We own an old Bruker AF300 console attached to an Oxford magnet of similar
vintage. Sometime last week this instrument stopped working properly and
we are somewhat puzzled by the symptoms. The sequence of events is
unclear, but the failure may have occurred during or immediately after a
helium fill. The symptoms are as follows:
1) The proton signal is weak and the line is broad. A water sample has a
half-height line width of approximately 100 Hz.
2) The deuterium lock signal is also weak.
3) The situation cannot be significantly improved by shimming. In
particular the Z shims have surprisingly little effect; either for better
or worse. The X and Y shims are effective only in as much as they can be
used to obliterate the already weak signal.
4) The Z1 shim behaves oddly is as much as it noticeably changes the field
position.
We have checked out the BSN-18 shim power supply by measuring the voltages
at the diagnostic connector. These are the voltages across the shim
current measuring shunt resistors, so we believe this to be a reliable
indication that correct current in being supplied to the shim coils. We
have also tried swapping in another shim tube assembly from a different
instrument. We are also confident that the vertical position of the shim
tube did not somehow mysteriously change.
At this point, our only working theory is that one of the cryoshims has
quenched. This does not seem very likely, but we're out of other
ideas. Before, we take the risk of putting in the stick and recharging the
cryoshims, I wanted to see if any one on this list has any other
thoughts. I would greatly appreciate any helpful suggestions.
James Gladden
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James A. Gladden
University of Washington
Chemistry Department
Box 351700
206-685-2081 (voice)
206-685-8665 (FAX)
gladden_at_chem.washington.edu
Received on Thu May 29 2003 - 15:24:18 MST