The R-390 is a vacuum-tube superheterodyne receiver with a frequency range of
0.5 to 2 MHz, using a tunable RF section for its initial selectivity.
Below 8 MHz, the receiver is triple-conversion; it is double-conversion
from 8 MHz to 32 MHz.
It uses two RF amplification stages, 6 IF amplification stages, and
two stages of audio amplification for each of two audio outputs.
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The R-390 VFO uses a Permeability-Tuned Oscillator (PTO), in which a
slug is moved into a solenoid coil to decrease the frequency at which
the oscillator runs. The R-390 VFO covers a little over 1 MHz; it is
designed so that exactly ten turns covers exactly 1.0 MHz, and so that
frequency increases (or decreases) linearly with the number of turns of
the Kilocycle tuning knob. This, with the odometer-style frequency
display, means that the R-390 has essentially linear tuning over each
1 MHz band from 1.0 MHz to 31 MHz, and that it is easy to tune to
within a few hundred HZ of a given frequency just by using the odometer
display. The very precise 100 KHz calibrator makes it possible for the
operator to align the odometer display precisely to any given multiple
of 100 KHz, and then use the receiver as a moderately-accurate frequency
meter.
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A mechanical corrector assembly inside the VFO, and coupled to the
leadscrew that is driven by the Kilocycle tuning knob, can be adjusted
so that the VFO frequency increases very nearly linearly with
leadscrew rotation. This is an elegant solution to a difficult problem,
and the designers should be proud of themselves.
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The RF section uses permeability-tuned filters before each stage of RF
amplification and before each mixer to increase selectivity and to
remove spurious frequencies. These tuning slugs for these filters are
moved by cams on the RF subchassis, and the cams are positioned by
a quite complex arrangement of gears driven by both the Megacycle and
Kilocycle tuning controls.
Click on this thumbnail top view of the gearing
to see a larger image:
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The Field and Depot Maintenance Technical Manual, TM_11-5820-357-35", gives an exploded
diagram of the gearing in Figure 73 and explains the theory of operation of the mechanical tuning
system in Section IV.
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Click on this thumbnail image of the RF section
to see a larger image:
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The R-390 is legendary for its sensitivity and its stability. It was
designed in the early 1950s by Collins Radio for the US military, and
was for a very long time the premier HF receiver in the US armamentarium.
It was succeeded by its (slightly) less complex and (slightly) less costly
cousin, the R-390A, which was very nearly as sensitive and selective, and
no less stable.
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The R-390 IF bandwidth can be set to 16, 8, 4, 2, 1, or .1 KHz. The
narrowest settings, 1 and 0.1 KHz, use a crystal filter, while the
remainder use LC bandpass filters.
Click on this thumbnail image of the IF section
to see a larger image:
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The R-390 audio bandwidth can be
set to wide, medium, or sharp. The "wide" position adds no filtering
to the signal chain, the "Med[ium]" position inserts a 3500 Hz low-pass
filter, and the "Sharp" position inserts an 800 Hz low-pass filter.
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RF Subchassis
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01 | V201 | 6AJ5 | 1st RF amp. |
02 | V202 | 6BJ6 | 2nd RF amp. |
03 | V203 | 6C4 | 1st mixer |
04 | V204 | 6C4 | 2nd mixer |
05 | V205 | 6C4 | 3rd mixer |
Oscillator Subchassis
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06 | V401 | 6AJ5 | 1st crystal oscillator |
07 | V402 | 6AJ5 | 2nd crystal oscillator |
IF Subchassis
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08 | V501 | 6BJ6 | 1st IF amp |
09 | V502 | 6BJ6 | 2nd IF amp |
10 | V503 | 6BJ6 | 3rd IF amp |
11 | V504 | 6BJ6 | 4th IF amp |
12 | V505 | 6BJ6 | 5th IF amp |
13 | V506 | 6AK6 | 6th IF amp |
14 | V507 | 12AU7 | Detector (A), Negative Peak Limiter (B) |
15 | V508 | 5749 | BFO |
16 | V509 | 6BJ6 | AGC amp |
17 | V510 | 12AU7 | AGC Rectifier (A), Positive Peak Limiter (B) |
18 | V511 | 12AU7 | AGC Time Constant Control (A), Cathode Follower (B) |
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AF Subchassis
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19 | V601 | 12AU7 | AF Amp. (A), Squelch (B) |
20 | V602 | 12AT7 | Local AF Amp (A), Line AF Amp. (B) |
21 | V603 | 6AK6 | Local AF Output (Phones, Spkr) |
22 | V604 | 6AK6 | Line AF Output (600 Ohm balanced) |
23 | V605 | 6082 | Voltage Regulator Pass Tube |
24 | V606 | 6082 | Voltage Regulator Pass Tube |
25 | V607 | 6BH6 | DC Amp. |
26 | V608 | 5651 | 90VDC Voltage Reference |
27 | V609 | 5651 | 90VDC Voltage Reference |
VFO Subchassis
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28 | V701 | 5749 | VFO |
Power Supply Subchassis (PP-621/URR)
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29 | V801 | 26Z5W | Rectifier (Part of PP-621/URR) |
30 | V802 | 26Z5W | Rectifier (Part of PP-621/URR) |
100 KHz Calibrator Subchassis
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31 | V901 | 12AU7 | Calibration Osc. (A), Buffer Amp. (B) |
32 | V902 | 12AU7 | Multivibrator (A,B) |
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