Inhaltsverzeichnis
Collins R-390A/URR
Manufactured by Collins Radio Comp., Cedar Rapids.
The R-390A is a military communications receiver which was introduced as a replacement for the Collins R-388 in the mid-fifties. The original R-390 is equipped with coil filters, only the A version comes with the famous mechanical filters. The manual describes the receiver as a „general - purpose receiver for use in both fixed and mobile applications“: It is designed for reception of CW, AM, FSK and, with some restrictions, SSB transmissions.
The set was built according to the same construction plans not only by Collins, but also by other American electronic equipment manufacturers (Motorola, Amelco) as contractors. All receivers had to achieve the same specifications, but the reception performance might vary between the sets of different manufacturers.
Technical data
- Frequency range: 0.5 - 32 MHz
- Frequency display: mechanical - digital display, 100 Hz
- Frequency memory: none
Power supply
- Mains operation: 115, 230 V
Dimensions
- 483 x 267 x 406 mm, weight 29.5 kg
Accessories
Operation
There are two different types of the R-390: As R-390 the set was manufactured in the years 1950 - 54 with coil filters, only in the version R-390A produced after 1958 the almost legendary mechanical filters are installed, externally this version can be distinguished by the bandwidth switch on the left side of the front panel in addition to the designation on the front panel. The receiver with a remote control for receiving 7 fixed frequencies got the designation R-391, while the R-392 was a more compact receiver with a similar concept for mobile operation with a 28 V power supply.
Space diversity reception is possible with two interconnected R-390 receivers. In diversity operation, two receivers are operated at two locations a few hundred metres apart on two antennas on the same frequency; by switching through the stronger signal in each case, selective fading can be almost eliminated.
The receiver covers the frequency range from 500 kHz to 32 MHz in 1 MHz segments. The received frequency can be read directly from a mechanical counter (mechanical digital display), which looks like an odometer. The MHz digits are selected with the large rotary knob „Megacycle Change“, the kilohertz digits from 000 - 999 are set with the knob „Kilocycle change“, 200 Hz dial lines are provided between 1 kHz digits, so that the maximum frequency accuracy is up to 100 Hz. Below 8 MHz the receiver works as a triple conversion superhet, above this as a double conversion.
The R-390 has a twinax connection for (symmetrical) dipole aerials; this signal is fed to the first amplifier stage via a preselector circuit tuned by the kHz / MHz knobs. From the second asymmetrical antenna input, which is intended for connecting rod antennas with their lower effective levels, the signal is fed directly to the first RF amplifier stage without having to go through the pre-selection circuit, and the signal from the internal crystal calibrator is also fed in here. An antenna relay is used to decouple the antenna signal from the first amplifier stage in standby/calibration and break-in mode (when operating together with a transmitter).
When operating in a band with a triple conversion, the frequencies between 0.5 and 8 MHz are fed to the first mixer stage after amplification and filtering via tuned circuits, which are also synchronised with the frequency tuning. After mixing with a 17 MHz crystal oscillator signal, the first IF between 17.5 and 25 MHz is generated. When receiving frequencies above 8 MHz, this step is bypassed and the signal is fed directly to the second mixer stage.
At this stage, a crystal oscillator signal selected by the MHz knob is mixed in so that the mixing product is always between 3 and 2 MHz, which is the second intermediate frequency. This is mixed with the signal from the receiver's VFO: The axis of the permeability-tuned frequency oscillator is rotated via the KHz tuning using numerous gear wheels, which also drive the mechanical counter. The excellent quality and linear frequency response of the PTO made it possible to end with an accurate mechanical frequency display. To prevent frequency drift when the receiver warms up, the receiver is equipped with a heating system that can be switched from the rear. As the PTO frequency varies from 3,455 - 2,455, the third intermediate frequency of 455 kHz is now available after the third mixing stage.
The intermediate frequency passes through crystal filters when bandwidths of 0.1 and 1 KHz are used; in the other positions of the bandwidth selector switch, the signal is passed directly to the first intermediate frequency amplifier stage. The signal is then passed through one of four switchable mechanical filters. This is where the famous mechanical filters are found, of which the 2 and 4 KHz filters in particular provide invaluable services in difficult interference situations, e.g. in the tropical bands. After amplification in the 2. - 4. IF amplifier stage, the signal is fed to the demodulator stage. The RF gain control directly influences the signal amplification of the second IF aplifier. The AF signal is forwarded from the diode demodulator to a noise limiter, which can be bypassed. The BFO injects a frequency from 452 - 458 kHz, so the control range is +/- 3 KHz. This makes SSB reception possible; in difficult situations, the RF gain must be carefully adjusted manually. After the noise limiter stage, an audio filter can be switched, which allows only signals around 800 Hz to pass for CW reception.
The signal is also fed to the AGC control from the fourth IF amplifier stage; the AGC control can be set to three decay times, thus meeting the requirements of different reception situations. The volume control is labelled Local gain, and in addition to the headphone output on the front panel, the receiver has a loudspeaker output on the rear. An internal speaker is not provided; due to the loudspeaker impedance of 600 ohms, a matching transformer is required to operate a low-impedance loudspeaker. The AF voltage for operating a radio teletypewriter, etc., can be adjusted via a second Line Out Gain control. The AF level is indicated by an instrument at the top left of the front panel, the sensitivity can be switched separately. At the top right is the S-meter, which has an unconventional calibration from 0 - 100 dB; 7.5 uV causes the S-meter to deflect to 20 dB, 30 mV to 100 dB.
In order to achieve high frequency accuracy, calibration is first necessary when changing to a different MHz band. The crystal calibrator provides a calibration signal every 100 KHz. The set is tuned exactly to a 100 KHz position using the KHz knob, then the tunign knob is uncoupled from the VFO by turning the Zero Adj. knob clockwise. With a bandwidth of 1 KHz and the BFO switched on and the pitch control in the 0 position, the tuning is now adjusted for zero beat, i.e. the KHz knob is turned until the whistle becomes lower and disappears, and the line level metre can be observed to drop to O at the same time. Turning the Zero Adj. knob anti-clockwise causes the tuning knob to be coupled to the VFO again; the frequency display is now set with an accuracy of 200 Hz. The signal from the crystal calibrator should result in a deflection of the S-meter of up to 60 dB on the lower frequencies and up to 35 dB on the higher frequencies; the antenna can now also be optimally adjusted using the „Antenna trim“ control, located above the frequency display, and adjusted to full deflection. The manoeuvre described also allows a rough check of an R-390A before an intended purchase at a radio flea market, etc.
For reception in standard situations, the main switch is set to the AGC position. The frequency is now tuned using the large MHz and kHz knobs. Inside, the synchronisation discs moved by gear wheels and the up/down movements of the preselector can be observed. I am equally fascinated by the degree of precision with which accurate preselection is achieved here, to which the receiver owes its mirror frequency rejection and large signal stability, and how smoothly the KHz tuning runs in the end. Many a Swiss watchmaker would be delighted with this marvel of precision mechanics. Even on the simplified diagram of the mechanical tuning system in the manual, the number of gear wheels is confusing.
The desired frequency, for example 6.165 kHz, is now set on the mechanical counter, and a „Dial Lock“ button allows the main tuning to be mechanically locked. The filter corresponding to the reception situation can be selected using the Bandwith switch: In the 16 Kc position, crystal-clear reception of stations is possible if there are no stations on adjacent frequencies, for example BBC London on 12095 kHz comes in here almost in FM band quality. I generally use the 8 Kc filter for uncomplicated AM reception; if one of the two 5 KHz adjacent channels is occupied, it is advisable to select the 4 Kc filter. This one and even more the 2 Kc filter give the receiver its legendary selectivity in tropical band reception. I am always impressed by the intelligibility and presence of even extremely weak signals. The lack of ECSS reception and the equally lacking features like passband tuning and notch filter is compensated for by the filter configuration. The audio quality corresponds to tube technology, any high-frequency noise (caused by the PLL circuit and other computer technology in modern receivers) is missing, which makes listening to weak signals less tiresome. As a replacement for the notch filter, for example, an external AF filter (GD 82) can be connected to the loudspeaker terminals; for CW reception, the R-390 has its own AF 800 Hz filter.
In practical use, I was able to compare my R-390 with my unmodified NRD-525 and operate the sets in parallel, with the NRD connected to a McDymek DA 100 active antenna and the R-390 (for the time being) improvised on a few metres of wire at the asymmetrical input. The signal strength of the private stations from the USA in the evening (WYFR, WEWN) on the high bands was comparable, the intelligibility of weak HCJB signals was significantly better with the R-390.
For band scanning, especially when no x999 - x000 KHz transition has to be made, I love to use the R-390; for quickly comparing several parallel frequencies of a programme at a known time, the NRD-525 has a clear advantage due to its electronic memories. The R-390 is turned on for deliberate „radio listening“, I use the NRD to check what is currently on in the 60 or 48 m band..
When buying an R-390, the most important thing is to make sure that it really is the A version with the mechanical filters. Of the various manufacturers, I think a set from Collins is preferable. Of course, a set that has been well maintained by its previous owner and, if necessary, recalibrated by someone who knows what he is doing (!), is more valuable. Before buying, make sure that the MHz and especially the KHz tuning has no play, the function can be easily checked with the internal crystal calibrator, the mechanical display must be calibrated using the markers as described above. Three tools are sufficient for all mechanical work on the set: A Philips screwdriver, a spanner and above all a special tool, a Bristo No.8 screwdriver, all screws, for example on the rotary knobs, are tightened with this tool with the profile of a six-pointed star. If the previous owner has used a normal Phillips screwdriver and spoilt the sensitive internal threads, not even the front panel can be removed…
Technical principle
Components
The set is equipped with tubes.
