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Powering an Old, old geiger counter...

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Inducktion

Sat Dec 17 2016, 11:59PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Even ceramics? A lot of the capacitors inside of it are ceramic ones (and one bumblebee?), but there was only one electrolytic and I already replaced it with a modern one.

Edit

So I replaced the bumblebee capacitor, after realizing that they too, are ones you need to replace. Value of it was WAY off.

That being said, I got excited! The counter was working perfectly, everything was great, and I started to put it all together so I could finally be done with it and move on to my next project...

But then suddenly, it stopped working right... No real indicator as to what happened, either.

Ugh.

I played around with it for a few hours, checking connections, checking resistors and capacitors and they are ALL within spec, to my knowledge. Then suddenly it starts working right, again, at least, sort of.

Wuh?

It works, but it seems like the tube is really "insensitive". Before when it was working correctly it would pick up a decent amount of background radiation, and give a healthy click every 1-3 seconds.

Now, it sometimes takes as long as 6-7 seconds to register any background radiation. Additionally, when I attempt to give the counter the proper 90 volts the one rail requires, the neon bulb gets dimmer and the meter starts to register less radiation... But the voltage itself doesn't sag?

Any ideas? :\

Proud Mary

Sun Dec 18 2016, 10:44AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Intermittent faults are the worst cos you can't detect 'em when they ain't faulting.

You've got a source, right? OK - place your source so you're getting a steady deflection on the meter, and tap each component in turn a few times with a non-conductive rod or pencil not forgetting the meter switch and the valves/tubes.
Jiggle VT2 and VT3 in their holders if you can and consider the possibility of dry solder joints (which look OK but aren't) to see if you can flush the fault out.

In other circumstances I would have suggested changing VT2 and VT3, but I know perhaps that isn't always that easy, so I have left the possibility till here.

I see on page 13 of the 'Operation and Maintenance Manual, Precision Geiger Counters, Model 107B Professional' (1955) that if VT1 is changed, it may be necessary to add to, or remove, a 22 megohm resistor from the string collectively designated as R3 on the circuit diagram in order to re-set the high voltage to 900V. It says that it is essential to use an electrostatic voltmeter for this task so as not to pull the voltage down during measurement.

Inducktion

Sun Dec 18 2016, 08:19PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

So... I came back and retested it again this morning, and the counter was back to working as it should... Correct sensitivity, meter was reading right, everything seemed to be working correct.

Then after about 10-15 minutes of being on, the geiger counter goes into this weird "insensitive" mode where the neon barely lights and flashes whenever the geiger tube is held close to the check source...

Sounds like a heat related issue, but even with a fan blowing on it it doesn't seem to fix it.

Maybe one of the tubes is leaky somehow? I can buy new ones on eBay for relatively cheap. $14 for two of them shipped to me.

Or go slightly cheaper and get 2 1AJ4 tubes for around $8.

Proud Mary

Sun Dec 18 2016, 10:07PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

OK - this is what I suggest.

Leave the set alone overnight as you have done before.

Refer to the fact box at the bottom of the circuit diagram "Typical Voltages To Ground"

Put your meter in voltage mode and clip its positive probe onto pin 2 (the pentode anode) of VT1 valve type 1U5. Clip the negative meter lead to the chassis.

Now switch on your GM counter. If it is working you should see a voltage around 95V - anywhere between 90 and 100 will do.

If the counter is working as before leave the meter connected until the circuit goes into failure mode. Did the voltage on the anode change when the set stopped working?

Inducktion

Sun Dec 18 2016, 11:53PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

well, this was before I read what you said Stella, but I actually checked what you suggested anyway;

So. I poked around, a lot more.

Somehow the meter kicked back into its normal mode of operation... I'm positive it's something to do with the amplifier section of the counter. Voltages (both the 90 volt and the 120 volt rail) stay rock solid, no matter what mode of operation the counter is in.

Upon shorting the 90 volt rail to pin 2 of VT2, the meter resumes normal operation for some time. After a while it would normally go back into the weird insensitive mode, but... hasn't this time around? I don't really know, it's not making sense to me.

Instead, now I'm dealing with some oscillation. It seems to be roughly 1 second interval, with the interval changing a bit when adjusting the calibration knob. I can stop the oscillations completely if I turn the calibration knob a bit more, but then the meter becomes relatively insensitive and doesn't register much background radiation. Nor does it correctly measure the radiation from my check source.

When it IS in the oscillation mode, the meter is EXTREMELY sensitive, and works how I think it should. But, oscillations. So... how do kill oscillation? And these oscillations happen even if the geiger tube is disconnected, so there's that too.

Proud Mary

Mon Dec 19 2016, 09:26AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

What is oscillating, can you tell? Keep in mind that VT2 is a multivibrator which turns the GM pulses into pulses of equal width and amplitude to be integrated by VT3. .

Inducktion

Tue Dec 20 2016, 02:41AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Alright, so.
Turns out there wasn't enough bias voltage on the input of VT2. Added in another 1 M ohm resistor to the 1.5 M ohm resistor, and the oscillations cease to be, and it's still as sensitive.

Now I'm just getting ready and putting it all back together nicely. :) Thank you so much for your help Stella. Your troubleshooting helped me keep going at it and it's paid off finally!

Proud Mary

Tue Dec 20 2016, 06:47PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Oh I am pleased!

I must say that the guts are in really beautiful condition for something so old - not a spider in sight.

The user manual goes right back to the early Cold War era when the US Govt wanted as many people as possible to go out prospecting to find new sources of uranium in the Lower 48 - a bit frightening really.

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