[RASMB] Wavelength problems

John Philo jphilo at mailway.com
Fri Mar 12 12:09:00 PST 2004


Dieter,

I've never heard of anything similar but here is are two hypotheses:

(1) you have a mechanical problem in the wavelength drive
motor-potentiometer assembly that is producing a large backlash when the
motor changes direction. In other words, there is a 30 nm difference in
wavelength calibration depending on whether the wavelength is increasing or
decreasing. That assembly is not something I would recommend trying to
repair or replace yourself.

However, I thought the instrument software was always supposed to approach
the desired wavelength from the same direction, to eliminate any potential
backlash. Perhaps this isn't true in all cases, or in all versions of the
software. (Really I should say firmware rather than software, because it is
the code in the instrument EEPROMS that does these things, not the PC.) 

(2) It is also possible that the instrument firmware has gone bad (bad
EEPROM on the data acquisition board) and is sending bad commands to the
wavelength drive. These EEPROMS do occasionally go bad. That could be tested
by swapping the EEPROMS from your two instruments, if they run the same
software/firmware combination on PC and instrument. 

Be forewarned though that this swap (or replacing the EEPROMS with new ones
you buy from Beckman) is not that easy nor without risk: it is very easy to
bend and break the pins on the EEPROMS, even when you have the chip
insertion/removal tools as I do. Further, these boards also have tiny
coaxial cables that must be removed and correctly re-attached (from the
photomultiplier and the wavelength and radial position potentiometers).

Lastly, as an aside, even though I think I'm pretty good at diagnosing and
fixing these instruments, I would never, never think of dropping my
maintenance contract, nor recommend that anyone else do so.

John Philo
Alliance Protein Laboratories

-----Original Message-----
From: rasmb-admin at rasmb-email.bbri.org
[mailto:rasmb-admin at rasmb-email.bbri.org] On Behalf Of D.Schubert
Sent: Friday, March 12, 2004 8:45 AM
To: rasmb at rasmb-email.bbri.org
Subject: [RASMB] Wavelength problems


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Dear colleagues,

we have a technical problem with one of our two XL-A. Since we do not have a
service contract any longer, we would like - if possible - to fix it on our
own. However, for that we need advice.

If we run a wavelength scan, everything seems to be o.k.; in particular the
absorbance maxima are where they should be. On the other hand, during radial
scans the "effective" wavelength is not what was programmed but approx. 30
nm lower. This effect, which occurs both in step and continuous mode, is
highly reproducible. It is independent of the monochromator used: exchange
against our second one does not alter anything. Also, reinstallation of the
XL-A software made no difference.

We could imagine that we are not the first ones to suffer from the problem
described, and we would appreciate very much if anyone could advice us how
to get rid of it.

Best regards,
Dieter

--
Prof. Dr. Dieter Schubert
Institut fuer Biophysik der JWG-Universitaet
Theodor-Stern-Kai 7, Haus 74
D-60590 Frankfurt am Main
Germany
Tel. +49 (0)69 63017073
Fax  +49 (0)69 63015838


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