I can now report a successful resolution to our mystery. Apparently our
supercon shim power supply did not have enough current in its main
heater circuit hence 99 amps nominal delivered did not produce 99
amps of current in the magnet. Replacing a resistor in the power supply
to increase the current in the main heater solved this problem. When we
re-ramped the magnet to 99 amps we easily found a signal at 300 Mhz and
successfully cryoshimmed. No one who contacted me mentioned having this
particular problem but its something to keep in mind if you can't find a
signal at the field you expect.
Jeff de Ropp
UCD NMR Facility
jsderopp@ucdavis.edu
Phone: 916-752-7677
FAX: 916-752-3516