A while back ( OK, a long while back) I posted a question about some
lock weirdness we were observing. I am reasonably convinced I have
found our answer, and so I am posting a summary.
The problem was major lineshape issues that seemed to occur with probe
changes.
Some of the suggested problems were:
thermocouple placement in the probes changing
broken shim wire(s) - most likely at the plate at the bottom of the
magnet. This can be tested by wiggling the cable or checking wire
conductivity.
Shim values not being loaded correctly.
On some systems it is possible to check the actual shim current values.
This would help diagnose both of these problems.
Something in the bore.
Overtightening the probe screws- this could either change the probe
position enough to affect things, or alter the shims by compressing
wires in the shim plate.
Something in the can that is shifting.
A discussion about this discussed VT and that that might contribute to
shim wires breaking or being intermittent. A vendor suggestion was made
to run air through the shims while doing VT.
This turns out to be my particular problem. The shims (specifically z4)
do not like to get cold. Even running at 6C was enough to make them
unhappy. Once they warmed up, all was fine. I have done 2 probe
changes at RT or above, and the shims were fine. 3 probe changes after
6C runs, with rt air flowing through the shims were fine. My solution
will be to run air through the shims whenever we do vt. (I did try
wiggling the shim cable with no effect, but have not (yet) actually
checked the wires.)
thanks to everyone who responded,
kas
--
Karen Ann Smith karenann_at_unm.edu
Director, NMR Facility Adj. Asst. Prof.
Dept. of Chemistry Clark Hall
University of New Mexico Albuquerque, NM 87131
505.277.4031 url: http://www.unm.edu/~karenann
Out of this world screensaver: http://setiathome.ssl.berkeley.edu/
"Something that has a one-in-a-million chance of happening...
happens once every second at 1 MHz."
Received on Fri May 24 2002 - 15:20:02 MST