counter stopped during a measurement

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duwaar
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Joined: Mon Jul 18, 2016 7:42 am

counter stopped during a measurement

Post by duwaar »

Hello MightyOhm,
I built one of your Geiger Counter kits and it worked fine for a while, but then it just stopped. The batteries seemed low (2.6v together), so I hooked the circuit up to my power supply at 3.00v. (I tried to be careful with the power supply, but the only way that I can think of that I could have fried this thing is by powering it with 5v instead of 3v.) It didn't work for about a minute, but when it started clicking again I figured that it just needed to warm up. It seems to work, but I know that something is wrong because it no longer responds to a radiation source that made the counter go crazy before. Also, there is no whine at startup, shorting across the tube clips does nothing (with or without the tube), and adjusting the potentiometer has no apparent effect either.

I tried all of the recommendations in your troubleshooting post (checking for correct placement of all the parts, reflowing my solder joints, etc.), but none of them seemed to help. I also read some other posts about similar problems where you asked for voltage readings, so here are my voltage readings between pin 3 of the pulse header and the specified point:

pin 1 of pulse header: 3.001v
TP1: 2.861v
TP2: 2.764v

U1 pins:
1: 0.000v
2: 2.266v
3: 0.001v
4: 0.021v
5: 1.854v
6: 0.000v
7: 0.000v
8: 3.001v

Removed U1:
U1 pin 4: 0.023v
TP2: 2.764v

With U1 removed, shorted U1 pins 3 and 8:
U1 pin 4: 0.052v
TP1: 2.875v
TP2: 0.859v
left leg of R8: 0.865v

All measurements were taken with a Fluke 175. I could only find input resistance (50 MegaOhms), but maybe you would have more luck.
http://en-us.fluke.com/products/digital ... #resources

Any idea what I can do to fix this thing?
Thank you
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mightyohm
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Re: counter stopped during a measurement

Post by mightyohm »

Try replacing Q3. It's a commonly available 2N3904. This is usually the first device to fail when HV is accidentally shorted to one of the nearby components.
duwaar
Posts: 4
Joined: Mon Jul 18, 2016 7:42 am

Re: counter stopped during a measurement

Post by duwaar »

Thanks for getting back with me so quickly.
I replaced Q3 as you suggested, but not much changed. The only major differences I found were the measurements on pin 4 of U1:
With the IC in: 2.215v
IC out: 2.392v
U1 sockets 3 and 8 shorted, pin 4: 0.041

Still does not whine on startup or respond to the radiation source.
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mightyohm
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Re: counter stopped during a measurement

Post by mightyohm »

I'll take a closer look this evening, but all of the other active devices are potentially at fault - U1 itself, the other two transistors in the HV supply, and the diodes.
duwaar
Posts: 4
Joined: Mon Jul 18, 2016 7:42 am

Re: counter stopped during a measurement

Post by duwaar »

I found a drawer of 555 timers, so I tried replacing U1. After doing that, the detector didn't seem to pick up any ambient radiation, but it did click when I touched J2 and J1 and when I tested the voltage between pin 3 of J6 and TP1. I then tried measuring TP2. I got erratic readings on my multimeter, and it won't click anymore like before I replaced U1 and Q3.
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mightyohm
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Re: counter stopped during a measurement

Post by mightyohm »

Make sure it's a cmos 555. A regular 555 won't work.
duwaar
Posts: 4
Joined: Mon Jul 18, 2016 7:42 am

Re: counter stopped during a measurement

Post by duwaar »

Oh, I see. I looked up the datasheet on my LM555CN; it doesn't say CMOS anywhere, so I assume it is not. Is that the part you would recommend I replace next?
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mightyohm
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Re: counter stopped during a measurement

Post by mightyohm »

Yes, you should try replacing U1. It's a TLC555CP from TI, but any CMOS 555 should work.
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