100% modulation and to continuously lower percent-
ages down to any modulation whatsoever, determined
by the setting of the noise limiter control.
Anoise limiter switch in conjunction with AVC
provides for use of the noise limiter on CW or on
modulated reception when interference is present.
Output Tube —The RCA 6K6GT output tube is
Resistance coupled from the A-F amplifier, an RCA
6SJ7 tube, and operates into an output transformer
which has taps for matching into a2.5 or' 600 ohm
load, or into headphones. The headphone winding is
designed so that amaximum of approximately 10
milliwatts of power may be delivered to 20,000 ohm
phones. Terminals are provided pn the rear apron for
the 2.5 and 600 ohm impedances. The output from
the 600 ohm winding is fed
terminals, neither of which is grounded. This wind¬
ing may be used to feed abalanced 600 ohm line. The
output from the 2.5 ohm tap is fed to the 2.5 ohm
terminals through atwo-position jack mounted on
the panel. The headphone winding also connects to
the jack. With the phone plug inserted into the jack
in the first position, the phones are in parallel with
the 2.5 ohm output and both are on. When the plug
is pushed into the second position, the phones are
connected to the phone winding and the 2.5 ohm
output is cut off from the rear terminals. If no load
is connected to the 2.5 or 600 ohm output terminals,
the phones should always be used in the second posi¬
tion; as under this condition aload resistor is shunted
across the 2.5 ohm tap to maintain impedance match¬
ing of the system.
Power Pack —The power pack mounted on the
receiver chassis consists of apower transformer, rec¬
tifier tube RCA-5Y.5GT, and filter. Atap switch is
provided on the rear apron for changing the power
transformer voltage tap. (See Figures 2and 6.) The
voltage for which the switch is set may be read di¬
rectly on the switch. The instrument may also be
operated from 6V. “A” and 250 to 300 V. “B” bat¬
teries, or Vibrator Power Supply Unit MI-8319.
Shielding —Interstage shielding is provided to in¬
sure stability under all operating conditions and to
minimize oscillator radiation., Complete external
shielding prevents coupling to any portion of the cir¬
cuit except through the antenna circuit.
Intermediate Frequency Crystal Filter —The first
detector plate circuit is tuned to the intermediate fre¬
quency and abalanced link circuit is used to couple
the first detector plate and first I-F grid circuits. A
455 kc crystal is connected in one arm of the link cir¬
cuit and aneutralizing capacitor is connected in the
other. The impedances of the coils in the link circuit
●'jned so that the crystal selectivity character¬
istic .not impractically sharp. The band width at two
times resonant input may be adjusted to 400 cycles,
1,500 cycles, or 3,000 cycles. For this adjustment see
“Operation.”
Intermediate Frequency Amplifier —Three stages
of I-F amplification are used; RCA-6SG7 tubes are
used in all stages and an RCA-6H6 tube is used for
AVC and second detector. The first I-F Transformer
has its primary and secondary tuned, and is coupled
through the crystal filter link. The second and third
I-F Transformers are composed of four tuned circuits
each. These circuits are varied in coupling by the
selectivity switch. The fourth I-F Transformer has
two tuned circuits.
The third I-F stage is not connected to the AVC
nor to the manual volume control so that agood AVC
characteristic with little overload distortion is ob¬
tained. This also permits the CW oscillator to be
coupled to the grid circuit of this stage, giving acom¬
paratively high detector excitation voltage with small
electrical coupling to the oscillator circuit.
Second Heterodyne Oscillator —The second het¬
erodyne (CW) oscillator is atriode RCA-6J5 tube
which is electrostatically coupled to the final I-F stage.
Apanel control is provided by means of which the
frequency of the heterodyne oscillator and resultant
audio beat note may be varied.
Particular care has been taken in the design of the
circuit constants to minimize oscillator harmonics.
Automatic Volume Control —The AVC voltage is
obtained from the second detector, an RCA-6H6 tube.
Avariable delay is obtained depending on the setting
of the R-F gain control.
The second heterodyne (CW) oscillator excitation
voltage is just lower than the A'VC diode bias voltage
so that it does not decrease the sensitivity of the
receiver.
Manual Volume Control —Two manual volume
controls are provided; an audio gain control which is
employed when the AVC is in use, to obtain the de¬
sired output level, and an R-F gain control.
Noise Limiter —The noise limiter circuit utilizes
an RCA-6H6 tube and limits the noise interference to
are
directly to the 600 ohm
Tuning Meter —The necessary wires for connect¬
ing atuning meter in the cathode circuit of the first
I.F. tube have been included in the cable wiring. If
and when meters become available, it will be asimple
operation to install atuning meter. A5milliampere
meter with zero deflection to the right is required.
V
PERFORMANCE
The performance data undef technical
and the data for the various artificial antenna of 200 mmf. capacity for band 1
and 200 ohms resistance for bands 2to 6inclusive.
The output was measured across aresistance of 2.5
ohms connected in place of the speaker voice coil.
The selectivity switch was placed in position 2.
summary
curves, are approximate
values taken on asample receiver. Variations in these
values are to be expected because of practical manu¬
facturing tolerances. The data were taken with an
7
