MKS Baratron Type 122B Pressure Sensor

[Cubdriver]

Hi All -

This is a bit out in left field, but I'm posting it on the off chance there may be someone here that's familiar with these and/or may have a schematic to aid in troubleshooting one.

It's a 100 Torr unit; mid 90s manufacture based on the date codes.  Regardless of the pressure applied, the output is around 11.4V (should be 10V at 100T, dropping to zero as it's pumped down into the mT range).

I've opened it up and see nothing blatantly amiss, and the capacitive part of the sensor seems to be working - as I pump it down, the capacitance (measured directly on the diaphragm connections with an HP 4262A LCR meter; Baratron circuit boards removed) decreases from ~160pF at -15" Hg to a low of ~57pF at mT pressures. (Granted I have no idea what it's SUPPOSED to be, so this may be off for all I know.  The fact that it varies with pressure makes me hopeful that mechanically at least it is ok.)

My efforts at online searches have gotten me a manual, but the extent of its information on the circuitry is a block diagram.  If anyone has or could point me to a schematic of the circuit, or could offer any troubleshooting tips, I would be most appreciative.  When I was at National Semiconductor years and years ago, they pretty much just worked, and if one died we replaced it.  I don't have that kind of budget, and would like to try to repair it if possible.

Thanks for any input.

-Pat