FEG filament

We have a Philips CM-200 FEG TEM. We have a problem with our FEG
filament.At start (Extaction Voltage=2.2 kV) its emission is about 30uA
but at operating mode(EXt=3.8kV)its emission goes down ,and while
working its emission drops to 1.2uA gradually. Aftrer closing V7 valve
its emission increases.
At first we think its filament is over,but we didn't observe such a
behaviour with one befor this Gun.
we want to know whether the gun life is over or is there any other
problem?

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