I replaced the filament and now everything works fine.
The problem was that the filament was blown - but the left and right
part of the broken filament still had contact.
So the HV and current reading was OK and check of the pins 1 and 10 gave
continuity (with disconnected leads in the teflon housing).
The two parts of the broken filament even "jumed back" into place after
moving them with a tweezer - and had contact again.

thanks,
Robert
-- 
Dr. Robert van Geldern
Leibniz Institute for Applied Geosciences (GGA)
S3: Geochronology and Isotopehydrology
Stilleweg 2, 30655 Hannover, Germany
phone: +49-(0)511-643-2313 (office), -2539 (lab)
fax: +49-(0)511-643-3665
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