on 6/6/2000 3:50 PM, Joost Van Haren at [log in to unmask] wrote:

> 2) We also have a VG Prism II which has not been behaving well. On and off
> over the last month a Electron volt PCB fault comes up on the screen and
> with that the source current is off scale and the filament current is 4.4 in
> stead of 3.8. Finally last week the instrument was running fine over the
> weekend the source fuse blew (the 5X20 mm, 5A 250V ceramic fuse). I replaced
> the fuse and ever since the fault has been coming up again. I also have
> taken the source apart 3 times over the last month to make sure that
> everything is put back perfectly and I have not been able to find anything
> wrong in the source itself. Has anyone had this problem with this instrument
> and if so do you have any suggestions where the problem might be?
>
> Thanks very much,
>
> Joost
>
> Joost van Haren
> Senior Research Specialist
> Columbia University
> Biosphere 2 Center
> 32540 S. Biosphere Rd.
> P.O. Box 689
> Oracle, AZ 85623
>
> Voice:  520-896-5018
> FAX:    520-896-6214
> [log in to unmask]
>

Allow me to address 2) above.  We have a PRISM II.  Your description may
indicate that you have a filament assembly that is shorted to the ion box
(source).  If you turn off your source HT, remove the source leads to the
vacuum feedthrough flange, and measure the resistance between pins 1 and 8
or between pins 12 and 8 you might see the short.  However, the short may
appear only when the filament is hot.  That would make it more difficult to
observe.  An insulation tester using higher voltages than a standard DMM
might help.  Also, there is a fuse on the eVolt PCB if you haven't checked
that.

--
Howard Berg
Development Engineer
UCSB Department of Geological Sciences
Santa Barbara, CA  93106-9630
Voice:  805.893.3487     Fax:  805.893.2314
mailto:[log in to unmask]
http://www.geol.ucsb.edu