Dear Dr. Wandless,           7th June 2010

     We have had a similar problem of an unresponsive HD collector of
our MAT 250 twice before.  On both occasions, it was due to a stiff
wire/rod to the collector cup moving over with time, until the
wire/rod contacted the housing, internally.  This effectively shorted
the collector to ground.  You can check to see if your collector is
grounded as ours was, by first removing the amplifier housing.  Then
connect an ohm meter from the external pin of your unresponsive
collector to the flight tube.  If you have continuity, the collector
is grounded out.  If you have no continuity between the collector pin
and the flight tube, then yours is a different problem than we had.

     To remedy the short, we had to vent the mass spectrometer and to
carefully unbolt and remove the collector housing from the flight tube
(clean, lint-free cloth atop the bench top, no finger prints, no dust,
etc.)  We then meticulously moved/bent the wire such that it no longer
was touching the housing.  As I say the first time we did this repair,
it worked for a time, but somehow the wire was touching the housing
again several months later.  So we bent the wire away from the housing
a second time.  Everything is in working order now.

     I hope this is of some help.  The collectors of the MAT 250 and
251 are essentially identical, so far as I know.


Frank Simpson
Novato, California
[log in to unmask]

On Mon, Jun 7, 2010 at 6:40 AM, Gregory A Wandless <[log in to unmask]> wrote:
> I have lost signal on cup 2 of my HD detector on our MAT 251. There is
> signal on cup 3 and it looks normal. I am pretty sure its at the collector
> because if I switch to CO2, I have signal for all cups, so I don't think its
> in the cabinet.  Any ideas?
>
> Has Roger Huested retired, his phone has been disconnected?
>
> Thanks,
> Greg Wandless
>