Small Pictures; Click the images for bigger pictures
This R-390A was given to me by a friend at work, who needed the house room since his daughter had moved back in. He got it from the Communications Branch manager at work, who had had it for some time, and who got it from a MARS station that was deactivated.

I had heard of the R-390A and R-390A receivers when I was in the Air Force, and had coveted one mildly from first hearing. Imagine my dismay when, the day before I returned from a TDY to Osan AB, Republic of Korea, I found that the folks in the MARS shack there were tossing several R-390A and/or R-390A receivers into a dumpster.

They told me I could have as many as I wanted, and if I hadn't shipped my hold baggage back to Camp Drake the previous day, I would have taken them up on the offer -- in spades.

It has been a long time since October 1969, when I left Osan, but now I have my R-390A. It's very nice.

The R-390A 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-390A 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-390A 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-390A 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_11-5820-358-35", gives an exploded diagram of the gearing in Figure 88 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: .

Like its older brother, the R-390, the R-390A is legendary for its sensitivity and its stability. It was designed by Collins Radio for the US military as a less expensive, less complex, more reliable, and easier to maintain replacement for the original R-390, and did yeoman service until very recently. It is said that the more modern solid-state receivers proved unreliable in the First Gulf War because their front ends were unable to withstand the high static charges caused by sand blowing across the antennas, and that the "Old Reliable" R-390A replaced Watkins-Johnson and Cubic receivers in many listening posts.

The R-390A 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 mechanical (magnetostrictive) bandpass filters. Click on this thumbnail image of the IF section to see a larger image: .

The R-390A audio bandwidth can be set to sharp or wide. The "wide" position adds no filtering to the signal chain, while the "Sharp" position inserts an 800 Hz low-pass filter.
R-390A Technical Manuals
R-390A Organizational Maintenance Manual R-390A Field and Depot Maintenance Manual
Serial Number Plate (1094655 bytes)
Front View (1554935 bytes)
Front and Top View (1643265 bytes)
Side View (1526117 bytes)

Tube Complement
RF section
01V2016CD6RF amp
02V2026C41st mixer
03V2036C42nd mixer
04V2046C43rd mixer
05V2055814A100 Kc multivibrator
06V2065814Acalibration oscillator and cathode follower
07V2075654/6AK5W1st crystal oscillator
IF section
08V4015654/6AK5W2nd crystal oscillator
09V5015749/6BA6W1st IF amp
10V5025749/6BA6W2nd IF amp
11V5035749/6BA6W3rd IF amp
12V5046AK64th IF amp
14V5065814AAGC time constant tube and detector
16V5085749AGC IF amp
17V5095814AAGC rectifier and IF cathode follower
18RT5103TF7filament current regulator
Power Supply
19V60125ZW/6AK5Wpower supply rectifier
20V60225ZW/6AK5Wpower supply rectifier
21V6015814A1st AF amp and AF cathode follower
22V6025814Alocal AF amp and line AF amp
23V6036AK6local AF output
24V6046AK6line AF output
25V605OA2voltage regulator