assuming the capacitors are the main problem here, so this will be the first attempt at fixing the problem. They are more than 35 years old, so the electrolytic ones have to go, but the question is, should i also recap the film capacitors also?

 

Situation report,

I removed all the electrolytic capacitors and tested them, the big blue one (C18) is definately defective, not sure about the other ones. I also tested the resistors, none of them drifted out of their tolerance values. I also found one more electrolytic on a CB inside the hand-held GM tube. i also de-soldered it.

Here are the results:

POWER SUPPLY BLOCK:

C18

C15

C16

C17

uGy/h Callibration block:

C55

C56

CB inside the GM tube:

I also got worried by this NTC because it looks like it has a small pinhole, i tested it and it showed 211 ohms, the resistance decreased while applying heat to it.

Now, the question is... what replacements would you recommend for those elements and where can i get them...

 

Ok, after thorough research, i managed to identify all the capacitors and their producer... All the capacitors except the Iskra electrolitic (big blue one) and the Siemens one are military tantalum axial solid capacitors made by "Elektro industrija Niš" (Ei), which sadly went bankrupt and they don't produce anything anymore...

What replacement capacitors would you recommend that are mil-spec to replace the old ones?

 

Replace the capacitors that you identify to be off nominal value.
Replace capacitors that are 10μF and higher.
Replace capacitors with similar composition, capacitance, and voltage or with higher voltage. Don't worry about the the original manufacturer of the capacitor.

Why do you desire mil-spec components? I would not be concerned about this. Commercial or industrial specs will be fine.

 

Hello,

The 15 volts capacitors can be replaced with 16 or 25 Volts capacitors.
When you take the 105 °C capacitors you are OK.

Bertus

 

Why do you desire mil-spec components? I would not be concerned about this. Commercial or industrial specs will be fine.

Well, the device was built to milspec standards with high-quality parts being used...
It was build for the Yugoslav army, the unit was hermetically sealed and could even be immersed under water to a depth of 1.5m and still work... It was also vibration proof as it was intended to be used in fast scouting vehicles for radiological survey of contaminated area.
In addition to it being a virgin unit, the units components are top notch.
Good quality fiberglass board with nice serviceable trace sizes.
Trim pots built on ceramic bisques with fired resistance elements .
Liberal use of GOLD flashed connectors.
CAL pot is a QUALITY unit.
No old carbon composition resistors . . . all are being stable carbon or metal film units.
Every capacitor that I see, other than the two yellow film caps are being metal or epoxy cased and if metal, they even have glass hermetic seals on the end !
The electrolytic caps look to be PREMIUM tantalum slug radial lead units, also with glass seals.
Two exceptions seem to be the E cap on uGy/h cal board that has an end blue marking on it and is a Siemens brand, that just seems to be a plain vanilla electrolytic.
The other was over on the top left corners power supply schematic and is an all BLUE E- capacitor.

 

That still does not give a valid reason for using mil-spec components, unless you are planning to sell it to the Yugoslav military which is very unlikely.

I don't see anything spectacular about the construction of the unit. Its only value is for a personal radiation meter or as a museum piece.

My primary aim would be to get it working if I care to do so.
I do not see any reason to replace any of the capacitors. C18 appears to be a more recent replacement and looks ok.
C15, C16, C17, C55, all appear to be tantalum capacitors in working order.

C56 (or is it C54) 0.68μF/63V appears to be reading higher than nominal. I would consider replacing this.

 

That still does not give a valid reason for using mil-spec components, unless you are planning to sell it to the Yugoslav military which is very unlikely.

I don't see anything spectacular about the construction of the unit. Its only value is for a personal radiation meter or as a museum piece.

My primary aim would be to get it working if I care to do so.
I do not see any reason to replace any of the capacitors. C18 appears to be a more recent replacement and looks ok.
C15, C16, C17, C55, all appear to be tantalum capacitors in working order.

C56 (or is it C54) 0.68μF/63V appears to be reading higher than nominal. I would consider replacing this.

C18 and C56 need to be replaced, the others seem to be in working order. (C18 also drifted out of it's tolerance values, it's rated for 220uF and it shows around 290uF)