Had to repair one of these HP/Agilent HP59864B units. It's very similar to Granville-Phillips 340 series. Usually the three big obvious 470uF filte r caps get baked out over the years, which lets 100KHz ripple get onto the op-amp +12VDC rail, then shorts out a couple of protection zeners.
SCHEMATICS: most of the unit is just variations on suggested circuits app notes in the spec sheets for various chips. Also, you can run the unit wit h a "fake gauge tube" test jig made from some resistors.
For lots of info see:
My measurements:
Gauge tube filament: 2amps, 1.2V ( about 0.6 ohms)
Gauge tube emission current: 10uA across 135V
Gauge tube collector electrode: +7.0pA to +7.0uA at 0.0 volts
AD input volts, pressure, collector current: -?5V? = 9.9 x 10-2 Torr = 7.0uA -2.79V = 1 x 10-2 Torr = 0.70uA -1.39V = 1 x 10-3 Torr = 70nA +0.0V = 1 x 10-4 Torr = 7.0nA +1.4V = 1 x 10-5 Torr = 0.70nA +2.4V = 1 x 10-6 Torr = 70pA +2.78V = 1 x 10-7 Torr = 7.0pA