Old Geiger Counter needing repair....

[mishima]

My class was recently donated an old but perfectly functioning Geiger counter. It was the type that came with a mono-speaker which made audible clicks when decay events were detected. It was left on in the cabinets for multiple days, and now there is a steady tone coming through the speaker and the reading on the meter is maximum on all 3 scales. I am wondering if there is any part I could check/test/replace to get this working again, I have little electronics knowledge in general. Schematic and other pictures below.

 

[Jeff Rosenbury]

Visually inspect the electronic parts for burned circuits.

Does the meter work properly?

 

[mishima]

The meter is at max when powered on, for all 3 scales. It is min when powered off.

 

[mishima]

Components do not visually look damaged, could it be the capacitors went through dielectric breakdown?

 

[rbelli1]

The two big black caps in the middle look like some kind of electrolytic type. They would be my first guess. The electrolyte dries out and the capacitance drops precipitously. C6 failing would also likely cause the symptoms you are describing.

Also in the top right corner the PCB trace is a different color. What is the situation there?

BoB

 

[dlgoff]

I got this quote from http://www.uraniumrocks.com/pages/cdv-700-model-information

The main problems with the circuit are transistor failure (likely due to inductive spikes) and the method they used to regulate the high voltage. Rather than using a voltage regulator tube like other models, they chose to use a couple of zener diodes to limit the voltage on the primary side of the transformer. Unfortunately, this means that a small change in the zener diode's voltage leads to a much larger change on the high voltage output.

Note: The recommended operating voltage for the 6993 tube is 890 vdc, so it could bite you.

image compliments of http://www.anythingradioactive.com/geigeraccess.htm

http://www.anythingradioactive.com/newprodpix/bitsanbobs/6993%20specs.jpg

 

[Jeff Rosenbury]

There seems to be a solder bridge connecting the emitter of V2 to something on V3. Perhaps it's shorting around L1?

But I'm probably mistaken if worked earlier.

There's a dark spot on the sub assembly. It's likely leftover flux, but it could be scorching. I can't tell from the photo.

Can you post a scope trace across the meter and the headphones?

 

[mishima]

dlgoff thanks for those great links. After browsing around a bit more it seems the cdv-700 models are fairly popular these days with modders and hobbyists.

rbelli I will have to check that top right corner tomorrow...could leaving the device on for days with hundreds of volts across the capacitors have dried them out?

Jeff I could rig up a soundcard scope but am a little concerned about voltages. I found https://www.orau.org/ptp/Library/cdv/eni700-6b.pdf this manual which gives a pulse voltage across the mic as 11 V. I would need to get that down to <0.7 for it to work with my computer, with a voltage divider. I think I could manage that.

But, I can't tell from the schematic what a working voltage across the meter would be, and with the nearby 930 V with possible failed components...I would feel more comfortable with a multimeter that could do a high voltage reading...perhaps mine can (I will check).

 

[Jeff Rosenbury]

dlgoff thanks for those great links. After browsing around a bit more it seems the cdv-700 models are fairly popular these days with modders and hobbyists.

rbelli I will have to check that top right corner tomorrow...could leaving the device on for days with hundreds of volts across the capacitors have dried them out?

Jeff I could rig up a soundcard scope but am a little concerned about voltages. I found https://www.orau.org/ptp/Library/cdv/eni700-6b.pdf this manual which gives a pulse voltage across the mic as 11 V. I would need to get that down to <0.7 for it to work with my computer, with a voltage divider. I think I could manage that.

But, I can't tell from the schematic what a working voltage across the meter would be, and with the nearby 930 V with possible failed components...I would feel more comfortable with a multimeter that could do a high voltage reading...perhaps mine can (I will check).

It's not worth risking test equipment. Some technicians keep an isolation transformer near their scope, but I guess those days are fading as USB scopes become more popular.

This device is battery powered. Leaving it on for days would cause dead batteries. However, capacitors do age. You can check for short circuited caps easily enough. If they are open, it's a bit tougher.

With the batteries out, measure the resistance across the capacitors. Do it in both directions since some of them are in || with diodes/transistors. If they read near zero in both directions, then they are shorted. They are more likely to dry open, but that's harder to check. You can remove them and check them with a capacitance meter if you have one.

 

[jim hardy]

My guess, as an old analog troubleshooter, is:
If it's not weak batteries,
you are hearing the power supply's oscillator.
It could be coming into the audio stage through either the high voltage circuit or the low voltage circuit. I think latter is most likely.

1. Check Low Voltage first... With device runing, place your DMM across C6 and write down what you get . You ought to read about 15.5VDC ..
2. Then switch meter to read AC volts, write down what you get. You ought to read less than 10% of the DC value.
If you read more AC than that , tack a capacitor of at least same value as C6 across C6 and repeat readings.
That will tell you whether C6 is open. And if your meter is "True RMS" it may report the DC content of voltage check the manual.

3. If steps 1 and 2 go okay,
check for noise on the the high voltage by temporarily connecting base and emitter of V2 together.
If that quiets things down, the noise is probably coming in through R12-C5 path. CR5 may be shorted or C8 may be open.

I think it's a long shot, but check R5 and R8. You can measure them in circuit with board de-powered. Read both directions...if you read more than the value of the resistor in one direction it's open. Other direction you'll read less because of parallel paths through the transistors.

Please post results of those three steps - there's lots of enthusiastic help here.

I like a Simpson 260 for troubleshooting...

To check for AC in the power supply, step 2, move meter red lead from "+" jack to "Output" jack above polarity switch. That connects a capacitor in series to block DC, specifically for that type measurement. Very handy. It's only a 400 volt capacitor so don't try that on your HV supply.

 

[mishima]

Measured C6 at 2.2 VDC, 13 VAC. Cut it out and replaced with 22 uF 25 V polar electrolytic, now getting 15.2 VDC and 0.03 VAC.

The meter is no longer a max constantly, but a min. Think I will go ahead and replace the other electrolytic, C1.

Edit: Forgot to mention, after cutting out the original C6 it was reading 0.22 nF on cap meter.

 

[mishima]

Replaced C1 and I am detecting events again. Nearest I had on hand was a 330 uF for it (originally 200). Had to crank the calibration pot to the max but readings are falling within the acceptable range for the built-in check sample (roughly 1.5 mrem/hr). Will test some other samples in the stock room tomorrow.

Big thanks to all the help here. This website makes my classroom more awesome on a regular basis.

 

[davenn]

good success story

thanks for reporting back

Dave