The RA6790/GM HF Receiver, is a fully synthesized, microcomputer- -based, tunable, solid-state receiver designed to provide reception capabilities for CW(A1), AM(A3), -LSB/USB (A3J), FM(F3) and ISB (A3B-Optional), emissions over the frequency range of 0.5 MHz to ' . 30 MHz. Frequency tuning is accomplished by either keyboard entry (numerical keypad) or single knob control for continuous tuning with selectable rates, FAST (1000 Hz), SLOW (30 Hz), FINE (1 Hz) increments and BFO continuous in 10 Hz increments. Front panel indication of frequency data are presented as 8 LCD (Liquid Crystal Display) digits of tuned frequency and 3 digits and sign readout of BFO data relative to IF Center ±8.0 kHz.
Other front panel displays are: RF Meter, AF Meter, Bandwidth, AGC, Mode, Remote and Fault indication (LED). Additionally, provision is made on the front panel, selectable by use of the RF Gain Control, to manually control the AGC threshold anywhere within the range of 110 dB above the preset AGC threshold level.
Full remote control (optional) of all Receiver parameters is available by either; (1) serial asynchronous, ASCII character-oriented with strap selectable baud rate of 50 baud to 19.2 kilobaud, selectable MIL-STD-188C or EIA Standard RS-232-C/RS-422/RS-423 compatible, 2-byte-serial, (2) byte-serial bit parallel IEEE standard 488C-1978 compatible or (3) other user specified interface format.
Built-in Test Equipment (BITE) is provided in the Receiver circuitry to find, test and report operational status to the lowest replaceable unit (LRU) with both local and remote notification.
The internal frequency tuning circuitry of the Receiver includes a single loop digital 1st LO Synthesizer, phase lock loop 2nd LO and BFO Synthesizers, which in combination determine tuned frequency to a resolution of 1 Hz. The synthesized BFO tunes ±8 kHz in 10 Hz increments with a pushbutton selectable parameter for immediate zero reference.
The Receiver is designed to operate with up to seven IF filters, using slots provided on the Main IF/AF circuit card assembly. Unless otherwise specified, the Receiver is factory equipped with six mechanical filters and one bypass link to provide seven selectable bandwidths. Audio output of the Receiver is either through a PHONES jack on the front panel or through an AF OUT connector on the rear panel. The PHONES jack provides a nominal 10 milliwatts into 600 ohms and is adjustable through the AF GAIN control on the front panel. The AF OUT connector provides a nominal 1 milliwait_ 600 ohm balanced line output and an output of nominally 1 Watt in 8 ohms, suitable for an 8 ohm speaker.
Rear panel features include BNC connectors for providing the Receiver second IF 455 kHz output, supplying or receiving external/internal reference signals used in conjunction with a slide switch. S2. A 25-pin D-type connector provides audio, AGC and fault status outputs while an N-type connector provides RF input from an external antenna. An optional remote control interface connector, either round 26 pin (188C/232C/422/423) or elongated 24 pin (IEEE-488), is provided when specified. A standard 3 prong male connector for connection of an ac line cord completes the rear panel assembly.
Input signals from the antenna are connected to a low pass filter which rejects signals above 30 MHz. The output of this filter is then coupled to a mixer stage where the RF signal is mixed with the synthesized local oscillator signal. The frequency of the local oscillator can be varied
from 40.955 to 70.455 MHz. This signal is brought through a filter and AGC controlled amplifier stages to the second mixer.
The 40.455 MHz first IF is combined with the fixed 40 MHz output from the second oscillator synthesizer to produce a 455 kHz second IF. After amplification, this second IF is routed to the plug-in 455 .kHz filters which provides the main Receiver selectivity.
A second AGC-controlled amplifier follows before demodulation takes place. In the SSB/ «' CW/AM modes, a product synchronous detector is used. In the CW mode, the BFO synthesizer may be varied ±8 kHz (above or below the -55 kHz center frequency) through front panel controls. For . . FM reception, the 455 kHz second IF is input to a limiting amplifier and subsequently to a separate '. FM detector. All three outputs from the synthesizers are referenced to an internal 5 MHz standard frequency source, or to an external reference of 1 MHz. The demodulated signal is fed through an audio crosspoint switch to separate AF amplifiers which provide outputs for a 600 ohm line, a headphone jack, and an external loudspeaker.
For ISB operation, the optional ISB board provides the LSB signal path, while the main IF/AF board provides the USB signal path. The LSB component of the 455 kHz second IF is processed through similar circuitry with the common BFO synthesizer providing the reinserted carrier for demodulation. A separate audio amplifier provides a 600 ohm line output.
All command signals, whether from the front panel controls or from an extended or remote operating position, are processed by the microcomputer assembly. Two separate buses carry control data and address information to and from the microcomputer control assembly to the synthesizers for frequency selection, and to the appropriate switching circuits controlling the different operating modes.
The RA6790/GM Receiver mounts in a standard 19-inch (48.3 cm) equipment rack, and occupies 5.25 inches (13.33 cm) of vertical space, 19 inches (48.3 cm) wide, 18.5 inches (47 cm) deep and weighs 30 pounds (13.5 kg).
A rigid, die-cast full width chassis is used as the base for the main frame of the Receiver. Mounted within compartments on the underside of this chassis are the mixer boards and the frequency generation system. The input low pass filter, main IF/AF, optional ISB IF/AF and power supply modules aré located on the top surfaces of the die-cast chassis while the control and digital I/O modules are attached on the Receiver main frame. All modules are accessible for maintenance and can be removed or replaced using simple hand tools without the use of a soldering iron.
Manual controls and indicators for operation and monitoring of the Receiver are contained on the front panel while input/output jacks and connectors are provided on the rear panel. A PHONES jack, for audio connection to optional headphones, is contained on the front panel for convenient access. A primary power fuse is accessible from the rear panel.
Photos by Amateur Radio Station K7PI
I have only recently been introduced to the Racal RA6790/GM Mil receiver, but, I am sure glad that I took the "plunge". This receivers has the cleanest layout of any receiver I have seen. It is easy to use and has the "feel" that a Mil-spec radio should have. This particular receiver is in very good condition and was not abused during its initial service. The receiver has been updated with new LCD displays, backlight LED's, and keypads. It has the rare ISB module with separate ISB, LSB/USB filters. There are 7 filters total. It has been updated with the VLF/LF EPPROMS but I am looking for an LF-version of the A2 card.
I use this radio (and the R-390A) with a Sherwood SE-3 AM detector. I have heard people complain of poor sensitivity with these receivers but I have not encountered that problem. I occasionally use the pre-amp in my multi-coupler but most times it is unnecessary. Bottom line is that I like this receiver as much as any receiver I have used. It has very good audio for a solid state receiver
Top view showing A4, A5, A6A1, A6A2, and power supply
A4 card, front panel, and A6 micro-processor boards (note battery in top left corner of A6 card)
Filters on A4 board
Rare A5 ISB board also showing 9621 lightning protection
Bottom view showing A2, A3, A7, and A8 boards