Notes on Restoring
National NC-300

by Ken - K6FC

 I recently obtained a National Radio NC-300 to add to my collection of vacuum tube receivers.  It was relatively clean and not much effort appeared to be needed to return it to good operational condition.  A nice thing about this receiver is the smooth main tuning dial and about eleven inches of bandspread, different from so many other receivers of its day.  The fact it was ham band only makes frequency readout a pleasure compared to the NC-183D that it was to replace at my AM operating position.

Upon removal from the cabinet I found a previous owner had done me the favor of replacing many of the old paper (leaky) capacitors, so one onerous task was already complete.  The next step was to remove and check the various tubes.  With one exception, all were per the schematic and in good condition.

The exception:  V7, the 6AH6 HF oscillator, had been replaced with a 6AU6.  Both are sharp cutoff tubes and although the 6AH6 has about twice the transconductance of the 6AU6, the circuit still oscillated when powered up.  Frequency stability however wasn’t great and a fault with the (notorious) 4H4-C filament regulator was suspected.  A quick check of the filament voltage showed the problem: the 6AU6 was running at 10 volts rather than the specified 6.3 volts.  The reason for this was evident after checking filament current specs at rated voltage.  The filament current for a 6AU6 is noted as 0.3 amps while the 6AH6 pulls 0.45 amps.  This “minor” tube change thus meant the 4H4-C was probably operating outside its functional envelope as the 6AU6 filament was getting most of the 13 volts available from the power transformer.  The fact the 6AU6 filament was drawing 2.5 times its rated power certainly didn’t add to oscillator stability!  A 6AH6 was found in the tube collection and put in place.  Frequency stability was now significantly improved.

Next the receiver was connected to an antenna to determine how it sounded on the air and several annoying issues were quickly identified.

First, the AGC and S-meter exhibited a very short time constant likely suited to 1950’s AM operation but not at all suitable for my intended general use.

Second, listening to CW or SSB was really hard on the ear, as signals were quite distorted except when the manual RF gain control was turned way down.  A look at the schematic told the story: there was probably way too much IF injection on the grid of V8, the 6BE6 oscillator/detector, and that resulted in significant audio distortion.

The fixes for both these deficiencies turned out to be relatively simple.

To improve AGC action a 0.56 cap was installed from pin 7 of V9 (grid of the S-meter amplifier) to ground to smooth the S-meter movement and a second 0.56 cap was installed from the AGC bus to ground at pin 3 of the accessory socket.

The product detector distortion was resolved with the addition of a 100 pf cap from pin 7 of V8 to ground, thus significantly reducing the IF injection level.

Now it was time to align the receiver.  An accurate signal generator is needed for this step and having an internal 100 KHz calibrator is an added plus.  Since this receiver did not come with a calibrator, one was made using a “Bud” box and components from the junk box.  With an added octal plug it fit right into the existing calibrator socket.

The second IF frequency is supposed to be at 80 KHz but was found to be in the 90 KHz range.  The reason for this is noted in the next paragraph.  Anyway, the second IF string was aligned at 80 KHz in accordance with the manual.

Next came the alignment of the crystal filter and second converter.  Following manual instructions to look for the crystal peak frequency, the peak was found to be at 2227 KHz rather than the expected 2215 KHz.  After 50+ years since its fabrication, the crystal had aged up about 12 KHz!  Likewise the second conversion oscillator frequency had also edged up either with age or by maladjustment, resulting in a higher second IF frequency.  The second converter oscillation frequency was reset to 2317 KHz so that the crystal filter was matched to the 80 KHz IF string.

Once the first and second IF stages were properly aligned it was time to address the front end of the receiver.  Again the manual instructions were followed and except for the annoyance of bending the little trimmer wire in the oscillator inductors (multiple times to set up frequency linearity), all the band segments set up nicely.  These RF adjustments are made with the receiver out of the cabinet and once the cabinet is back in place there are minor changes, but the zero adjust knob takes care of that issue.

So, a relatively easy restoration job for a nice 50’s era receiver.  The added time constant to the AGC is not so much as to affect AM reception (in fact I think it helps) and listening to SSB and CW is no longer a constant battle with the RF gain control.  Tuning is smooth and with a calibrator installed, frequency readout is really quite accurate.  After a short warm-up, frequency stability is impressive for a receiver of this vintage.  The pentagrid RF converter tubes are noisy compared with modern designs, but then the receiver is not going to be used for moon bounce QSO’s!

The NC-300 is a neat receiver. It is a nice addition to any station.

More information on the NC-300 available here.




15 December 2011