Geiger Counter Problem

[Chronosock]

Unfortunately I only have an analog one.

[mightyohm]

What meter do you have? I'll look up the specs on it.

[Chronosock]

Sorry for the wait again, I don't get to get to my little work area often. It is a GB Instruments GMT-312.

[Chronosock]

Bit of an update, I now have a digital multimeter. What were the voltages you wanted me to check?

[mightyohm]

Sorry it's taking me so long to reply, but I have been traveling and haven't been able to get time in the lab to measure voltages. I should have an update soon (still searching for my multimeter).

[mightyohm]

I tested some voltages of the high voltage supply circuit with two digital multimeters: my Fluke 87V with 10 megaohm input impedance, and a cheap Cen-Tech meter with 1 megaohm input impedance (this is typical for cheap digital meters). The input impedance of the meter will affect the high voltage measurement.

All measurements are in volts (V) and with fresh batteries.

Fluke 87V
Using the Fluke 87V, with the ground lead connected to pin 3 of J6 (the PULSE header). Pin 3 is the pin all the way in the upper right corner of the board, closest to the mounting hole for the case.

Battery pack + terminal: 2.9 (should be around 2.9-3.1V)
Battery pack - terminal: 0.0

If you measure between pins 1 and 3 of J6, you should also measure the battery voltage (2.9-3.1v).

More voltages, ground lead of the meter still connected to pin 3 of J6.

TP1: 2.9
TP2: 214V (note how this is lower than 400V, this is because the meter is loading down the voltage, since I did not add a 1 gigaohm resistor in series with the meter)

U1 pin
1: 0.0 (ground)
2: 1.5
3: 0.95
4: 2.8
5: 1.9
6: 0.0 (ground)
7: 0.5
8: 2.9 (same as battery voltage)

Cen-Tech (typical cheap multimeter)
Using the Cen-Tech meter:

TP1: 2.88
TP2: 95V (note how this is lower due to the lower impedance of this meter)

U1 pin
1: 0.0 (ground)
2: 1.5
3: 0.95
4: 2.6
5: 1.8
6: 0.0 (ground)
7: 0.5
8: 2.9 (same as battery voltage)

The good news is that apart from the high voltage (TP2) measurement, both meters give pretty much the same readings. I don't have an analog meter to test with, but I suspect it would probably give drastically different results due to loading of the meter coil on the circuit.

I'm really interested to see what voltages you are getting on your kit. Your results should help us troubleshoot further.

[Chronosock]

So, got the voltages, here are the results:

Battery terminal to pin 3 of J6: 3.119v

TP1 to pin 3: 3.118v

TP2 to pin 3: goes between 106.5-109v

For U1 I wasn't quite sure what pin was what number, so I measured them from left to right if you were looking at it from the top

Pin 1: 2.68 Pin 2: 0 Pin 3: 0.784 Pin 4: 3.121

<==*Dot is on this side of the chip

Pin 5: 3.093 Pin 6: 2.026, but went down to 0.8 because it beeped randomly. No different response from the uranium I have from United Nuclear. Pin 7: 1.649 Pin 8: 0

[mightyohm]

Here's the pin numbering. Pin one is closest to the dot, it's on the lower left of the chip if you are looking at the kit "right side up". The pin numbers increase to the right, then wrap around to the upper side of the chip. I'm not sure I follow your voltages, can you check the diagram and re-order the pins to match?

What kind of digital multimeter are you using?

[Chronosock]

Battery Voltage: 3.113v
1: 0v
2: 1.643v
3: 2.032v
4: 3.084v
5: 2.061v
6: 0v
7: 0.791v
8: 3.113v

And I am using a Mastech MS8268 Digital Multimeter:

[mightyohm]

I looked up the manual for your meter (http://www.panda-bg.com/datasheet/646-3 ... MS8268.pdf) and it says that your meter has a 10 megaohm input impedance in the DC range, so you should be reading voltages closer to 200V than 100V. How does the voltage on TP1 vary as you adjust VR1?

From the voltages you gave me, it seems like Q3 isn't switching properly.

Try this:

Remove U1.

Then measure the voltage on pin 4 of the socket, it should be close to the battery voltage 3V. TP2 should also read around 3V.

Then take a small piece of wire (like a resistor lead) and short U1 socket pins 3 and 8 together. Then measure the voltage on pin 4 again. It should be less than 0.5V. The voltage on TP1 should be close to 0V.

This will tell us if Q3 and Q1 are working.