Hi Folks,
I too have a shim that is not working. I traced the trouble to a broken wire at
the junction on the base of the shim stack. It looks to me that the screw broke
the wire being that is is very close. I have done a lot of VT work , real cold,
and perhaps this might have contributed to the breaking. Maybe coating the joints
with something might reduce oxidation.
I really dont like taking it apart again.
Jim
Jane Strouse wrote:
> Dear AMMRL'ers,
>
> I have had exactly the experience that Ben describes below. We do a
> lot of VT work and I have fixed these connections more times than I would
> like to remember. I agree 100% with Ben that Bruker should consider making
> these connections a lot more robust.
>
> Jane Strouse
>
> >Date: Thu, 11 Apr 2002 09:05:16 -0400
> >From: Ben Bangerter <ben.bangerter_at_yale.edu>
> >X-Mailer: Mozilla 4.76 [en] (Win98; U)
> >X-Accept-Language: en
> >To: Karen Ann Smith <karenann_at_unm.edu>
> >CC: ammrl <ammrl_at_chemnmr.colorado.edu>
> >Subject: Re: Another shimming weirdness
> >
> >Karen Ann,
> >
> >Is it possible you have a bad connection to one of the shims, perhaps at the
> >base of the shim coil asssembly? If this were the case, when you have the
> >bad lineshape, one of the shims would have no effect. I think you have
> >Brukers, right? From my experience with Bruker shim stacks, the soldered
> >connections where the wires of the shim coils are attached to the PC board
> >where the wires to the shim cable attach, at the base of the shim stack, are
> >prone to fail. I have seen this a couple of times. I believe the culprit
> >is electrolytic corrosion that occurs when there is water condensed in this
> >area during low temperature operation, when the shim assembly is cooled
> >below the dew point. A small amount of mechanical disturbance can break or
> >open the connection, and when a connection is remade it can "weld" slightly
> >due to the current density at the point of contact. The first time I
> >experienced this, it was maddening until we removed the shim stack,
> >disassembled the base, and examined the connections. Lots of corrosion
> >products were evident, and an open connection.
> >
> >If this is the cause of the problem you are experiencing, it is easily
> >fixed. You would need to remove the shim assembly to the bench, disassemble
> >the base to examine the connections, clean off any corrosion products, and
> >repair any break. I have long thought Bruker ought to passivate these
> >connections because of the potential corrosion problem, perhaps with a
> >coating of lacquer of some sort.
> >
> >Of course these comments may be way off the mark in this case; if so,
> >"never mind!"
> >
> >Ben
> >
> >*****************
> >
> >Karen Ann Smith wrote:
> >
> > > AMMRLers,
> > >
> > > I have been seeing a pretty weird problem on my 500. Sometimes when I
> > > swap probes, the shim goes to (really bad). As in linewidths of (many)
> > > 10s of Hz. But only sometimes.
> > >
> > > The most recent example is that last week the 5 m probe was in and
> > > shimmed pretty well. Swapped it for the 2.5 on friday and all was
> > > well. Swapped the 5 back in on Monday and again, all was well. I did a
> > > little shimming on Monday and saved the file. Tues. I swapped the 2.5
> > > back in. The lineshape was terrible, the field had to be changed to get
> > > the system to lock, and the o1 necessary to put water on resonance was
> > > off by about 1100 Hz. Today I put the 5 back in, read in Monday's shims-
> > > and the field was fine, but the shims... not so good.
> > >
> > > Last month, this happened about every probe swap. I looked up the bore
> > > and didn't see anything. I cleaned the bore anyway, and the problem
> > > seemed to go away - until yesterday. When this first started, I blamed
> > > the surrounding- esp. as it seemed that the nearby (touching 5 gauss
> > > lines) 250 also seemed to be changing. Now, I don't think so. I can't
> > > find any changes in the nearby labs- and the shims are fine as long as a
> > > probe is left in the magnet. I am sure the probes are aligned the same
> > > every time- I have marks on the probe and magnet.
> > >
> > > Last month, z4 was the big changer. Today, z4 was not too bad, but
> > > lower x,y,z all needed some adjustment.
> > >
> > > Ideas, thoughts, suggestions??? Please note that not-swapping probes is
> > > not an option.
> > >
> > > kas
> > >
> > > --
> > > Karen Ann Smith karenann_at_unm.edu
> > > Director, NMR Facility Adj. Asst. Prof.
> > > Dept. of Chemistry Clark Hall
> > > University of New Mexico Albuquerque, NM 87131
> > > 505.277.4031 url: http://www.unm.edu/~karenann
> > > Out of this world screensaver: http://setiathome.ssl.berkeley.edu/
> > > "There is nothing a good day of cycling won't cure."
>
> Dr. Jane Strouse
> Department of Chemistry & Biochemistry
> UCLA
> Los Angeles, CA 90095-1569
> (310)-825-9841 - voice
> (310)-825-0393 - FAX
> strousej_at_chem.ucla.edu
--
Work phone (days): 802 656 0196
Web Site: http://www.uvm.edu/~jbreeyea
University of Vermont
Received on Sat Apr 13 2002 - 09:14:41 MST