Dyna FM-3 - Detailed Circuit Descriptions; Circuit Sections Explained

2. An oscillator-mixer stage using a 6AT8A high con-
version
triode-pentode with screen injection.
3.
Four I.F. amplifier stages with progressive limiting
using two 6BA6 pentodes and two 6AU6 pentodes.
4. A symmetrical wide band discriminator with matched
semi-conductor diodes.
5. A
cathode follower providing discriminator isolation
and low impedance source to the multiplex and audio
channels using one-half of a 12AX7jECC-83.
6. A feedback audio amplifier using one-half of a
12AX7jECC-83.
7. A 19KC amplifier and doubler using a pentode-triode
6BLBjECF-80.
8. A balanced bridge synchronous detector using four
matched semi-conductor diodes.
9. Stereo audio output stages consisting of feedback plate
followers using a 12AX7 jECC-83.
10. A dual indicator EMM-801 acts as a tuning eye and
as a stereo indicator.
11.
The power supply, which uses a 6V4jEZ-80 full wave
rectifier.
The Antenna Circuit
Provision is made to match either a 75 ohm unbalanced
or a 300 ohm balanced transmission line, thus accommo-
dating any standard antenna array used for FM reception.
The R.f. Stage
The cathode coupled dual triode R.F. amplifier circuit
was chosen for the input stage. It combines the high gain
and low noise figures of the commonly used grounded grid
input with a tuneable input-a necessary feature to pre-
vent overloading from strong signals which might cause
cross-modulation and spurious responses.
The Oscillator-Mixer
The triode section of the 6AT8A is used in a "tickler
feedback" tuned grid oscillator circuit. Careful temperature
stabilization and choice of operating parameters result in
an oscillator circuit with sufficient stability to obviate any
need for automatic frequency control with its inherent
degradation of the audio signal.
The pentode section of the 6AT8A is used as the mixer.
The oscillator is injected into the screen circuit to provide
complete isolation of the oscillator tuning circuit from the
signal tuning circuit at the mixer grid. Oscillator drag
(change in oscillator frequency as the mixer tuning is ad-
justed) is virtually nonexistent, grea't1y simplifying the
adjustment and tracking of the fiont end. This isolation
also reduces re-radiation of the oscillator energy into the
antenna which might cause interference in other nearby
receivers and television sets.
Circuit constants of the oscillator and mixer circuits
have been adjusted to give uniform sensitivity over the
entire FM band.
The 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 de-
signed 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 per-
mits useful reception of weak signals even without full
limiting action.
A 6BA6 variable-mj.L 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 cir-
cuit 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.
The Discriminator
A symmetrical discriminator configuration is used in
place of the conventional unbalanced circuit. This circuit
balances out any noise and 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 and
matched semi-conductor diodes are used to obtain the full
benefit of
this design.
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.
The discriminator signal goes first through one-half of a
12AX7 functioning as a cathode follower and then through
the second half of this stage which is used as an audio
amplifier with plate to grid feedback. The output of the
cathode follower also is used as a source to a portion of
the multiplex circuitry.
The Multiplex Integrator
The first portion of the multiplex section is fed from the
cathode follower. This signal is Q-multiplied and amplified
at 19KC in the pentode half ot the 6BLB. Then it is
further amplified and doubled -to 38KC in the triode sec-
tion of this tube. This triode section also acts as a limiter,
holding a
constant 38KC level regardless of signal strength
and pilot amplitude. Such limiting greatly reduces noise
through the stereo "gates" and preserves equally good
separation at all level signals.
The audio signal from the plate circuit of the 12AX7 is
shaped in a 67KC band reject filter. Then it is mixed with
the reconstructed 38KC signal in a balanced transformer.
A
four-diode bridge provides synchronous detection of left
and right channels which then are passed through a com-
bined
38KC rejection filter and audio de-emphasis network.
A signal from the
19KC amplifier section is used to acti-
vate the upper beam of the EMM-801 tuning eye which
illuminates the word STEREO when there is a stereo trans-
mission.
The action of the multiplex section is automatic as the
38KC channel is quenched in the absence of a pilot signal
from the broadcast station. However, it can also be shifted
to locked mono reception by use of the push-pull Stereo-
rnatic-mono switch which can deactivate the automatic
stereo feature.
The Audio Section
A 12AX7 is used for audio output, with one half for each
stereo channel. This tube has plate-to-grid feedback and is
used as a plate follower. It has low impedance output, per-
mitting up to 25 feet of shielded cable. It has very low
distortion and minimum frequency discrimination.
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