I
1
I 0 + 180V
c4
I/ I
I
0 +140V
c1
I
Figure 3-2. Simplified Schematic Diagram of High Frequency Oscillator Section
1
feedback signal in phase with the grid signal on V1 and the high frequency oscillator from the amplitude con-
oscillation occurs. Locating the phase shift networks trol circuit and the AVC Amplifier stage V5.
between the oscillator tubes rather than in cascade
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permits the tube capacitance to be employed as part A type 6AK5 pentode tube was used for V4 in many
of the phase shift network. instruments. A type 5654 was used later and is re-
commended for replacement of V4 in all instruments.
On the 100 KC to 1 MC range an additional series re-
sistance, not shown in the simplified diagram, is I
In early instruments, plate and screen of V4 were tied
switched into the coupling circuits between the tubes
together and the tube functioned as a triode. In later
which causes the phase shift to take place at one-tenth
instruments, the circuitry was changed with the tube
the frequency of the highest range.
connected as a pentode. Modernization notes for mak-
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These additional resistors, as well as the plate load ing the necessary triode to pentode circuit modifica-
resistors, are precision composition type which are tions will be found ir, paragraph 3-9 of this manual.
factory matched and selected. Occasionally, small
low value resistors are placed in the circuit to com- 3-4 AVC AMPLIFIER
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pensate for slight tolerances of the precision resistors.
The AVC Amplifier shown in the simplified schematic
Calibration adjustments are provided for the high fre- of Figure 3-3 is driven by buffer stage V4.
quency end of both ranges of the high frequency oscil-
1
lator. The trimmer capacitors on the three sections The output signal from V5 is rectified by germanium
of tuning capacitor C1 are adjusted at 10 MC. Ceramic diode CR3 and is then used as grid bias for V1, V2,
capacitor C3 in the grid circuit of V1 is adjusted at and V3. This maintains a constant output from the
1 MC with the Frequency Range Switch in the "X100KC" oscillator as well as the buffer stage V4 which is in-
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position. side the AVC control loop.
3-3 BUFFER AMPLIFIER The high frequency oscillator output level and fre-
quency response can be controlled by adjustment of I
The signal from the high frequency oscillator tubes components in the AVC amplifier stage. For example,
is fed into the buffer amplifier stage V4 which isolates if the output of the high frequency oscillator increases
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R22
t
I
c IC
I
I
I TO
AMPLITUDE
CONTROL i
- - -
I
Figure 3-3. Simplified Diagram of High Frequency Oscillator AVC Control System
-8-
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ACEC
