[ RadSafe ] GR-130 problem

[Colin]

Hi,

I've not done any repair work on the GR-130, but it sounds to me like a
high voltage drift problem.  High meg resistors used in these things
commonly drift with age, causing the high voltage to drift.  Sometimes,
depending on the fine detail of the electronic design, the HV readings
on the instrument are made through the same resistive divider network
that controls the HV, so the reading of HV on the instrument will stay
the same, but the actual PMT voltage drifts and changes energy
characteristic and sensitivity.  When measuring the HV, you need a high
voltage divider probe with a high resistance input.  A normal multimeter
will load down the HV too much and give false low readings.

The fact that the two instruments have failed at similar times supports
this idea, as the operating life of the high meg resistors is some
function of time and voltage.  I have no real evidence that is the
situation in your case, but it seems to be the pattern I have seen in
many other instrument repairs.  If you can find a local electronic
technician used to dealing with high voltages on cathode ray tube
television sets, they will likely have the HV probe needed, and may not
be too expensive.

Good luck with the problem.

Colin Beeforth
Lasielle Consulting
Melbourne Australia