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 2.GQ Geiger Muller Counter
 GMC-500+ High CPM - Dead tube?
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1 Posts

Posted - 05/02/2022 :  11:34:16  Show Profile  Reply with Quote
My GMC-500+ have started showing unreasonably high CPM on the first tube. I've had it for a few years so far, and initially everything was fine.

Some time ago CPM1 started going off the charts: the usual background range is 10-20 CPM, and CPM1 now fluctuates between that and couple thousand. CPM2 is 0 to 5 at the same time.

I haven't changed any HV settings prior to this.
Resetting to factory defaults didn't fix this.

Is tube #1 dead? Any chance this can be fixed with a firmware update?

Thank you.

Device revision 2.23

Edited by - Bazooka on 05/02/2022 11:37:37
Reply #1


25 Posts

Posted - 05/02/2022 :  15:31:59  Show Profile  Reply with Quote
It seems to me that the GM M4011 counter is to be replaced (it has discharges that are not extinguished). You can do such a repair yourself in a simple way - by purchasing the GM M4011 glass counter

GC: GMC-500, GMC-500 plus, GMC-600, Exploranium GR-110G, Terra MKS-05 bluetooth, DoseRae 2, RadiaScan 701A, Automess 6150AD6, Soeks Qantum, PRM-9000, Radex Obsidian, Radex RD1008, MKS-03SA, DKG-21M, Raysid
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Reply #2


1134 Posts

Posted - 05/02/2022 :  23:30:48  Show Profile  Reply with Quote
A new tube won't help if for any other electronic(?) defect the voltage has gotten out of hand.

There were quite a few reports recently about high anode voltage. Surely can't hurt to measure voltage.
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Reply #3


2219 Posts

Posted - 05/10/2022 :  09:51:57  Show Profile  Reply with Quote
@ullix is right, you may need to check if the voltage is still in the working range. You can try to decrease the tube voltage percentage by 5-20% to see if there is any improvement. But I most likely the tube needs to be replaced. Still need to check the voltage though because if it is too high, you do not want the new tube to die faster, or not work properly.
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Reply #4


1134 Posts

Posted - 05/21/2022 :  23:36:45  Show Profile  Reply with Quote
... it does not depend on the tube but on some electronics problem.

This may well be the case, but we won't know until we know what the true voltage is to either determine that this is the cause or exclude it.

I believe the anode voltage of the two tubes is the same, and cannot be set individually? To measure you need a decent DVM and a 1 GOhm resistor.

If you don't know how to do it or do not have the equipment you can take some "iterative" approach:

Whatever the setting of the voltage setter is, reduce it to half. If then you don't get any counts, keep increasing the voltage step-wise and check counts at each step. The background should be near 20 CPM - anything from 10 to 30 CPM is likely ok.

If you have no better radioactive source, get Potassium. It can bring some CPM=100. It may be in your household already. For details see "Potty Training"

Make yourself a nice cup of hot chocolate. But before you prepare it, measure the counts from the cocoa powder ;-)
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