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.
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