Hi Regina
I have seen a similar problem a few times on the instruments I have here.
It happended on a VG SIRA-12 and once on the DeltaPLUS.  What sometimes
happens is that the VFC (more likely) or the head-amp amplifier saturates
(max output) and doesn't reset back to zero after but holds to that point.
The only way to reset it, is to reset the electronics by switching the power
off to the boards for 10sec then back on again.

To check which one is the problem (VFC or Amp), you can reverse cup 1 and 2
or 3 of the head amplifier at the VFC card on the SBC frame.  Make sure the
source is off first so that you have no voltage signal at the collectors.
If the problem transfers to a different cup then you know it is from the
head amplifier.  If not it is at the VFC.  If you have no idea what I mean
by this, call your local FinnMAT engineer to help you do this.

Gilles

> -----Message d'origine-----
> De : Regina Galarza [mailto:[log in to unmask]]
> Envoyé : 13 novembre, 2001 07:35
> À : [log in to unmask]
> Objet : [ISOGEOCHEM] Help
>
>
> Dear listmembers,
>
> I need some ideas about what is happening with our Delpta
> plus. We have a
> Finnigan Delta plus to do continuos flow N2 and CO2 analyses.
> We have problems with the signal in cup 1 (mass. 28 for N2
> and mass 44 for
> CO2).
> Three weeks ago; the background signal in all cups was O.K. When the
> reference gas ocurred, the signal in cups 2 and 3 was stable
> but not in cup
> 1. It had valleys. When we tried to do a stability test, we
> obtained the
> following delta values:
> delta13/12 for CO2 between 0.18 and 1.81
> delta29/28 for N2 between 0.18 and 0.43.All of them greater
> than 0.15 per mill
> We thougt that it could be because of contamination in the
> references gas.
> We changed them and purged the system but the problem persisted.
> At this moment, the background signal in cup 2 and 3 are OK
> but in cup have
> a value of 14V!!!!!. Is cup 1 break down????? What does it means?
> Any suggestions as to the origin of the problem and a
> possible solution
> would be greatly appreciated
>
> Thanks                  Regina
>