Operation & Service Manual
Section III
Model 1805B RF Control Unit
Theory & Operation
LISTEN INDICATOR.
The LISTEN indicator (DS6) is a red LED located in the bottom center of the front panel. This
indicator is only illuminated when the Model 1805B is addressed as a listener via the IEEE-
488 bus.
REMOTE INDICATOR
The REMOTE indicator (DS7) is a red LED located in the bottom center of the front panel.
This indicator is only illuminated when the Model 1805B is under IEEE-488 bus control.
STANDBY INDICATOR
The STANDBY indicator (DS5) is an amber LED located near the lower left-hand comer of
the front panel. When the POWER switch is toggled to the STANDBY position (middle
position), This Indicator illuminates, indicating that +30V power has been applied to allow
pre-heating of the precision resistor network within the Model 1805B, and that the +20V
temperature controller circuit for the thermistor mount has been activated.
!
WARNING
Ensure that input connector and voltage selector/fuse assembly is set for the proper
voltage before switching the POWER Switch to ON or STANDBY; otherwise, damage
may result to internal circuitry.
POWER SWITCH
The POWER switch (Sl) is a three (3) position toggle switch located near the lower left-hand
corner of the front panel. In the down or OFF position all power is removed from the Model
1805B internal and controlled external circuitry. In the middle or STANDBY position, +30V is
applied for pre-heating the internal precision resistor network, +20V is applied to the
temperature controller circuit for pre-heating the thermistor mount, and the amber STANDBY
indicator is illuminated. In the up or ON position, all required power for operation is applied to
the Model 1805B, and the green ON indicator is illuminated.
ON INDICATOR
The ON indicator (DS4) is a green LED located near the lower left-hand comer of the front
panel. When the POWER switch is toggled to the ON position, this indicator illuminates to
indicate that all Model 1805B internal power supplies have been activated.
DC SUBSTITUTED POWER SWITCH
The DC SUBSTITUTED POWER switch (S3) is an eleven (11) position limited-rotation
rotary switch located in the left-hand center of the front panel. When the Model 1805B is
under manual control this switch controls the level of the dc substituted power being supplied.
The selectable levels are 0.5, 1,2, 3, 4, 5, 6, 7, 8, 9 and 10 milliwatts. Under IEEE-488 bus
control, this switch is inactive.
RF ON INDICATOR
The RF ON indicator (DS2) is a green LED located in the upper left-hand comer of the Model
1805B front panel. When the RF ON/OFF switch is toggled to the left (ON) position, this
indictor illuminates, indicating that the selected dc substituted power level is supplied to the
bridge, and the appropriate RF control signals are routed from the rear panel of the Model
1805B. If the Model 1805B is under remote IEEE-488 bus control, this indicator illuminates
when the RF ON functions have been performed by the software and system controller.
BRIDGE BALANCE METER
The BRIDGE BALANCE meter (Ml) is located at the top center of the front panel, and is
scaled from a low of -10 microwatts to a high of +10 microwatts relative to a mid-meter scale
reference of zero (0). A 0 reference level reading on the meter indicates that the level
controlled by the Model 1805B corresponds exactly to the dc substituted power level selected
on the DC SUBSTITUTED POWER switch. Any deviation from the selected level is read on
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