We just went through the same thing, and it was painful. We ended up finding that a relay supplying the high voltage to the gauge was dead, replaced it and everything works again. You can easily figure out if this is the problem by unplugging the cable at the gauge head and looking for all the required voltages.
A good place to start on the Combivac is to remove it from the 252 and then take the board out of the case, hold it up to the light and look for heat marks. We ended up replacing everything that looked sketchy (couple of capacitors, bridge rectifier, resistor), and then after a bit of work with the multimeter found the guilty relay, which also had faint heat marks. I have the schematics to the Combivac IT 250, which was somewhat useful, I can send it offlist if you're interested.
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Gray Bebout
Sent: Thursday, 2 February 2017 5:17 a.m.
To: [log in to unmask]
Subject: [ISOGEOCHEM] MAT 252 gauge controller
Our 23.5-year-old Finnigan MAT 252 is (again) ailing, this time due to an issue with a gauge controller, specifically an old Leybold-Heraeus model (Combivac IT 251). Looking at old ISOGEOCHEM messages, we see that similar issues have arisen elsewhere.
Does anyone have (or know of) a decommissioned 252 from which we might extract this controller, or have an extra of this controller laying around the lab, or boxed up somewhere gathering dust (I know, a long shot....).
We also are considering doing some work ourselves on the faulty controller, but we lack a schematic to work with. Does anyone have the schematic for the 1993-vintage IT 251?
Thanks, and best regards,
Gray E. Bebout, Professor
Department of Earth and Environmental Sciences Lehigh University
1 West Packer Avenue
Bethlehem, Pennsylvania 18015-3001
Office phone: 610-758-5831
Laboratory phone: 610-758-6168
Research webpage: http://www.lehigh.edu/~geb0/geb0.html