Noise spike in Tecmag Ac250

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I sent this off to Tecmag supprot, but perhaps someone out there has a
solution:

I am running MacNMR 5.6 with Mac 8 on a former AC250.

There is a serious proble with a noise spike.  
It is not a center glitch. 
It shows up in both proton and carbon spectra and is independent of O1.
It shows up even if I terminate the receiver input at the probe.
It does change size with RG.
It does not appear to increase with the number of scans.

Its position is related to the parameter "number of points"

aq. points     no. pts      spike position
  8k            16k         point number 8567
  8k             8k              4281
 32k            32k             17114
  8k            32k             17114

The spike is very sharp (has truncation wiggles if I just FT).
It is both positive and negative.
The position may move a couple of points- it can be hard to tell.

The spike is large enough that it is dominate in .1% Ethyl Benzene.  I
have to tell students to ignore this particular point- which is hard if
they have peaks in the middle of the spectrum.

What can be done to eliminate this problem?


thanks,

kas
-- 
Karen Ann Smith               karenann@unm.edu
Director, NMR Facility        Adj. Asst. Prof.
Dept. of Chemistry            Clark Hall
University of New Mexico      Albuquerque, NM 87131
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