Dear All,

If you replace the LM317T , you may also have to replace the resistor
network around this
chip to obtain the same regulated voltage as you had before the
electrronics failure.

Glen



                    Andrew Tait
                    <[log in to unmask]        To:     [log in to unmask]
                    LA.AC.UK>              cc:
                    Sent by: Stable        Subject:     Re: Europa 20/20
                    Isotope                electron energy set to -90V only
                    Geochemistry           reaches -72V.
                    <ISOGEOCHEM@LIS
                    T.UVM.EDU>


                    08/24/01 02:44
                    PM
                    Please respond
                    to Stable
                    Isotope
                    Geochemistry






Hi Myles,

I had exactly this problem about 2 months ago...

At 12:39 24/08/2001 -0600, you wrote:

>Our Europa 20/20 has developed a peculiar problem. If the electron energy
>is set by way of the software to be -90V for example, the source monitor
>shows that the electron energy is actually different than -90V. This
>problem becomes increasingly worse as the electron current is increased.
At
>first I thought that this is a result of a failing filament but I replaced
>the filament and the symptoms are still very strange. Note that the
>following measurements (made by the source monitor) occurred with an
>electron energy setting of -90V:
>
>When electron current is 50uA, electron energy is  -89.3V (should be -90V)
>when electron current is 100uA, electron energy is -87.1V (should be -90V)
>When electron current is 200uA, electron energy is -78.9V (should be -90V)
>when electron current is 300uA, electron energy is -72.6V (should be -90V)

This is exactly what I saw too.

The problem is on the source motherboard under the right hand panel of the
20/20 mass spec.  In the middle of the board there are 4 power amplifiers
and 4 capacitors.  Take out the board and replace the whole lot.  The total
cost will be about 12UKP.  You should be able to get the LM317T (I'm at
home, so check the numbers from the existing components!) and 22uF
electrolytic (check this too!) capacitors from any decent electronics
supplier.  The chances are that just the filament supply is weak, but you
might as well do the others at the same time.

To prevent this circuit from failing again, it's worthwhile doubling up the
two filament leads.  Get some decent quality HT cabling and run it in
parallel with the existing cabling.  It'll save you about 0.3 of an ohm,
which reduces the load on the emission circuit by about 25%.

I hope this helps,

Andrew