Hello ALL,
I can only say that three instruments of early 70s vintage  (more correctly
highly modified versions thereof) and even older units totally made in-house
are in routine operation in our laboratory in addition to more recently
purchased CF-C-IRMS equipment.
Generally speaking, these instruments aren't nearly as sensitive as more
recently available equipment but (1) they are capable of acceptable analyses
for many projects (2) they are ideal for giving students hands-on experience
and knowledge of the basic principles of IRMS , and (3) we have grown to
understand them and can repair them in-house.
Electronics failure tends to be a major factor in our environment where
humidity tends to be low. Components failed simply because after 25 years,
they "dried out" Another factor was burnouts and brownouts because lightning
storms caused whopping power surges over the years.
Trying to obtain certain specific components tends is often impossible and
for sure expensive. With specific reference to the GD150, we used one for
H/D analyses. The filament supply failed and the cause was breakdown of the
chopper amplifier system. The choppers were claimed to be no longer
available from the company. Consequently, we had some units customed made
but they did not work satisfactorily after waiting months for their arrival.
Coincidently I recall that when Jim O'Neill was at Menlo Park he operated
such an instrument in which the bulk of the electronics had been replaced
with supplies built "in-house.
In earlier times, we did not purchase complete instruments because one was
buying older technology than necessary. We bought the basic components and
designed much of our own electronics and sample handling. With our 602s and
903s, ion pumps were used rather than oil diffusion pumps, thus realizing
table-top instruments many years before a manufacturer claimed to have the
first. We used the 310J amplifier modules for ion current measurement a
couple of years before Micromass started to use them. Rather than fiddle
around with decade potentiometers,etc for balancing ion currents, we used
voltage to frequency conversions of the total signals, counters, and
interfacing to computers leaving the computer to deal with calculating ion
current ratios and del values. Initially we used Apple computers and we went
IBM compatable last year using Olivetti computers judged to be obsolete to a
Business Education Dept in a local high school and FREE.  By modifying Nupro
valves, we avoided the expense of waste lines on the  dual leak inlet
systems. Our variable volumes were made from commercial stainless bellows
units at a fraction of the cost of those offered by the MS companies. We
have more recently developed a quartz stabilized digitally controlled
filamant supply, a transimpedance amplifier for ion currents, and a
versatile MS-PC interface, the costs of their construction being very
reasonable. Mike Wieser as part of his graduate theses upgraded a solid
source instrument built in the late 1960s to include Hall probe magnetic
field stabilization  and computer control through optical coupling of many
functions including filament current, magnetic field, and data acquisition.
I know that other laboratories, certainly in former times before budget
cuts, engaged in such developments.
The bottom line is that older mass spectrometers can be effectively
refurbished with modern electronics and components at very reasonable costs.
Amazing electronic devices in the $10 range can serve as the basis of
amplifiers, counters,etc. and can replace those bulkier units with  their
many components which eventually fail.. Performance will probably not be as
good as some of the current offerings but can exceed that of the instrument
as originally purchased.
I believe that a point is reached where replacement of complete electronic
modules using modern devices is preferred to constantly fixing the original.
Some companies offer replacement electronic packages  but the more that can
be accomplished in-house, the more independent and secure is the operation..
Many of you of course hold the opinion  that replacement of the total mass
spectrometer is the answer...  but with the success rate for instrument
requests from granting agencies being lower than desirable, rejuvenation of
older equipment should be given serious consideration.
I am sure that colleagues in other countries who did not have access to even
the older instrumentation would make use of donated equipment and probably
make it operational. It would also be nice if somehow they could be supplied
with devices to help them refurbish such instruments.
Roy Krouse