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Model 5065A
Circuit Diagrams, Theory, and Maintenance
TEMPERATURE CONTROLLER A11 THEORY
The A11 Assembly incorporates two temperature con
troller circuits that work with the sensing and heating
elements in A12 RVFR Assembly for temperature con
trol of the Rb
87
cell, and the Rb
87
lamp.
The two regulator circuits are nearly identical so dis
cussion will be confined to the upper regulator circuit
on A11 schematic (which feeds the cell heater).
Q1 and Q2 comprise a Wein bridge oscillator with
emitter follower and detector providing a dc signal volt
age representing the required correction for the dc
controller section. Oscillator frequency (about 3 kHz) is
determined by a phase-shifting network connecting to
Q1 base through C8, R1, R3, C1, and C2. Amplitude is
determined by the required heater power necessary to
maintain proper operating temperature of the cell oven.
Information on cell oven temperature is conveyed to the
resistive leg of the Wein bridge by A12TR1 in the cell
oven. Temperature setting of this heater control circuit
is done by selecting A11R5. The cell oven thermistor
has a negative temperature coefficient, so a decrease in
oven temperature causes thermistor resistance to in
crease, thereby increasing oscillator amplitude. Diodes
CR1 and CR3, and C21 translate oscillator output to a
negative dc level applied to Q5.
The negative signal at Q5 base reduces Q5 collector
current; which in turn increases Q7 collector current
and the heater current output of chassis-mounted Q3.
In this manner, heater current is increased to com
pensate for a temperature drop. When equilibrium is
established (oven at required temperature), the Wein
bridge is balanced with oscillator amplitude at a level
corresponding to the required heater current.
Diode
CR7 provides temperature stabilization for Q5.
Diode
CR5 protects Q1 base-to-emitter junction from reverse
voltage.
A11 MAINTENANCE
NORMAL OPERATION
The A11 Assembly contains two oven controller circuits.
These circuits control the temperature of the spectral
_
lamp and the Rubidium Absorption cell inside the RVFR
Assembly A12.
Proper temperatures are maintained by controlling dc —
current in the oven windings.
The output of the A11 Assembly are at pins 2 and 14.
Nominal output voltage is approximately 18 V at these
—
terminals.
OPERATIONAL CHECK
_
Proper operation of this assembly can be easily checked
by (1) placing your hand on the RVFR Assembly and
noting that it is warm to the touch and, (2) checking _
front panel meter indications in the CELL OVEN and
LAMP OVEN positions. The meter should read between
15 and 35. (NOTE: These readings will change with
ambient temperature.)
—
TROUBLESHOOTING AND REPAIR
Disconnect ac and dc power before removing A11 —
Assembly.
ASSEMBLY REPLACEMENT
_ ~
a.
Disconnect ac and dc power (Battery fuse or
Option 002).
b.
Remove A11 Assembly.
Remove A11R5 and
install it on the new circuit board.
c.
Remove A11R6 from the old board and install —
it on the new one.
d.
Install new board and restore power.
8-54
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