The National NC-270 is a ham-bands-only receiver covering the 80 through 6 meter bands. Has BFO, product detector, five choices of selectivity (5, 3, 2.5 each for upper and lower side band, and 0.6 KHz) and separate RF gain and volume controls. The NC-270 also a built-in crystal calibrator and notch filter. The receiver uses dual conversion. The first IF is 2215 KHz using crystal control. Second conversion is 230 KHz using a "Patented Ferrite Filter". Changing the coupling impedance between the two halves of the filter provides the selectivity choices.
The NC-270 was introduced in 1960 at a price of $249.95. It was reviewed in QST magazine in January 1961 and in CQ magazine in May 1961. The matching NTS-3 speaker was an optional accessory at $19.99. The NC-270 and its speaker both have the "flip-foot", a flip out stand for the front of the set to angle the desk-mounted receiver and speaker up toward the operator.
The first cosmetic version of the NC-270 did not have the function or selectivity controls labeled on the front panel. The RF Gain and Audio gain were labeled by the initials R.F.G and A.F.G. The perforated cover was also the same lighter gray-blue color as the flip-foot. This version shows the later cosmetics with the darker blue perforated cover and the full labels.
The overall cosmetic condition of this set was very good as purchased and the set was working.
Other than a thorough cleaning and alignment including the usual safety checks and application of deoxit, this set did not need any repairs as such. The capacitors in this set are mostly ceramic so only a few needed replacement. However, after alignment I tried to turn on the crystal calibrator. It was frozen. Apparently the former owner had not used it. I applied a bit of light oil to the bushings, turning the radio on its side so that the oil would only go where needed. After lubrication, I was able to turn the switch on but thought the calibrator was not working. A quick scope check showed it was working fine. Disconnecting the antenna temporarily allowed me to hear the calibrator on the lower bands although on 10 meters it was very faint. It is not audible on the 6 meter band.
While the schematic has an arrow that says "to antenna terminal", the output wire actually goes to an unused terminal on the antenna portion of the bandswitch for just a bit of capacitive pickup. The CQ reviewer experienced the same calibrator results so this is apparently normal for the set.
The 230 KHz second IF needed considerable adjustment. The ferrite filter has a thin long horizontal screw coming from each side. The one side was relatively easy to turn by hand. The other side is more difficult to reach and was already slightly bent allowing me to see if I was making progress with my long nosed pliers. I re-aligned it to a close 230.25 KHz.
The first IF is crystal controlled. No adjustment needed. Alignment of the various ham bands followed. A couple of the upper bands were off by quite a bit. My experience with the NC-155 made the alignment relatively easy since the two procedures are nearly the same.
After alignment, the set was a joy to tune for SSB conversations. I listened to a number of SSB conversations on the 20, 40 and 80 meter bands. However, I noticed that the power transformer was getting uncomfortably hot after about an hour or so.
Yet another hot transformer
Like the NC-155, the manual for the NC-270 specifies a voltage rating of 105 to 125 volts. It also specifies a power draw of 75 watts and 0.68 amp at 115 VAC. Like the NC-155, the actual power draw at 120 volts was 90 watts. Also like the NC-155, the B+ voltage line is specified at 160 volts on the schematic and with my variac at 115 VAC the B+ was about 10% high.
I modified the NC-270, adding a 120 ohm resistor between the 5Y3 rectifier and the first electrolytic. Like the NC-155, that reduced the B+ to the specified 160 volts with 115 VAC input. As expected, it also reduced the power draw a couple of watts. The transformer still gets hot but not uncomfortably so when operated at 115 volts maximum. A variac or a bucking transformer to run these sets at 110 volts or even 105 is a very good idea. Performance is not effected to any noticeable degree.
Despite the heat, I have not heard that the transformers in the NC-155 or NC-270 have a record of failure. It is possible that the transformers have insulation of a higher temperature capability. Nevertheless, I am operating these sets at reduced voltage.
Line to chassis resistor
The NC-270 has the same 470K ohm resistor and a capacitor wired between the unswitched side of the power line and the chassis as does the NC-155. I again polarized the power cord so that the unswitched side, and that resistor-cap combination, would only see the neutral side of the power line.
The NC-270 proved to be a very good receiver for SSB. In comparison with the NC-155, the switch selectable lower or upper sideband is handy. However the NC-155 S-meter can operate on SSB but the S-meter of the NC-270 cannot.
Like the NC-155, the NC-270 was quite stable with minimal drift after a relatively short warm-up time. The big fly-wheel dial was smooth in tuning and a pleasure to operate. The overall sensitivity is good to excellent except for 10 and 6 meters. In fact 6 meter AM sensitivity is not even on a par with a good Heathkit Sixer "Lunchbox". This may be a matter of my expectations since the CQ review said the 6 meter sensitivity was excellent.
Performance on 80 and 40 is where the receiver really shines. There is so much sensitivity that the RF gain cannot be maxed without major distortion. Since the set has no AGC on SSB, best results are had by maxing the volume control and adjusting RF gain for the best signal.
Follow-up Notes, hot NC-270 (and NC-155) transformer
Dave WA6VVL, in a note to the Antique Radio Forum, measured the excitation current of the NC-270 power transformer and came to the conclusion that the transformer has too few turns in the primary for the voltage applied. The excitation current is the current taken by a transformer at no load. I decided to duplicate his experiment with my second NC-270 which has the earlier appearance and is somewhat cosmetically challenged (although it works quite well). I pulled out all tubes and pilot lights to eliminate any load. Input volts versus excitation current was measured as follows.
By way of reality check, I tested a transformer with the identical physical size and lamination stack as that of the NC-270 taken from a defunct Knight C-27 citizen's band radio. Its excitation current was only 95 mA at 115 volts and 120 mA at 120 volts.
Again, despite the high excitation current and the heat, the transformer is not known for a high failure rate. Operating the radio at reduced voltage is advised.
Early and later panel comparisons
Here is a picture of my other "cosmetically-challenged" NC-270 showing the early panel labels for knob functions. It looks a lot better than when it first arrived on the bench but needs paint and some TLC. It works well, otherwise it might have become a parts donor.
A manual and schematic for the NC-270 can be found on BAMA. See the homepage for a link.
Date 6-16-12, follow-up notes 6-20-12, 6-22-12
The National NC-155 receiver was the previous item on the bench.