Jin Wang wrote the following about optima vacuum problem:

> We have a Micromass Optima with Isochromcontinueous flow interface here at
> Iowa State University. Recently we are experiencing a vacuum problem. When
> CRT7 valve is closed, the vacuum reading in ion gauge is only 3.6E-6mbar
> (it's normally about 1E-8mbar). This happened gradually. If we reopened and
> then closed the valve, we could sometimes get a vacuum of 1E-7mbar. But now
> no matter the valve is colsed or open, we can only get 3.6E-6mbar. Today we
> changed a new copper seal seat, knife edge and wavy washer for CRT7 valve,
> but the vacuum is still 3.6E-6mbar. We degased the ion gauge, the reading
> didnot change. When we tried to degas again, the computer gave us message,
> it reads " ion gauge bad calibration" and meantime the vacuum disply for ion
> gauge disappear. I rebooted the computer, it didnot help. Now everytime when
> I try to turn the ion gauge on through the software, computer keeps giving
> me the same message and I can't monitor the ion gauge vacuum!
> Any suggestions regarding to these two problems are greatly appreciated.

Well, I'll do this back-to-front.  The problem with the Ion Guage is
most likely to be the power supply in the Vacuum Guage Controller
unit.  The two power transistors, TR5 and TR6 and their associated
Zener diodes are prone to failing if the unit is run at high pressures
for extended periods.  On our Prism III, we need to bypass the Ion
Guage completely to run continuous flow :(  Our Optima has run into
this same problem 3 times now over the past couple of years.  VG did
some modifications to the later units, but they are pretty extensive,
and involve completely replacing the (overcomplicated) IG power supply
with something more robust.

You can uprate the two Zener diodes to a higher current rating, which
I've done on the Optima and so far it's been fine.

Your first problem sounds like a very bad leak somewhere.  Once you
get the IG sorted out, you should tune the source to mass 28, and use
Nitrogen externally to leak check everything.  Did you try stoppering
the CRT7 inlet valve where the capillary connects, and pumping out the
small volume there?  That would eliminate the valve passing.

Let me know how you get on.

Totty  <8^)