The are two quite similar types of the R-390 receiver: the original 1950-1954 R-390 comes with LC filters, only the later R-390A comes with the legendary Collins mechanical filters. The R-389 was a VLF variant, the R-391 features a motorized frequency remote control and the R-392 was a compact receiver based on a similar circuit but with 24V DC power supply for mobile use.
Using two R-390A receivers, You can use the space diversity technique to eliminate selective fading effects on the reception: Two receivers on the same frequency are fed from two different antennas and only the signal from the receiver with the better signal level is switched to the headphones.

The receiver covers the range from 500 kHz to 32 MHz in 1 MHz segments, You can read the exact frequency from a mechanical - digital "odometer" type readout. The big rotary control "Megacycle Change" is used for switching the correct MHz - segment, with the main tuning knob "Kilocycle Change" You tune in the the correct Kilohertz setting in the range of 000 to 999, small 200 Hz interpolation lines give You an accuracy of better then 100 Hz.
The main antenna connector of the R-390A is a Twinax type meant to conect a (symmetrical) dipole antenna, it's signal has to pass through a tuned preselector stage (tuning of this is coupled to the kHz / MHz controls by many brass gears) before it's been handed over to the first RF amplifier. Signals from the second asymmetrical connector, meant to connect to a much shorter vertical antenna with lower signal levels, the signal will pass directly to the first RF amplifier without having to pass the first tunes preselector stage. At the same point, the signal of the internal crystal calibrator is coupled in here. With an antenna relay, the antenna signal with be disconnected from the first RF amplifier when the set is switched in break in and standby / calibration mode.

In frequencies up to 8 MHz, the Collins R-390A uses triple conversion, in the frequency ranges above 8 MHz double conversion circuitry.
In the frequencies between 0,5 - 8 MHz, the signal will be filtered and amplified (the preselector circuits are tuned mechanically accordingly to the tuned frequency) and will be mixed with a 17 MHz crystal signal to a first intermediate frequency in the 17,5 - 25 MHz range.
In frequencies above 8 MHz, this step is bypassed and the signal will directly be fed to the second mixer stage. In the stage, the signal is mixed with a crystal controlled signal switched by the MHz control to achieve a second intermediate frequency lying in the 2 - 3 MHz range. This IF will be mixed with the signal of the receiver VFO: the axis of the permeability tuned frequency oscillator will be moved coupled to the mechanical odometer - kilohertz - display by turning the big kilocycle tuning knob. The perfect linearity and mechanical stability of the permability tuned oscillator (PTO) made realisation of the mechanical digital frequency display possible. To avoid frequency drift while the receiver is warming up, the set is equipped with an oven switchable from the back of the receiver. The PTO frequency can be varied from 3.455 - 2.455 kHz, so after the third mixer a third IF of 455 kHz will be reached.
Ths IF will pass Quartz filters in the 0,1 and 1 kHz bandwidth positions, in the other bandwidth positions, the signal will pass one of the famous Collins mechanical filters in this place. Specially the 2 and 4 kHz IF filters give perfect results in difficult reception conditions in the tropical bands.
After additional amplification in the 2. - 4. IF amplifier stages, the signal is passed to the demodulator stage. The RF gain control will affect amplification in the second stage directly. From the fourth IF amplification stage the AGC level will be controlled, three different decay times are switchable.
The signal then is passed to the diode demodulator in AM mode, for CW reception, a signal of 452 - 458 kHz from the beat frequency oscillator is added, so that You will hear a beat note when receiving A1 signals. You can also use this for SSB signal reception, when You regulate the RF gain carefully, You will get acceptable results. Much better SSB reception is achieved using an external SSB demodulator as the Tech. Material CV-591 or the Sherwood SE-3, the Collins made SSB demodulator CV-157 is very scarce. There is also a switchable Noise limiter and a 800 Hz audio filter for CW use.
From the diode demodulator, the AF signal is fed to an AF amplifier, the "Local Gain" control will affect the signal level at the speaker jack at the rear of the set. Caution: remember connecting a high impedance 600 Ohm speaker, use an AF output transformer if You plan to connect a modern low impedance speaker. A second "Line Gain" control affects the audio level on the "Line out" jack, this can be used to control a FSK converter or an external power amplifier. The AF level is displayed by the meter in the left upper corner of the front panel.
The other meter in the right upper corner marked "Carrier Level" will display signal strength and has a 0 - 100 dB calibration, with 7,5 uV the meter will show 20 dB and with 30 mV, it will reach the 100 dB line.

To achieve perfect frequency accuracy, You have to recalibrate the mechanical frequency display after switching the MHz segment. The internal crystal calibrator will give You a calibration signal avery 100 kHz, so tune the receiver to the nearest 100 kHz point. Then disconnect the PTO from the mechanical counter using the "Zero Adj" knob, this will act as a clutch when turned clockwise. Use a IF bandwidth setting of 1 kHz, switch on BFO, BFO Pitch set to 0 and tune to "Zero beat", this means turn the kHz control until the whistling noise will become low pitched and will eventually disappear. Then turn the "Zero Adj." knob counterclockwise to couple the PTO to the mechanical display and You should end wit ha frequency display accuracy of better then 200 Hz.
The crystal calibrator signals should give a S-meter deflection of 60 dB in the lower frequency bands and of around 30 up to 35 dB in the higher frequency ranges. Use the "Antenna Trim" control to adjust antenna coupling to maximum S-meter / "Carrier Level" meter deflection.
This manoeuvre will give You a good impression whether Your Collins set is working properly, useful when You consider buying one at a hamfest.

To operate the set turn the main "Function" switch to AGC position, When You tune in the correct MHZ and kHz - digit with the "Megacycle Change" and "Kilocycle Change" knobs, You can see all the marvellous gears moving behind the front panel, it look like a clockwork - never take the mechanical parts apart or try to reassemble the stuff as long as You are not a trained watchmaker... Turn the "Kilocycle Change" until the desired frequency is displayed, e.g. 6 - 1 - 5 - 5, You can lock the dial mechanically on that frequency. Use the "Bandwidth" switch to select the IF filter appropriate for Your reception conditions, the 16 kc filter gives You very crisp reproduction of the signal if there is really no adjacent channel interference, I usually chose the 8 kc setting for uncomplicated conditions and the 4 kc one, if there is adjacent channel interference. This one and the 2 kc filter are responsible for the excellent selectivity of the set in the tropical bands. The readibility of poor signals is surprising everytime I use my Collins. Not that the Collins lacks the posibilities of improving the readability by Passbandtuning and HF Notch filters as found on modern amateur radio receivers quite often - but the IF filters help You a lot.
The audio quality is typical to the one of hollow state sets, no high pitch hiss from the internal microprocessor, nor any "birdies" associated with computer technology. To replace the missing Notch Filter, You might try an external Audio Frequency Notch filter as the famous Dierking GD82 /GD84, for CW reception, the R390A has an internal 800 Hz AF filter.

In practical use, I did compare my R390A quite often to my NRD-525 without filter modification, a great set that is known as quite hissy and has suboptimal audio. The signal strength of the private / christian broadcasters on higher frequencies in the evening were similar on both sets, with the faint HCJB signals from Ecuador, the 390 did a better job.
Just to check, what's on in a band, I prefer the '525 with it's direct frequency input keypad, when I find good conditions, I like to turn on the Collins and cruise the bands with it - as long as I don't have to overrun 1 MHz segments, to go from 4980 to 5035 kHz is quite cumbersome, You will have to Switch the MHz 1 higher and turn the kHz-dial all the way done - no problem, If You don't plan to go back to 4980 for a second... That's why I prefer the '525 for checking parallel frequencies - but the R390 for comfort listening of stations with poor signals.

If You consider buying a used R390A, make sure You get a A-version with the mechanical filters if You intend shortwave DXing. I prefer my Collins set, usually the prices of the other contractors are a bit lower.
Of course, a set beeing serviced and realigned professionally by it's former owner has a better value. I have given You a description how to check the main functions using the internal crystal calibrator before. The mechanical frequency readout and the gears should move without backlash and the frequency display should be calibrated within the calibration range.
For all mechanical work on the set, You need only three tools: a Philips screwdriver, a small spanner and - most important - a Bristo No.8 wrench. If You're lucky, You find these tools still in place in a holder on the rear of the set. Make sure that the previous owner hasn't spoilt the screws on all the knobs using an ordinary Philips screwdriver instead og the Bristo one, You might find a set so ruined so that You cannot even take away the front panel.
Make sure to look out for the special antenna corrector.
Quite often, the meters are mising, as the radium coloured indicator give a small amount of radiation, mane meters have been taken away, replaced or You find sets with lead covers over the meter.
I found another problem: the voltage compensator of the set will "draw" a small amount of current when the set is switched on: this will regularly cause my RCD switch (Residual Current protective Device or Ground Fault Circuit Interrupter (GFCI)) to switch off. As new houses in Switzerland are equipped with RCD/GFCI security switches for the whole building or floor, You might find it difficult to operate an R390 here. I think it's a wanted operation of the compensator that will cause the protection switch to switch off because it's happening on all my R390's, and I don't suspect all my R390's to be faulty. A possible workaround is to use a "crossover transformator".