HP 606A Operating And Service Manual page 25

Section III
Paragraphs 3-15 to 3-20
3-15. PERCENT MODULATION METER CIRCUIT.
The modulating signal goes to pin 2 of V9A, a cathode-
follower. This rube feeds shunt diode CR5 which
rectifies the signal. A dc voltage corresponding to
the modulation is fed to the PERCENTAGE MODU-
LATION meter. Clamp diode CR4 prevents the cathode
of V9 from going negative to protect capacitor C56,
whose voltage rating is 25 volts. This point would
otherwise go toward - 200 volts if
\ 19
was removed
or was weak.
3-16. The modulating signal also goes to pin 2 of V8.
The signal level to V8 can be controlled by the AT-
TENUATOR VERNIER. When the signal level is
varied the output modulation is varied, as explained
in the following paragraph. This circuit is also used
to reduce the voltage on the rf amplifier during switch-
ing. As soon as the RANGE switch is rotated out of
the detent position, S7 disconnects the+300 volt power
supply, hence grid voltage of V8 drops to ground
potential. The B+ is not reconnected until after the
turret has made contact in its new position. This
action keeps he amplifier tubes from drawing exces-
sive screen current when the turret is disengaged
from the plate circuit. When the + 300 volt dc supply
is disconnected R64 is substituted as a load to keep
the power supplies in regulation.
3-17. DIFFERENTIAL AMPLIFIER. The external
modulation signal fed into the MODULATION jack
is combined with the dc reference level in the same
manner as the internal modulation. Either of these
signals is applied to a resistive network consisting
of R57, R62 and R68 and is added to the dc reference
voltage. The combined voltages appear at the AT-
TENUATOR VERNIER ( R63). This control varies
the dc, which controls the carrier, and the modulation
signal at the same rate. Thus the percentage of mod-
ulation remains constant regardless of carrier level.
3-18. The dc ( carrier level) and ac ( modulation) sig-
nals are fed through an rf filter to pin 2, grid of the
triode section of V8, as a reference signal to be com-
pared to a signal from the output which is fed to the
pentode grid ( pin 7). This triode plus the pentode
Model 606A
section of the same tube form a differential ampli-
fier. The ac level of both of these signals is propor-
tional to the modulation. The modulating signal is
the reference voltage and the actual modulation of
the output is compared to this reference. The dc
level of the output ( proportional to the rf)is compared
to a reference dc level that is proportional to the de-
sired rf level, as set by the ATTENUATOR VERNIER
control.
3-19. Since the cathodes of V8 are connected together,
the reference signal applied to the triode section will
also appear on the cathode of the pentode section.
This signal will be compared to the signal from the
output which is applied to the grid. Any deviation
from a fixed voltage difference between these two
signals results in an output signal which has a polarity
such as to reduce this difference. Thus the original
conditions are restored. For example, if the rf level
drops, the voltage on pin 7 of V8 will become more
negative. This will reduce the amount of current
flowing through this tube so the plate will become
more positive. The grid voltage of V6 is proportional
to the plate voltage of V8. As this grid voltage be-
comes more positive the current through V6 will in-
crease. But this current is also the cathode current
for the rf amplifier, so the output will increase until
the original conditions are restored. By this action
the output is stabilized and is constant to better than
±1 db over the entire frequency range. The rf level
can be changed by varying the ATTENUATOR VER-
NIER ( R63) which will change the reference bias. In
a similar way the modulation is held constant. Since
the crystal detector circuit has a time constant fast
enough to follow the modulation envelope, the output
modulation is compared to the modulating frequency
and distortion is minimized.
3-20. BIAS SUPPLY. One half of V7 is used as a
constant voltage source to furnish the plate potential
needed for the triode section of thedifferential ampli-
fier. The other half of V7 supplies the screen po-
tential for the pentode section of the differential ampli-
fier. The screen potential is adjusted for rf amplifier
cut-off when both grids of the differential amplifier
are at ground potential. This establishes the zero
point of the ATTENUATOR VERNIER control.
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