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Radex RD1503+ (To fix or not to fix)

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Dragon64

Mon Jan 26 2015, 06:58AM

Registered Member #1438 Joined: Sat Apr 12 2008, 12:57AM
Location: Canada
Posts: 218

Hello everyone, I recently ordered a supposedly working RD1503+ geiger counter from eBay but found to my dismay that the counter registered a constant 0.00uSv/h with no sensitivity to any radioactive sources.

I opened the counter up see if the high voltage supply to the tube was broken or if the tube itself was broken.

After disconnecting the tube's high voltage supply from the tube, I measured the voltage between the ground and high voltage terminal. This test was inconclusive as my meter drained the supply before I could get an accurate voltage.

I then tried hooking up a S29BG (ÑÐ¸29Ð±Ð³) to the respective high voltage terminal and ground with the geiger counter suddenly responding to my radioactive sources. At this point it seems like the tube was faulty.

The price of an SBM-20-1 tube that this counter uses costs a minimum of $25.99 USD (I bought this counter for $111.64 USD) and the average cost of a working RD1503+ counter costs $173.9 USD.

Would it be worth it to fix this counter or to return it?

Sulaiman

Mon Jan 26 2015, 02:50PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I would email the supplier and ask for a replacement.

It's NEVER a good idea to open up and try to fix a new faulty product under warranty,
(unless that's why you bought it)
they could claim that you broke it.

ALSO if you only want the relative measurement of cps it's ok
BUT if the meter is 'calibrated' in Sv etc. then you will need to re-calibrate the meter.

Dragon64

Mon Jan 26 2015, 04:13PM

Registered Member #1438 Joined: Sat Apr 12 2008, 12:57AM
Location: Canada
Posts: 218

Sulaiman wrote ...

I would email the supplier and ask for a replacement.
.
It's NEVER a good idea to open up and try to fix a new faulty product under warranty,
(unless that's why you bought it)
they could claim that you broke it.

ALSO if you only want the relative measurement of cps it's ok
BUT if the meter is 'calibrated' in Sv etc. then you will need to re-calibrate the meter.

I bought this on eBay used without the original warranty it came with and only eBay's buyer protection.

There was already signs that the geiger counter has been opened based on scratch marks all around the lips connecting the two parts and broken tabs connecting the two pieces.

The counter does have some sort black cylindrical connector on top which I'm guessing was how the manufacturers connected and calibrated it but I have no clue what type of connector that is.

Sulaiman

Mon Jan 26 2015, 07:11PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I buy quite a lot of things via eBay and if the item is listed as 'used' then you can get your money back if it is not working. If it was marked 'spares or repairs' then you can't.

The seller would have known if it didn't work before shipping
and would not like negative feedback.

In this case I would explain that you tested the GM tube by replacing with one of your own spares, I doubt that the seller will assume that you broke it as folks with spare GM tubes would generally not be considered incompetent.
You may come to some partial refund agreement with the seller.

For home use Sv is nice but relative cps is usually adequate.

radhoo

Mon Feb 16 2015, 10:51PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

The Radex-XXXX dosimeter family is not particularly my favourite, as they are a little better than DIY dosimeters in terms of power efficiency and features.

Still, I'd opt for repairing the unit by getting a new tube. The only issue is you would need the SBM-20-1 variation of the popular SBM-20, which was both end caps removed. Those are more expensive as they are harder to find.

Nevertheless, you can get a regular SBM-20 and cut the caps with a fast spinning diamond disk (the glue is too strong). Look for newer SBM-20 tubes, the date is written on them in the form YYMM or MMYY . Newer ones are from '93, older ones can go as low as '81.

As for calibration that is a bit over-rated, and the old Victoreen yellow-bricks are to be blamed for that - their inverters were unregulated, and used some pots to adjust the voltage. Also their tubes had a short plateau, so you had to carefully place the tube in the optimum voltage interval.

The new digital dosimeters properly generate the required regulated high voltage. This combined with the aprox. 100V nice plateau that is almost horizontal for the SBM-20, will place you on the safe side regarding the CPM - > uSVh conversion of the Radex dosimeter, which is a very inexact approximation anyway. So not much need to worry about this "calibration" or related problems when changing the tube.

Conundrum

Fri Feb 27 2015, 08:44AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

I did notice that some "bad" tubes have oxidized casings and/or pins and this is quite common with ZP1313/ZP1314 due to the materials used.
Two of mine exhibited this symptom and had erratic counts which didn't seem to correspond with radiation levels.
I fixed it with silver paint and now both are totally stable and reliable.

Also worth noting is that dented SBM-20's are typically OK if they work with specified voltage.
If you are unlucky enough to have an unstable one I came up with a circuit of a 1000pF and 10M resistor in series which seems to do the trick and avoids wasting an otherwise useable tube.

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