732B/734A
Instruction Manual
4-8
The first circuit (U3A, R2, R3, VR2, and Z1-R10 on the reference hybrid) supplies
current to the 10V high side of the 10V beta string resistors. The second circuit (U2B,
R8, R9, and Z1-R8 Z1-R9) sinks current from the 10V low side of the 10V beta string
resistors and Reference Amplifier collector current. The third circuit (U2A, Z1-R5
through Z1-R7, R5, and R6 on the reference hybrid) sinks the current from the anode of
the Reference Amplifier zener. Thin film resistors Z1-R8 through Z1-R10) are laser
trimmed to values that are a function of the Reference Amplifier voltage and collector
current.
Buffering of the 10V Output 4-10.
The 10V high output stage contains the series-pass components that supply load current
to the binding posts and bias current to the Reference Amplifier, and other critical
circuitry inside the ovenized assembly. The 10V high output stage also contains an
output current-limit circuit that prevents loads from drawing more than 12 mA.
Components for the 10V high output stage are located on the Flex assembly and include
Q201 and Q202, C201 through C204, R201, and CR201.
Oven Temperature Controller A4 4-11.
The oven temperature controller is a voltage-to-temperature closed loop amplifier. The
set point is established by a voltage divider on Z401. The set point voltage is
approximately 6.43V. This corresponds to a temperature of 45°C inside the oven.
U401B serves as a summing junction and amplifier. The setpoint divider, the feedback
thermistor on HR3, and a gain resistor on Z401 form a bridge that is insensitive to
changes in Vcc when the bridge is balanced or close to being balanced.
The feedback element is a precision, stable NTC thermistor. The stability of this element
is critical to the stability of the oven temperature.
U401A acts as a voltage amplifier with a gain of 240. The output of this amplifier feeds
a common-emitter stage where the emitter resistors are the heaters. Q401 is the control
transistor for the heaters and its power dissipation also adds to the power that the heaters
produce.
To provide good thermal coupling, the feedback thermistor is mounted on the back side
of one of the heater substrates, HR3.
RT401 and R412 serve as a temperature sensitive voltage divider that provides a voltage
signal, TEMP-SENSE, that feeds the IN CAL LED logic.
A precision thermistor mounted close to the Reference Amplifier on the Reference
Amplifier substrate enables the user to monitor oven temperature through the
MONITOR/EXT BAT IN connector on the rear panel. (Figure 3-7 shows the connector
pinout.) At an oven temperature of 45°C, the thermistor resistance is approximately 41.3
kΩ.
A1 Front Panel Assembly 4-12.
The Front Panel assembly contains the following two functional blocks:
1. LED Circuitry
2. Overvoltage Protection Components
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