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MODEL
8559A
SERVICE
35 MHz Calibration Oscillator (B)
The 35 MHz Calibration Oscillator consists of a differential amplifier formed by Qll and 412. A frequency
determining tank circuit
(L21, C45, and C46) is connected to the base of Q11. The base of 412 and one side of
the tank are at RF ground due to C48. Capacitor C45 temperature
-
compensates the oscillator; R13 controls the
bias current and output amplitude. As the base voltage of
Q11 increases, the voltage at the emitters of Q11 and
412 increases. Since the base of 412 is effectively at signal ground, the increase in voltage at its emitter reverse
biases its emitter
-
base junction, shutting 412 off. As 412 shuts off, the voltage at its collector increases and is
fed back in phase to the tank at the base of
Q11 through C47. The output is taken from the collector of Q11,
filtered to lower harmonic content, and sent to the CAL OUTPUT connector on the front panel.
Power Supplies
(J)
Three supply voltages power the Third Converter Assembly A10:
+
15
VF,
-
10V, and
-
10.6
VF.
The
+
15
VF
is derived from the
+
15V supply line and is filtered as it enters the board. The
-
10V and the
-
10.6
VF
originate from the
-
12.6V supply line. After filtering, the
-
12.6V supply feeds a shunt regulator comprising
R48, C53, VR1, and VR2 and develops the
-
10V supply. The filtered
-
12.6V supply also feeds three
-
terminal
regulator U2, which develops the
-
10.6 VF supply. Regulator U2 improves isolation of the 300 MHz Oscillator
from the supply lines and reduces spurs caused by oscillator harmonics.
THIRD CONVERTER ASSEMBLY A10, TROUBLESHOOTING
Spring contacts are used on the circuit board to ground portions of the
circuitry to the aluminum extrusion walls. Care is required when removing
the circuit board to prevent damaging these springs. The circuit board must
be installed in the extrusion before attempting to adjust the 321.4 MHz
Bandpass Filter (block C).
Low Gain:
Most common failures are: CR1, CR4, the PIN diodes in the IF preamplifier, and the Flatness
Compensation Amplifier.
300 MHz Oscillator off F~equency:
Most common failures are C16, C17, and C18.
35 MHz Oscillator off Frequency:
Most common failures are C45 and L21.