Your National Model NC-303 is an amateur-band receiver incorporating all the design features necessary to provide optimum performance consistent with present day operating requirements. Elimination of general frequency coverage and all circuits not pertinent to amateur operation has enabled National to produce a compact, efficient receiver with the built-in rugged dependability characteristic of National Company prod- ucts. Contributing to the utility of the receiver are a Q multiplier rejection notch, an S-meter, variable IF selectivity plus selectable SSB with a fast-attack-slow-release AGC circuit, a muting circuit, a heterodyne detector, and separate noise limiters for AM and for SSB and CW reception. These operational features as well as basic design improvements result in a receiver with exceptional oscillator stability, high sensitivity, and extreme versatility.
Your NC-303 features a double-conversion super- heterodyne circuit employing 15 tubes including the rec- tifier, voltage regulator and current stabilizer. Through careful selection of intermediate frequencies, excellent selectivity and image rejection have been achieved. Fre- quency coverage includes the 160, 80, 40, 20, 15, 11 and 10 meter bands plus the "X" band. The "X" band functions as a tunable IF in the 30- to 35-megacycle range for use with broad-band crystal converters avail- able from the National Company. Individual scales provide direct reading calibration for 6, 2 and 1 1/4 meters. Each amateur band has an individual calibrated dial scale. Tuning is facilitated by the fact that only the calibrated dial scale that is in use is visible. Band- scanning is accomplished with ease as a result of the use of a heavily-weighted tuning knob coupled to a free-moving gear train. The tuning knob has a rim- drive vernier to aid in fine tuning. Careful attention to mechanical design has resulted in tuning action without any trace of backlash.
The outward appearance of the receiver is com- pletely modern and functional. Cabinet and panel con- struction permits the receiver to be removed from the cabinet and rack mounted without the addition of any parts other than mounting hardware. Adequate ventila- tion is provided by the use of convection cooling through holes in the chassis and perforated cabinet lid.
|First IF Amplifier||6BJ6|
|Second IF Amplifier||6BJ6|
|AM Detector, Noise Limiter, AGC||6AL5|
|SSB-CW Noise Limiter||6AL5|
|S-meter Amplifier and AF Amplifier||12AT7|
|Audio Output Amplifier||6AQ5|
|High Voltage Rectifier||5Y3GT|
|AGC Clamp Diode||1N1692|
FREQUENCY COVERAGE: Your NC-303 tunes all the amateur bands from 160 through 1O meters plus a 30- to 35-mc band designated as the "X" band for use with external converters for the 6-, 2- and 1 1/4-meter bands. Each dial scale is cali- brated for only one band. Each scale has band-limit markings plus underscoring to indicate the phone por- tion of the band. The dial drum rotates with the band switch so that only the dial scale associated with the band being tuned is visible. The following table gives the frequency of each band tuned by your NC-303.
|160||1.8 - 2.0 mc|
|80||3.5 - 4.0 mc|
|40||7.0 - 7.3 mc|
|20||14.0 - 14.4 mc|
|15||21.0 - 21.5 mc|
|11||26.5 - 27.5 mc|
|10||28.0 - 29.7 mc|
|6||49.5 - 54.5 mc (with converter)|
|2||143.5 - 148.5 mc (with converter)|
|1 1/4||220.0 - 225.0 mc (with converter)|
|WWV||10.0 mc (with calibrator)|
IF SELECTIVITY: The IF passband may be set at S(400 cps), SB1(2 kc), SB2(2 kc), M(3.5 kc), or B(8 kc), at the operator's discretion. The S, M and B positions are centered on 80 kc, with the SB1 and SB2 positions centered at 81.5 and 78.5 kc respectively. With the ever increasing amount of activity on the amateur bands, variable selectivity is a must on any modern communications receiver. For copying CW signals in the midst of heavy QRM, the 400-cycle bandwidth is invaluable. The SB1 and SB2 positions are special features of your NC-303 which allow instantaneous one-knob selection of the correct sideband for SSB reception. These positions provide optimum bandwidths of 2 kc centered on the received sideband. Either sideband or a DSB signal may be selected for optimum reception. The M position is provided to improve the readability of phone operation when interference is heavy, and the B position makes it possible to copy net stations or round table discussions with a minimum of retuning. Attention to the 8-kc bandwidth is also important when the converters in the VHF ranges are being used.
Q MULTIPLIER: An adjustable rejection notch is invaluable in re- ducing interference caused by heterodynes in the recep- tion of phone signals. The Q multiplier in your NC-303 provides a very sharp rejection notch that may be posi- tioned anywhere in the IF passband to reject interfering carriers. Use of the rejection notch does not affect the intelligibility of the desired signal.
NOISE LIMITERS: Noise suppression in your NC-303 is accomplished by two separate noise limiters, one for AM reception and the other for CW and SSB reception. The dipping level of the AM noise limiter is set at an optimum value of 40% to 50% modulation and automatically maintains the best signal-to-noise ratio regardless of signal strength. One of the outstanding features of your NC-303 is a double-ended noise limiter separate from the AM noise limiter with a manually adjustable dipping level for CW and SSB, This limiter dips both positive and negative noise peaks.
MUTING CIRCUIT: A built-in muting circuit silences your NC-303, when desired, without resorting to the use of relays. Any point in the transmitter, which supplies approximately minus 22 volts with the transmitter on, may be connected to the muting terminals at the rear of the receiver. When the transmitter is energized, the audio amplifier in the receiver is cut off to prevent feedback or key clicks.
SINGLE-SIDEBAND DETECTION: Incorporated in your NC-303 are unique features which provide stable, distortion-free SSB reception with- out noise bursts and/or ignition interference. The IF selectivities of SELECT control positions SB1 and SB2 provide optimum bandwidths for SSB reception. The operator may select either sideband with one control and may switch sideband without retuning the receiver or CWO control. Frequency stability is assured in your NC-303 by using a first conversion oscillator with regu- lated plate voltage, heater current stabilization and a carefully temperature compensated high-C circuit. The use of a crystal controlled second conversion oscillator eliminates any possibility of frequency drift from this source. The heterodyne detector oscillator (CWO) utilizes another high C tuned circuit to provide stable, distortion-free audio from the incoming SSB signal. A special fast-attack--slow-release AGC circuit pro- vides essentially constant-level output at all times. A separate noise limiter has been included to improve SSB and CW reception. Your NC-303 has provision for a slow tuning rate by means of a push-to-engage, pull-to-release vernier rim drive for the main tuning knob. This vernier reduces the tuning rate to one-seventh of that provided by the main knob. The SSB signal may be rapidly tuned in by the main knob, then the vernier may be engaged to easily tune for maximum intelligibility. The vernier may be disengaged at any time without changing the receiver tuning by pulling the vernier knob outward.
AUDIO RESPONSE: A tone switch has been provided to allow selection of the best audio response for the existing signal con- ditions. Provision is made to attenuate the low and high frequency portions of the audio spectrum, either individually or together.
VOLTAGE AND CURRENT REGULATION: The filament current of the first conversion oscillator in your NC-303 is regulated through current stabilizer tube V-13(4H4-C). The result of this regulation is less than a 50-cycle shift in oscillator frequency on the 10- meter band for a 10% change in line voltage. Plates and screens of the 1st and 2nd conversion oscillators are voltage regulated by the voltage regulator tube V-11(OB2).
CONVERTERS AND CABINET: The National NC-300C1, NC-300C2 and NC-300C6A broad-band, crystal-controlled converters tune the 1 1/4-, 2-, and 6-meter bands respectively. The slide rule dial of your NC-303 incorporates three separate dial scales to provide direct-reading frequency calibration when the converters are used with the receiver. All operating voltages required by the converters are available at the accessory socket of your NC-303. The National NC-303 converter cabinet is designed to provide a convenient and attractive means of mount- ing the converters. From one to three converters may be mounted in the cabinet which matches the receiver cabinet and is identical in size to the NC-303 speaker cabinet. A four-position switch is mounted on the front panel to permit operation of the receiver or any one of the three converters without changing connections to the receiver or antenna.
S-METER: The S-meter gives the operator a true indication of relative strength of incoming signals. The meter is calibrated in S units from 0 to 9 and in decibels from S-9 to 40 db over S-9. A potentiometer is provided on the chassis to zero-set the meter.
POWER REQUIREMENTS: Your NC-303 requires a 105-130 volt, 50/60 cycle AC source of supply. Power consumption is approx imately 70 watts.
CALIBRATOR SOCKET: A socket is provided on the chassis to permit insertion of the National XCU-303 or XCU-300 crystal calibrator unit. When the OFF-CAL-WWV switch on your NC 303 is in the CAL position 100 kc markers are generated for checking dial calibration. When it is in the WWV position, the calibrator can be checked against the 10 mc WWV signal. Calibration of the crystal calibrator frequency against WWV is available only with the XCU-303 calibrator.
ACCESSORY SOCKET: The accessory socket provided at the left rear corner of the chassis is designed to accept suitable external accessories within the power limitations of 1.5 amperes at 6.3 volts AC and 25 milliamperes at 150 volts DC.
ONLY $44.99* DOWN UP TO 24 MONTHS TO PAY.
Suggested cash price $449.00.
*Most National Distributors Offer Budget Terms and Trade-In Allowances.
Sources: National NC-303 Instruction Manual and 1958 QSTs.
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