Service manual

PN 4237242C
2.5-4
INSTRUMENT DESCRIPTION
DATA ACQUISITION
dark current from the detector. There is interaction between these two adjustments, so they
must be adjusted together.
Inputs
r J1 - Hgb photodetector
r J2 - ±15 V
Outputs
r J2 - Hgb output voltage
r J3 - Hgb lamp output: currently not used
Adjustments
r R7 - Hgb Preamp adjustment (adjusts output voltage)
r R8 - Preamp offset adjustment (adjusts Hgb zero)
Test Points
r TP1 - orange, Hgb Preamp card output
r TP2 - black, Hgb Preamp card ground
Vacuum Sensor Card
The Vacuum Sensor (Vacuum Sense/Vent Valve) card is located in the lower chassis
(Figure 2.5-2) and provides a means of accurately reading count vacuum. It accepts a vacuum
input from 0.0 in. Hg to 7.0 in. Hg and converts it to an electrical signal from 0.0 V to 5.0 V.
The card also is host to two solenoid valves, LV1 and LV2. LV1 connects the vacuum sensor to
the VIC or to atmosphere. LV2 controls the VIC vent to atmosphere, which is required when
the chamber is being drained. A block diagram showing the functioning of the Vacuum
Sensor card is shown in Figure 2.5-2.
The main component of the Vacuum Sensor card is U1, a differential pressure transducer that
outputs a voltage relative to the pressure difference at its input ports, P1 and P2. Port P1 in
the MD II implementation is always open to atmosphere, while P2 is connected to LV1, which
determines the pressure source to be measured. The voltage output of U1 is fed through a
low-pass filter and amplifier section that filters out electrical noise and sets the gain. This
voltage is fed through a summing amplifier along with the offset reduction source voltage to
produce the final VAC OUT voltage level.
The offset reduction source is a negative voltage close to 1.0 V. Since the pressure transducer
outputs about 1.0 V when there is zero pressure differential, the offset reduction lowers this
output to near, but greater than, 0.0 V, producing better resolution with the expected output
range of 0.0 V to 5.0 V.
To measure vacuum, LV1 is first energized, which connects P2 to atmosphere. A reading is
taken that establishes the zero reference voltage. LV1 is then de-energized, connecting P2 to
the VIC, and the high reference voltage reading is taken.