My R-390 (predecessor to the R-390A) HF radio receiver, overhauled, aligned, and tweaked by Rick Mish of Miltronics

Click on the thumbnail to see a larger image (149645 bytes)
Thumbnail of R-390 (non-A) front panel



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.

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.

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.

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: .
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.
Click on this thumbnail image of the RF section to see a larger image: .

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.

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: .

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.

The tube complement of the R-390 is:
RF Subchassis
01V2016AJ51st RF amp.
02V2026BJ62nd RF amp.
03V2036C4 1st mixer
04V2046C4 2nd mixer
05V2056C4 3rd mixer
Oscillator Subchassis
06V4016AJ51st crystal oscillator
07V4026AJ52nd crystal oscillator
IF Subchassis
08V5016BJ61st IF amp
09V5026BJ62nd IF amp
10V5036BJ63rd IF amp
11V5046BJ64th IF amp
12V5056BJ65th IF amp
13V5066AK66th IF amp
14V50712AU7Detector (A), Negative Peak Limiter (B)
15V5085749BFO
16V5096BJ6AGC amp
17V51012AU7AGC Rectifier (A), Positive Peak Limiter (B)
18V51112AU7AGC Time Constant Control (A), Cathode Follower (B)
AF Subchassis
19V60112AU7AF Amp. (A), Squelch (B)
20V60212AT7Local AF Amp (A), Line AF Amp. (B)
21V6036AK6Local AF Output (Phones, Spkr)
22V6046AK6Line AF Output (600 Ohm balanced)
23V6056082Voltage Regulator Pass Tube
24V6066082Voltage Regulator Pass Tube
25V6076BH6DC Amp.
26V608565190VDC Voltage Reference
27V609565190VDC Voltage Reference
VFO Subchassis
28V7015749VFO
Power Supply Subchassis (PP-621/URR)
29V80126Z5WRectifier (Part of PP-621/URR)
30V80226Z5WRectifier (Part of PP-621/URR)
100 KHz Calibrator Subchassis
31V90112AU7Calibration Osc. (A), Buffer Amp. (B)
32V90212AU7Multivibrator (A,B)

for a total of 32 tubes, plus one Amperite 3TF7 ballast tube to regulate the filament current in the BFO and AGC amplifier and one selenium rectifier -- the only semiconductor device in the entire receiver.
R-390 Technical Manuals
R-390 Operator's Manual (956159 bytes)
R-390 Organizational Maintenance Manual (788827 bytes)
R-390 Field and Depot Maintenance Manual (3048971 bytes)