14 - 600 Khz.
This is a 1960s era U.S. Navy shipboard VLF receiver, the RCA designed and built AN/FRR-21, the rack-mounted, fixed-station version of the AN/SRR-11. This fairly compact receiver none-the-less weighs 69 lbs. It is about 3/5 as high as the R-389 or R-390. It contains a 28 tube dual-conversion (60 Khz, and 200 Khz IFs), superheterodyne receiver of unique construction. It is very highly modularized, making service much easier than for most other receivers, and uses sub-miniature tubes throughout which were designed for extreme reliability. The tubes are soldered into miniature modules which are then plugged into modular sections, i.e. ANT, RF, MIXER, OSC, 1st IF, IF, AUDIO, CALIBRATOR, POWER SUPPLY, etc.
The receiver covers the range 14 - 600 Khz, in five bands, 14 - 30 Khz, 30 - 63 Khz, 63 - 133 Khz, 133 - 283 Khz, and 283 - 600 Khz. There are 3 modes of operation, A-1 (CW or SSB) Broad, Medium,and Sharp, A-2 (AM), and F-1 (RTTY). There is a diode detector for the A-2 mode, and an automatically switched-to "BFO-Mixer" for the A-1 and F-1 modes. The BFO-Mixer is actually a product detector, and thus the receiver works quite well on CW or SSB. There is an automatic noise limiter for the modes which use the BFO-mixer, and an adjustable peak-limiter in the audio stage. It also has an adjustable "Silencer", or squelch for use in the AM mode.
Antenna input impedance is easily changed from High Impedance (approximately 300 ohms) to Low Impedance (approximately 70 ohms) by means of an internal jumper.
There are outputs on the rear panel for IF (to an IF connected RTTY converter), AC power, RF input (an "N" connector), and two audio outputs for connection to an external audio amp/speaker, labled Audio Phone, or to an audio-type RTTY convertor, labled Audio Line. These audio outputs have their own front panel Output control.
Connectors for the rear-panel connections are standard Amphenol Mil 97-3106A type connectors available from Newark and other parts suppliers. I bought mine from Newark.
All inputs and outputs are very effectively filtered by integral robust band-pass filters so that the receiver is isolated from nearby transmitters or other spurious signals.
Audio output is by either two front-panel mounted 1/4" phone jacks with their own separate Level Control, or via the two rear panel mounted connectors previously mentioned.
The very accurate dial readout mechanism is quite unique in that it is uses a projection system and an etched glass disk giving an effective linear analog readout approximately 12' (feet) in length, combined with an extremely accurate and easily resettable logging scale. The photo below gives a somewhat poor idea of what the dial looks like, since the markings are actually much darker and clearer than I was able to show in this photo
There is a built-in accurate 10 Khz. crystal calibrator of unique design which switches all outside signals away when it is switched on.
Selectivity is accomplished via cascaded and switched complex filters in the IFs and separate audio filters. Selectivity is very good. For instance, the 1st IF bandpass filter is 2.6 Khz wide at 6 db down and 3.7 Khz wide at 60 db down. There are up to three separate filters in the second IF, labled "broad", "medium", and "sharp". These are then followed by a sharp audio filter which is switched in or out as the operator wishes.
The built-in power supply accepts 105, 115, and 125 VAC at 50 to 400 Hz, depending on jumper-settings, and provides regulated and unregulated voltages for the receiver.
This particular receiver is in unusually good condition, and apparently was not used very much. The factory installed tools are still in their holders!
Electrical/electronic condition is quite good, although as with any other receiver of this age, its alignment should be checked and tweaked. Interior is clean and it works properly on all bands and modes. All rotary switches in these receivers are ceramic, not phenolic as so many others are, and all rotary switch contacts are silver. The crank-arms which operate the switches, which are one of the very few things that can go wrong with this receiver, have been checked and are all in good shape.