Dynatuner FM-1 Istruzioni per il montaggio Operazione di allineamento - Pagina 4

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Tlte I.F. Amplifiers and Limiters
Phase shift in an FM signal corresponds to amplitude
non-linearity or distortion in an AM signal or in an audio
amplifier. Accordingly, the I.F. amplifier circuits were
designed for minimum phase shift across the pass band.
Since this occurs with undercoupled transformers, the
alignment of the I.F. section is greatly simplified. Simple
peak tuning (tuning for maximum signal) is the optimum
adjustment for the undercoupled I.F. transformers. This
minimum phase shift approach maintains low distortion
of the audio signal all the way down into the noise and
permits useful reception of weak signals even without full
limjting action.
'
A 6BA6 variable-mu pentode is used for the first and
second I.F. stages, and 6AU6 sharp-cutoff pentodes are
used for the succeeding two stages. Each I.F. stage acts as
a limiter when the signal input to that stage reaches a
pre-determined point, Thus no automatic volume control
circuit is needed and no additional recovery time constants
exist to introduce delay on fading signals.
There is suffi-
cient gain in the receiver so that the last limiter is effective
on input noise.
Tlte Oiscriminator
A balanced-bridge discriminator configuration is used
in place of the conventional unbalanced circuit.' This circuit
balances out any noise ane signal rectification occurring
in the plate circuit of the last limiter tube and is the key
to the phenomenally low distortion figures achieved by the
Dynatuner. A wide band discriminator transformer is used
to obtain the full benefit of this design, and matched semi-
conductor diodes are used in place of vacuum tubes to
avoid heater hum at this critical point in the circuit.
The balancing action of this circuit reduces the inter-
channel noise (between stations) by a factor as great as
10 db in level over many conventional tuners which use
limiter-discriminator circuits.
Tlte Audio Section
The first section of the 12AX7 dual triode is used as a
direct-coupled cathode follower between the discriminator
and the de-emphasis network. This prevents loading of the
high impedance discriminator circuit by the de-emphasis
network with its consequent reduction of high frequency
transient (square wave) response.
A low impedance
de-emphasis network is used to feed the volume control
of the tuner directly. Also a multiplex take-off point is
provided at low impedance which will not affect the regular
operation of the tuner.
The second section of the 12AX7 is a feed hack "plate
follower" audio amplifier compensated for the very wide-
band response inherent in the rest of the tuner circuitry.
Tlte Tuning Eye
In operation this eye acts as an indicator showing when
a station is tuned properly, and is connected through the
appropriate network to the last limiter grid circuit. During
a portion of the alignment procedure, the eye is connected
to other specified parts of the circuit to indicate correct
alignment. This is the only instrument required during the
alignment procedure. The operating point of the eye has
been set for extremely precise indication of the optimum
tuning (or alignment) condition, and is a more accurate
indicator than tuning meters.
The action of the eye in tuning is very sensitive. It wil
indicate a signal as low as 1 microvolt; it approaches
maximum closure at 10 microvolts-yet it cannot be over-
lapped at higher signal strengths. The center of channel is
always indicated precisely. There is no "flat spot" in
tuning.
Tlte Power Supply
The power supply is a conventional full wave "pi"
rectifier using a 6V4/EZ80 rectifier.
Additional power
handling capability is included to power a Dynaco multiplex
adapter in the space provided on the chassis.
The Dynatuner is engineered to
provide the finest
available overall performance. It is also designed to main-
tain this performance level for many years. Unlike other
tuners which must be factory aligned to give rated perform.
ance, the Dynatuner can be adjusted to, and kept at
optimum performance by its owner.
USING YOUR DYNATUNER
Your Dynatuner has been designed to give you the
highest quality of FM reception combined with permanence
of its excellent operating characteristics. A few moments
learning how to use it will amply repay you with increased
satisfaction.
The tuner must be connected to either a preamplifier or
directly to an amplifier using shielded cable of the type
supplied. It is acceptable to use cable lengths up to 25 feet
if the tuner is required to be located away from the associ-
ated equipment.
The audio cable can be plugged into either of the two
output sockets as nonnally wired. The second socket can
be used to connect to the other channel of a stereophonic
system or to a tape recorder, or it can be left unused.
Later, if you wish to add a Dynaco multiplex adapter, the
extra socket will be the output for the second stereo channel
by connecting it internally to the adapter, for which space
has been left on the chassis. The extra socket can also be
connected to the multiplex output of the tuner for use with
external adapters.
See
the Appendix for specific informa-
tion on this.
The front panel controls of the Dynatuner have been
made as simple to use as possible. The function of the
on-off switch is obvious. This switch also controls the AC
outlet on the rear of the tuner so that it is possible, if
desired, to turn both tuner and amplifier, for example, on
and off simultaneously. It is also possible when using the
Dynatuner with equipment which has additional switching
facility, such as the Dyna PAS-2,
to
leave the on-off switch
"on" and use the auxiliary equipment for all the switching.
The tuning knob of the Dynatuner enables tuning frome\
one end of the FM band to the other. By observing the
tuning eye, it is simple to see precisely when a station is in