Repairing an old radiation detector.

[Lupus_Vulpes]

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.

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:

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?

 

[Martaine2005]

Well, for starters, those caps are nearly 40 years old.

Martin

 

[Lupus_Vulpes]

Well, for starters, those caps are nearly 40 years old.

Martin

Do you think it would be wise to replace all the capacitors and then re-calibrate the device?

 

[Martaine2005]

That would be my first initial observation.
Are you looking at restoring or just repairing?.

Martin

 

[Lupus_Vulpes]

That would be my first initial observation.
Are you looking at restoring or just repairing?.

Martin

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...

 

[Martaine2005]

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

 

[Bluejets]

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

One would hope it was never used in or around Chernobyl.

 

[Lupus_Vulpes]

Well, it will be a painful process to

One would hope it was never used in or around Chernobyl.

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

 

[Bluejets]

I'd tend to deal with that problem if it comes up rather than pre-empting.

 

[Martaine2005]

I'd tend to deal with that problem if it comes up rather than pre-empting.

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

 

[Lupus_Vulpes]

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...

 

[Martaine2005]

I would urge you to get a second 'expert' opinion....

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

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

 

[Lupus_Vulpes]

I would urge you to get a second 'expert' opinion....


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

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

 

[davenn]

Do you think it would be wise to replace all the capacitors and then re-calibrate the device?

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]

I got my wor karea ready... let's see what we can find...

I will take out all the electrolytic capacitors, also test all the resistors for their values and check for any suspicious elements (components with pinholes, discolorations, corrosion etc.)

 

[73's de Edd]

Sir Lupus_Vulpes . . . . .


I just took a quick look at components and the units schematic and relevant design, and the most glaring thing I see is the units 1980'ish vintage and the units use of GERMANIUM transistors in the power supply . . . .in the schematics top left corner.
BUT ! at that time, the logic for that was the unit operating on a mere 3 VDC power supply, which must also be true on the step down of the 24/12 VDC supply down to that same level also.
All is well down in the GM det circuitry, just below, where they correctly stepped up to using silicon transistors in its design..

VOLTAGE TESTING . . . .

Why not use A Square test point for your DC metering ground and probe test points A-C-D-E and F Square test points to see how close they are to those voltage values given, with the unit being powered on.
H Square is the units main power, Minus 3 VDC, so check it also.

As far as the units build . . . .

I am looking at that unit with my learned and experieced eyes.
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
Quality film capacitors used in the high voltage doublers C13 and C14.
The most critical capacitors in the GM circuitry would be the C1 and C2 caps . . . BUT . . . you say it works / tests fine
Looking around I see some AA131 diodes being used, ALSO BEING GERMANIUM . . . . BUT, being used for a lower sensitivity threshold..
Now moving over to the possibly problem related area at the schematic top right corner D4 and D3 are silicon types . . .GOOD . . . but the associated C4 and C6 capacitors should have no DC leakage.

Drop down to center right for the Ionization Chamber and its post amplifier, is using a low filament voltage micro peanut tube .
That should be all sealed up, but you can see where a switch connects to an internal resistor going to the tubes grid.
Don't think it will be a problem, but one would not want any collection of linters or dust that could become humidity dampened and resistively leak to ground, at that connection..



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 first mentioned, top left corners power supply schematic and is an all BLUE E- capacitor .

That's what I am seeing . . . . .



73's de Edd . . . . .


The easiest way to make a fire with two sticks is to make sure that one of them is a match.

 

[Lupus_Vulpes]

Sir Lupus_Vulpes . . . . .

I am looking at that unit with my learned and experieced eyes.
In addition to it being a virgin unit, the units components are top notch.

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... I was surprised when i took it appart how every wire, bolt and potentiometer was smeared with that green paste to stop it from coming loose...

Drop down to center right for the Ionization Chamber and its post amplifier, is using a low filament voltage micro peanut tube .
That should be all sealed up, but you can see where a switch connects to an internal resistor going to the tubes grid.
Don't think it will be a problem, but one would not want any collection of linters or dust that could become humidity dampened and resistively leak to ground, at that connection..

The unit was hermetically sealed, and before i opened it, it still had the wax seal from the factory when it was produced in 1986 so no body opened it or tempered with it since, and there are apsolutely no traces of dust or corrosion inside

Why not use A Square test point for your DC metering ground and probe test points A-C-D-E and F Square test points to see how close they are to those voltage values given, with the unit being powered on.
H Square is the units main power, Minus 3 VDC, so check it also.

Could you explain more in detail how to do this because i am a complete beginner in this

 

[Lupus_Vulpes]

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...

 

[Lupus_Vulpes]

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?

 

[73's de Edd]

DANGER ALERT . . . . .DANGER ALERT !

I saw that you had the units GREY thermistor pulled and I NEED to warn you that it must be handled like a . . .? . . .? . . . . like a . . . .soap bubble.

It is being formed as a slug / disc with its polymorphous metallic amalgam, with it additionally , not being readily acceptabe to connections. So, they use either a high voltage vaporized and sputtered metal deposition in a vacuum chamber or they use a surfactally abraded compressing of a suitable conductive metal to each of its opposing faces to make solder connection capability..
Then soft soldering is used to connect 45 degree splayed wire leads to each of the prepared sides of the unit. ( HEY ! . . . . I see that they just put your leads on center and didn't go for the splaying. )
Then, it is either left naked or gets a conformal protective / insulative coating.
I see that yours got the coating . . . . complete with an errant air bubble. . . . no extra charge.
BUT the unit it is still QUITE delicate in handling or being subjected to the pressure of a distinct touch / bump.
SO when handling it, hold its opposite wire connector sides in a death grip, between finger and thumb.
I was so afraid that you would see that cracked paint on one leg and just have to straighten that lead and have the whole end connection come off in your hand IF not FIRMLY gripping it as I just mentioned.

THAT is being one item, that you would NOT, just pick up another one of easily.
BUT if you were worrying about it, in its design application and its degree of power involvement . . . . . . it is about 1000% power overrated.
Hoping that nothing has befallen the leads/end cap connection(s) already.

Moving on to the units E- capacitors. . . . . .

If you were to put one of my suspicioned two questionable E-caps in one hand and the one of the higher microfarad rated ,metal cased, tantalum caps in the other hand for a comparative weight evaluation, the common E-cap will weigh only a fraction of the premium tantalum unit

Will fill you in on other aspects tomorrow, all out of time now.


73's de Edd . . . . .

Don't lend people money. . . . it gives them amnesia.