US Military R390A-URR
Date of Manufacture:1954–1985
Manuals and schematics :
Download Here (very large, will take some time)
Tube and Semiconductor Complement:
6DC6 6AK5W(2) 6C4(3) 6BA6W(6) 6AK6(3) 5814A(7) 26Z5W(2) 3TF7 0A2
Please note: The manual download is over 30 Mb.
The Collins designed R390A/URR (URR = Universal Radio Receiver) was manufactured by a number of manufacturers, including Motorola, Teledyne, EAC (Dittmore Freimuth), Stewart Warner, Capehart and Collins themselves. The one depicted was made in 1961 by Capehart. There was a belief that some of these sets were also made by Helena Rubinstein - but it is unsupported. Fowler began making these sets again in 1984 as a result of special military requirements.
The R390A was developed from the R390, a set manufactured by Collins, as a result of a cost-cutting exercise.
These radios saw action in the Vietnam war,and later in the first Iraq War. A slightly different version was made for the Navy and a 24-volt version - the R392 was a watertight version made for the US Army.
A version using a solid-state frequency synthesizer was made by Manson Labs for the US Military, and a few were used by NASA.
The LF version is the R389.
The radio is a double superhet for the 8-32 MHz range and a triple superhet from 0.5 - 8 MHz. It is essentially a crystal-controlled converter followed by a receiver tuning 2-3 MHz. The triple/double conversion arrangement minimises the number of crystals required. This means the set has 32 1MHz-wide wavebands controlled by a crystal oscillator followed by a linear VFO controlled variable intermediate frequency. The final intermediate frequency (IF) is 455kHz. The R390A VFO (Variable Frequency Oscillator) is usually referred to as a "PTO" (Permeability Tuned Oscillator). In the illustration you can just make out the oven containing 15 crystals in the centre.
All tuning is performed by varying the inductance of the tuned circuits, as opposed to the more commonly used capacitive tuning used in most radios. Tracking of the circuits is accomplished by a complicated system of gears and cams linked to every circuit that needs to track.
The PTO has a complicated system of adjustments that allow for linearisation of the tuning over the 2-3 MHz range of the tunable intermediate frequency.
The set has an antenna trimmer, but on the set in the illustration, this does not have to be adjusted from its central position. The earlier R390 receiver did not have an antenna trimmer.
The set was not designed for SSB reception, an outboard SSB converter is available. Many sets were converted for SSB reception using the "Captain Lee" or "Dallas Lankford" modification. The set in the illustration has the Captain Lee modification, in which the final 6BA6 IF stage is replaced with a 6BE6 product detector. (In my view this degrades AGC performance).
The set has 6 selectivity settings - 100Hz, 1kHz, 2kHz, 4kHz, 8kHz and 16 kHz. It also has a narrow audio filter for CW reception. This is achieved using 2 crystal filters and 4 Mechanical filters.
In addition to the crystal filters, there is an ultra-sharp audio filter for CW.
The carrier insertion oscillator (BFO) is also inductively tuned.
A single pentode (6DC6) is used in the RF stage.
Using the Set
The combination of switched MHz and tunable kHz knobs may not suit everybody. Because I generally look for stations on certain broadcast or amateur bands, I don't find the combination to be any trouble at all. Some users have reported the kHz tuning to be stiff - it shouldn't be. If the gearbox is correctly assembled and the drive bushing through the front panel is correctly in line, there should be no problem. Admittedly, it will require slightly more effort than an Eddystone or RA-17, because of the drive to the Veeder-Root counter displaying kHz. The frequency is very easy to read. There is a crystal calibrator, so you should never be out by more than a hundred Hz or so.
The drift is specified as 300Hz over a 20 deg temperature range. As in the Racal RA-17, this drift will be almost entirely from the VFO. (The drift for the RA-17 is not specified in the same way - simply 50Hz over constant supply voltage and ambient temperature). Suffice to say, drift is negligible on both radios after a short warm-up period. (I don't have an RA-17, so can't confirm directly that it has negligible drift.)
SSB signals are easily resolved on a modified radio. On an unmodified R390A, the RF gain will have to adjusted until the best reception is achieved. The BFO is very easily pre-set to either USB or LSB. AGC speed an be adjusted to slow. There is a "moment of silence" when switching out of slow mode. There are converters, such as the CV-157 and CV-591 that correctly resolve SSB by reinserting the carrier in the correct phase relationship to the signal, in theory making the copy very good quality.
The set has the option to switch the crystal ovens on or off. I keep them off.
Sensitivity is quoted as 5 microvolts (AM) and 1 microvolt (CW) for a 10db rise.
Although it is (just) possible to run a loudspeaker from the set using a matching transformer, most people use an outboard amplifier- here a Mullard 3-3.
The signal strength meter is simply calibrated 0-10. On the majority of R390As, it does not hold its zero position well and is very hard to adjust. Some people have replaced the single-turn adjustment potentiometer with a ten-turn type.
A 3TF7 barreter is used to stabilise the current to the filament of the PTO valve. The 3TF7 is not easy to find, and is unreliable (I have blown 2 of them in 10 years). It can be replaced with a suitable 42 Ohm (39 Ohm is the nearest preferred value) 10 Watt (for safety) resistor.
The other hard-to-find valves are the rectifiers. They can be replaced with silicon rectifiers.
If possible - don't attempt to mechanically align the set. The gears can be cleaned with a stiff brush and sewing machine oil.
To avoid a possible problems with the filters, the .01uF capacitor feeding the mechanical filters needs to be replaced with a 600 VDC good quality polystyrene type.
Please don't use Allen keys or Torx wrenches on the grub screws holding the knobs and for adjustment of the tuned circuits. The correct tool is the Bristol Wrench - as in the above enlarged illustration. Using the incorrect tool will spoil the set and maybe your reputation.
There is an easy way to tune the BFO - set the selectivity to 2kHz and as you tune, you will notice the noise (or sharsh) varies in pitch. With the control in the central position, tune for the lowest pitch.
R390A Y2K Manual from BAMA. This article gratefully acknowledges the efforts made by the Authors.
Racal RA-17 Service Manual - also from BAMA