A number of AMMRLers indicated interest in how my shim problems get resolved, so I'm posting an update after getting the worst of the problems fixed. This will be the final general post on the subject unless I figure out the second problem. Again huge thanks go out to everyone who has helped me try to sort this out.
As a reminder, my main problem was that axial shims Z2-Z7 weren't working correctly with negative DAC settings. After swapping the main driver board with no luck, a couple of people suggested that I measure my test point voltages under load as well as just powered on (obvious in hindsight). This was the key piece of information: the –12 VDC output from the shim power supply (TP12) dropped immediately to –1.5 VDC when any resistance at all was added.
With that piece of information, Dennis Edwards pinpointed the problem: a bad voltage regulator on the negative supply for the axial shims. To verify, he suggested measuring the other voltage in that part of the path (TP38); if it increased along the the other voltage to maintain a constant potential difference, it had to be the voltage regulator. This proved to be the case.
Our electronics shop was able to replace the voltage regulator (a discontinued part) by modifying a differently packaged by electronically equivalent component. This only happened yesterday – they have at least a one week backlog, and since we're a medical school the hospital gets priority. So far, my axial shims are working again.
My other problem is that about the same time, my X and Y shims both changed by more than 12k DAC units and now need significantly more current. This hasn't changed. In a way, that's good, because it means the shims are truly isolated from one another. It still seems unlikely that I dumped a supercon and would be able to compensate with only RT shims, although the X and Y coils only had ~1.5 and 2 amps at install. While replacing the voltage regulator, our electronics shop guys generally checked things over and didn't find any other problems. If anyone has suggestions as to what else could be going on, I'd love to hear them so I can figure out what's going on. However, this isn't a huge problem: both X and Y are responsive and can be shimmed, and most of our other shims don't require very large DAC settings so cooling isn't a problem.
Cheers,
Andrew
--
Dr. Andrew Fowler | University of Iowa
Associate Director | B291 Carver Biomedical Research Building
Medical NMR Facility | Iowa City, IA 52242
319-384-2937 (office) | 319-335-7273 (fax)
andrew-fowler_at_uiowa.edu
________________________________
Notice: This UI Health Care e-mail (including attachments) is covered by the Electronic Communications Privacy Act, 18 U.S.C. 2510-2521, is confidential and may be legally privileged. If you are not the intended recipient, you are hereby notified that any retention, dissemination, distribution, or copying of this communication is strictly prohibited. Please reply to the sender that you have received the message in error, then delete it. Thank you.
________________________________
Received on Thu Jul 14 2011 - 05:34:20 MST