Radiation Alert, Monitor 4 repair

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Monitor 4, "Radiation Alert"
S. E. International Inc, Summertown TN

These little GM counters have a typical failure:  the 500VDC voltage falls  
low, as the zener regulation diodes become damaged.

In the power supply circuit on the PCB, three zeners in series are used as  
feedback reference:  one of 1N5272 at 110V, and two of 1N5281 at 200V (to c
reate a 510V zener.)

I found that all three were damaged and leaky, resembling resistors of valu
e between 1M and 4M.   They still checked out as diodes.  But they were con
ducting at much lower voltages than their rating.   They caused the -HVDC v
alue to be around -260V, so the geiger tube wasn't counting.

On the PCB, the first diode is connected between a 470K resistor and a 4.7M
 resistor which leads to the GM tube.  All three diodes are in series.

I didn't have any 1N5281 on hand, so I tried a 1N5279 (180V, not 200,) and  
the unit woke up and worked fine.   It might not give calibrated meter read
ings though, since the energy of each pulse might be down to ((510-40)/(510
))^2, or 85% (since the HV supply is down by about 8%, if low by 20+2040%

Note that there's a trimmer pot on the PCB.  Possibly for meter calibrate?  
 I didn't change its setting, measured at 362 ohms.

Re: Radiation Alert, Monitor 4 repair
On Wednesday, December 25, 2019 at 2:08:45 AM UTC-8, William Beaty wrote:
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For HV use, these parts should have long lives (they're glass/metal packages, no epoxy).
How old IS that unit?   Has it been in any odd environments (neutron flux?) ?

Replacement with 1N5388 might be a good idea; the axial parts are stil metal/glass,
just have an epoxy overcoat.   Surface-mount packages, though, I'd be less sure of.

The glass-package parts DID have some light sensitivity, sometimes, so testing
for leakage can be an interesting challenge.

Re: Radiation Alert, Monitor 4 repair
On Wed, 25 Dec 2019 02:08:42 -0800 (PST), William Beaty


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Offhand, that sounds to me like a dirty, leaky, or water logged PCB.
Did you test the diodes with at least one lead lifted from the PCB?
If you measured the resistance while the diodes were on the PCB or you
did some soldering on the PCB, clean both sides of the PCB first with
distilled water, and then with 91% (or better) isopropyl alcohol.  If
there's no conformal coating, try acetone:

   I have known them fail due to HV leaking from the tube  
   across the PCB. The board has to be very clean, free of  
   any flux etc. Often commercial ones are attached with  
   Teflon spacers to minimize charge leakage.

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Again, you could be measuring board leakage, not diode leakage.

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What you might have done is vaporized any water (or contamination)
around the solder pads.  

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Dunno.  I couldn't find an inside photo or a schematic.  My guess(tm)
is that the pot is in series or across the meter to set full scale.

Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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