The responses I got were very helpful in our trouble-shooting
process. Although it turned out to be a trivial (but tricky) problem,
I think all of us could benefit from these responses. They are posted
below.
Thanks again!
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Jiejun Wu, Ph.D. (714)856-6010 (phone)
NMR Facility Manager (714)856-8571 (fax)
Department of Chemistry jwu@uci.edu (internet)
University of California
Irvine, CA 92717-2025
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Summary:
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Greetings,
We had a similar problem with a Unity300 some time ago. Tracing the
problem was fairly easy, though our difficulty was intermittent which
required a bit of thought to deal with. Our machine would work
perfectly and bomb briefly and then return.
Ultimately, our probelm was traced to a reference frequency generator
which was causing BOTH the lock and the actual frequency to jump by a small
amount.
What we did was to have the spectrometer acquire a 16 transient spectrum
every 10 minutes or so all night long and then look in the morning.
We did this three times, over a weekend, once locked and with the shims turned
on; again with the shims "on" but unlocked; a third time unlocked and with
the shims turned off. Someone had to show up to start the third experiment.
Lineshape was obviously poor with the shims off but certainly good enough
to see the line shift.
A simple D2O sample was used thoughout. By sorting through these spectra
we quickly saw that something was affecting BOTH the lock AND the
spectrometer frequency. By the next day the only common reference
generator was replaced and all was, and has been fine.
I realize that our experience is not likely to be the same as yours.
I only hope that our experience can speed your 300 to a happy recovery!
Cheers,
Ron Crouch
__________________________________
Ron Crouch, Burroughs Wellcome Co.|
RTP, NC (919) 315-3840 |
crouch@bwco.com |
__________________________________|
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I would suggest TURNING THE LOCK OFF (after shimming, if you wish),
and acquire without a lock. If the lock is "push/pulling" the field,
it won't do it if the lock is turned off!
If the artifact still exists with the lock disabled, it is most
likely something in the shim unit.
Good Luck,
Richard Shoemaker, Ph.D. Phone--(402) 472-6255
Instrumentation Director, Chemistry FAX---- -9402
Research Associate Proffessor, Chemistry
University of Nebraska-Lincoln
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You could try looking across the sense resistor at the output of the Ho and
shim power supplies with a scope. This is easily done, as I have done it
many times. The appropriate test points are shown in the BSN-18 manual.
Tom Pratum
Dept of Chemistry
Univ of Washington
pratum@uwchem.chem.washington.edu
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you may want to check the quarz/lo board-try unplugging the
cable then reconnect.
tom n
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This seems similar to whatt we observed when the PTS 10 MHz clocks become
unstable. If you have another synthesizer available, try its clockto see if there is
any difference.
Rudi
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We saw a similar problem on an AC300 some years ago, which was followed by
intermittent complete loss of signal. The problem then was the 10 MHz
crystal source (in the PTS synthesiser). Although these are pretty
reliable we have had a second failure (on an XL300) since then. You might
like to try substituting an external 10 MHz reference, for example from
another spectrometer if you have one (this just plugs into the back of the
PTS; set the switch by the socket to EXT). This is an easy test to
perform and might just identify your problem, whereas tracking down
instabilities in shim power supplies is a bit trickier. The next place to
look is probably the power supply to the shim amplifiers; it should be
fairly easy to spot trouble here. A subtler source of problems in the
AC300 lies in the shim and Z0 DAC's, which have quite a high failure rate.
These are just a few of many possibilities; if they don't help and you
need some more suggestions just let me know.
Best wishes,
Gareth Morris
Dept. of Chemistry
University of Manchester
Oxford Road
Manchester M13 9PL
United Kingdom
Tel. (0)61 275 4665
Fax (0)61 275 4598
Gareth.Morris@man.ac.uk
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I'm glad that you got my note. A final comment that I maybe did not make
clear yesterday: We did 16 transients in a few hundred separate spectra
because our problem was intermittent. When a "bad" transient occurred a
spectrum with multiple lines was recorded, rather than just one!
If your problem is more constant, trouble shooting will hopefully go much
faster. Good luck and best regards.
__________________________________
Ron Crouch, Burroughs Wellcome Co.|
RTP, NC (919) 315-3840 |
crouch@bwco.com |
__________________________________|