AMMRL,
I'd like your help with a couple of odd problems that I think are related to our instrument's lock system. We're experiencing them on a 500MHz Bruker Avance-III with DCH cryoprobe. The system has a BACS autosampler, and all users' spectra are acquired in automation.
1) When there's no sample in the magnet, the lock signal jumps between two stable states: one that is "locked", according to the green Lock button in the BSMS window, and has a lock level of ~14%; and one that is "unlocked", with a red Lock button, and exhibits a lock level of ~32%. So something appears to be wrong in the lock system, but we're having trouble identifying what it might be. We've updated our firmware, and though we think the source is in a board, we haven't ruled out a misset software parameter. We haven't ruled out that this behavior is normal for this system and that we're just ignorant.
2) Every now and then, one of our users gets a spectrum in which the peaks have multiplied. See this page for an example:
http://imserc.chem.northwestern.edu/NMR/Troubleshooting.html
In a standard NS=16 spectrum, for every normal resonance line one observes 16 peaks, most of which are regularly spaced ~30 Hz apart. Each shows some ringing on one side, suggesting the field is being swept during acquisition. This suggests the system was unlocked during this acquisition, and the field changed linearly over the course of the experiment. This problem occurs infrequently enough (once every several days on this ~80%-utilized high-throughput spectrometer) that most of our users have never encountered it, and very few have observed it twice.
It seems like these problems could be related, and that their source lies within the lock system. We're swapping in new boards, but we're having some trouble with that, and we'd like your help identifying a probable origin. This system has two lock boards (a transmitter and a receiver), and a controller board (ELCB). It's been suggested that we replace the BSMS/2 cooling fans, but we're reluctant to for a few reasons: the fans feel like they're going strong, the neighboring shim boards' temperatures are normal, and fan replacement seems prohibitively difficult, involving dismantling the lower portion of the chassis and warming up the cryoprobe because of the connected pneumatics.
What do you think? Have you observed this? Any help or guidance would be appreciated.
Thanks.
- Josh
Josh Kurutz, Ph.D.
Instructor and Senior Scientist for NMR
IMSERC, Chemistry Department
Northwestern University
2145 Sheridan Rd.
Evanston, IL 60208-3113
847-467-1681
imserc.chem.northwestern.edu
NMR Blog: www.imserc-nmr.org
Other: www.joshkurutz.com
Received on Thu Dec 01 2011 - 10:51:34 MST