8-276. Q6 is necessary primarily for the condition when the oscillator has been stored at -55OC. Since U3 (at
-
55°C) cannot supply enough base current for Q8, Q6 provides the added current gain required.
8-277. PRECISION VOLTAGE REFERENCE. U2 is a
10.OV voltage reference. It provides a stable voltage
source for the bridge and
U1.
A
change in the bridge reference voltage changes the voltage across the thermistor
and hence, the power it dissipates.
8-278. OVEN CONTROLLER TURN-ON CURRENT LIMITING. The turn-on current limiting circuit con-
sists of U18 and associated components. From an initial turn-on condition, the thermistor senses the oven
temperature to be LOW. To correct this situation, U3 attempts to drive heavy amounts of current through the
Q7 and Q8 heaters. If allowed to continue this way, excessive current
will
flow. When Vcc is applied to the oven,
U1B forces the voltage across
R38
and R39 to equal the voltage at UlB(2) by sinking the base current from Q6.
By sensing Vcc, the circuit transforms the heater transistors into what appears to be a
fured heater resistance of
47
Ohms typical.
8-279. HEATER TRANSISTOR BALANCE. Because heater transistors Q7 and
Q8 are not equally spaced
from the crystal, it is necessary to offset the power dissipation between the two transistors. Amplifier
U1A
references a voltage-divider across Vcc (R25 and R26) and a second divider (R27 and
R28)
referenced to the
midpoint between the heater transistors. This arrangement allows
U1A to control the base current of Q7 to en-
sure the voltage at the midpoint between the heater transistors is a constant percentage of Vcc
(~0.57
x
Vcc
+
2%).
8-280.
DISASSEMBLY AND REASSEMBLY
8-281. Prior to performing disassembly and reassembly procedures, perform the following:
a. Set POWER ON-STBY switch to
STBY position.
b. Remove the power cable from
5334B's LINE socket.
WARNING
WHEN THE COVER IS REMOVED FROM THE HP
53348,
LINE VOL-
TAGES ARE EXPOSED WHICH ARE DANGEROUS AND MAY CAUSE
SERIOUS INJURY IF TOUCHED. DO NOT REMOVE THE COVERS UN-
LESS
IT
IS NECESSARY.
NOTE
Refer to the exploded view in
Figure
6-1
when performing the disassembly
and reassembly procedures.
HP
5334B
-
Service Manual
8-37
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