Chassis and underside pictures
The National NC-173 is the best all around receiver in the moderate price field. It is engineered for the host of application for which one stage of high gain r.f. amplification and 3.5 watts of audio output power are adequate. The Amateur will find this receiver chock-full of features to widen his scope of activity. The NC-173 has proved itself thoroughly dependable in a great many applications. Short wave listeners can now enjoy true world wide reception; all users can be proud of owning a mighty attractive piece of equipment, an excellent example of modern product design. The frequency range of the NC-173 is exceptional in that it includes the conventional 540 kc. to 31 Mc. range plus the 48 to 56 Mc. portion of the spectrum which covers the Amateur six meter band. The tuning system employs separate directly-calibrated dial scales with associated control knobs for General Coverage and Bandspread tuning. Both dials are well-illuminated and have auxiliary linear scales for logging purposes. Calibrated bandspread tuning is provided for the main Amateur bands, i.e., 6, 10-11, 20, 40 and 80 meters. Band changing is accomplished by means of a highly efficient band-switch system. Essentially the circuit consists of one stage or radio frequency amplification, a first detector and a separate stabilized high frequency oscillator, two intermediate frequency amplifier stages, a diode type second detector, an audio limiter, a high gain type audio stage and an audio output stage plus a separate AVC amplifier, a stabilized beat frequency oscillator plus voltage regulator and rectifier stages. A crystal filter is connected between the first detector and first i.f. stage.
|1st i.f. Amplifier||6SG7|
|2nd i.f. Amplifier||6SG7|
A new highly flexible crystal filter provides and adjustable selectivity characteristic with a wide range form broad-band broadcast requirements to sharp Amateur single-signal CW reception.
A new concept in noise limiter design is introduced in the NC-173 Receiver. This new limiter could be termed "double action plus" and the noise limiting action is equally effective on either phone or CW reception. A panel-mounted threshold control permits adjustment of the level at which limiting action starts.
A voltage regulator tube effiiciently minimized frequency drift in the high frequency oscillator and also in the beat frequency oscillator. This voltage control, plus temperature compensation, assures frequency stability for both phone and CW reception.
NARROW BAND FM ADAPTER
The NFM-73 Adaptor makes the NC-173 an excellent receiver for narrow band FM. A 6H6 tube is employed as a noise suppressing ratio-type discriminator and a 6SK7 i.f. coupling stage eliminates undesirable loading of receiver circuits. Instant selection of AM or NFM by phono-radio switch.
Main Tuning; Bandspread Tuning; Bandswitch; RF Gain - AC ON/OFF; AF Gain; Send-Receive; AVC-MVC; Tone; CWO; CWO Switch; Limiter; Phasing; Selectivity; RF Trimmer.
Additional refinements include and S-Meter with adjustable sensitivity, a continuously variable tone control and a phono input jack for connection to external apparatus such as a turntable pickup and the NFM-73 narrow band FM Adaptor. The AVC system, crystal filter network, noise limiter and tuning system are identical to those used so satisfactorily in the NC-183. The panel of the NC-173 contains an illuminated S-meter calibrated in S units from 1 to 9 at approximately 5 db. per unit and above S9 form 0 to 40 db. There is also a pick-up jack on the panel which feeds into the high gain 6SJ7 1st audio stage: audio gain and tone controls are operative with this connection through the audio system which is essentially flat from 75 to 6000 c.p.s. Power output is about 3.5 watts with terminals for 8 and 500 ohm impedance loads on the rear apron. Inverse feedback is used to reduce audio hum to an exceptionally low level. The antenna input circuit of the NC-173 is arranged for single wire, balance feed or low impedance concentric line; average input impedance being roughly 500 ohms. The panel TRIMMER control readily compensates a wide range of antenna loading. The panel also contains a full complement of operating controls.
The selectivity switch of the wide range crystal filter permits a choice of six progressively narrower i.f., pass-bands. Maximum and minimum selectivity characteristics are as follows:
|6 db. down||20 db. down|
|Selectivity Switch "OFF"||3.9 kc.||8. kc.|
|Selectivity Switch "5"||80 cycles||400 cycles|
2.0 microvolts or better for a 6 db. signal/noise ratio throughout the frequency range.
Signal/image 25 db. or better at 30 megacycles.
Approx. 80 watts at 115v., 50/60 cycles, 1 phase a.c. (easily adaptable to 220/240 volt service as well as emergency operation from batteries).
Table Model, 19 3/4" x 10 1/8" x 12 1/2" , 46 lbs., Gray Enamel Finish.
Rack Model, 10 1/2" panel height, 46 lbs., depth behind panel 14 3/4" overall, Black Wrinkle Finish.
|NC-173T Table Model (with speaker)||Net $189.50|
|NC-173R Rack Model (with speaker)||Net $189.50|
|NFM-73 Narrow Band FM Adapter||Net $17.95|
|TB-3 Tilt Base||Net $3.95|
|NC-173TS (Table) or NC-173RS (Rack) speaker||Net $10.00|
Source: National Radio Products 1948 Catalog; Photos from KK5FE
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