Racal RA6793A Receiver

Front Panel view of Racal RA6793A Receiver

General

        This receiver was made by Racal USA from 1991.
        Keypad is rather hard key and have 100CH memories.
        Frequency range: 500Hz to 30MHz. Serial No.185

        Military version of RA6793A is R-2320/URR and have 2 phone Jacks.
        Different connectors for Power and Remote, have AF VR for ISB and phone.
          RA6793A-6(R-2320/URR) referred from Tront Surplus Scientific Inc. HP
 
  Microproccer
       Z80.
       Clock is 2MHz.
       ROM is only 1.

About circuit.

          RF circuit is the similar to RA6790/GM, so that pin-assignment of rear connector is exactly the same.
         This receiver employs upconversion system and no RF amplifier built in.
         1st IF 40.4555MHz, 2nd IF 455kHz.
         Input circuit has a LPF  from 0 to 30MHz and No preselector.
          Roofing Filter (Xtal filter) derives from two filters:  BW 20kHz and 25kHz at 40.455MHz.
         2nd IF Filters are either 7 Xtal filters or  Collins mechanical filters.
         My one has 5 filters: 0.4kHz CW, 1.2kHz RTTY, 3.2kHz SSB, 6.8kHz AM, 16kHz wide AM/FM.
         Automatic Band Width measurement displays 0.4, 1.5, 3.2, 7.9, 18kHz respectively.
         I built in 2.3 kHz BW Collins MF for SSB receiving.

           
                     Rear view                                                                        5 crystal filters

           
                    Bottom view                                                                   Bottom view cover removed

           
        Used an SD5000I DMOS Quad FET tip in the 1st Mixer(cover removed)                1st Mixer and LO Amp (A2 Board)
        Roofing filter of 1st Mixer is 20kHz BW.
        Frequency of 1st LO is 40.455MHz to 70.455MHz, the output is +11dBm.

2nd Mixer (A3 Board)
2nd Mixer is MC1496P and LO is 40MHz/-1dBm.
Roofing filter is 25kHz BW.

           
                   First LO Synthesizer (A7 Board)                                      Second LO/BFO Synthesizer (A8 Board)

        Synthesizer is base on PLL, and Ref. OSC is 5MHz TCXO.
        It can be locked by external Ref. OSC. Input is +2dBm.
        Have RS-232C unit.
        ISB is Option.
        AF output is 600ohm line and 8 ohm speaker.
        Dial step is 1Hz, 30Hz, 1kHz.

About Display.

        Right hand DISPLAY
        Frequency  Hz
        BFO          (CW MODE)+-8kHz
        Threshold level (0 to 150)
        Memory CH
        Left hand  DISPLAY
           RF, AF LEVEL    0 to 120 dBu RF/-10 to +10 dBm AF
           MODE             AM/ CW/ USB/ LSB/ FM/ ISB
           FILTER           BW1 to BW5 5filters(AM/ CW/ FM)
                               USB, LSB
           AGC              SHORT/ MED/ LONG
           AGC control    (MAN)

Front Panel description

                     

Left-hand keypad:
mode, meter,RF digital meter, manual,  AGC,
BW, scan, sweep, threshold, memory use

                                Right-hand keypad:
                                tune rate, lock, BFO, BFO center,
                                enter, local/remote,
                               10 keys for frequency, memory, BFO, squelch level

Specifications     Racal RA6793A Receiver

        Noise figure: Less than 15 dB.
        Intermodulation (Out-of-band): For signals 100 kHz or more from receiver tuned frequency.
        The third order intercept point is greater than +30 dBm.
         Second order intercept point is greater than +60 dBm.
        (Below 1.5 MHz these limits may be exceeded.)
        Intermodulation (in-band): Better than -50 dB for two -10 dBm input signals within
        the IF passband when measured at the IF or line AF output.
        Cross Modulation: The level of a 30% modulated signal, 50kHz off-tune necessary to
          cross modulated an on-tune carrier to a depth of 3 % shall be greater than +21 dBm (2.1 volts).

        Receiver Test Data by Sherwood Engineering(RA6790/GM)

• Noise Floor                                                            -128 dBm

• AGC Threshold                                                         0.3 uV 1 dB

• 100 kHz Blocking                                                      145 dB

• Sensitivity                                                               0.7 uV

• LO Noise Spacing                                                     130 dBc 10 kHz

• Front End Selectivity                                                Broadband

• Filter Ultimate                                                         85 dB

• Dynamic Range Wide Spaced                                     95 dB/20 kHz

• Dynamic Range Narrow Spaced                                  66 dB/  2 kHz
  

     The author conducted sensitivity testing of my receivers. December 2008
          Mode CW/BW=0.3kHz/AGC off/F=29.9MHz/BFO +650Hz/s+n/n 10dB used HP 8948A SSG and Shibasoku 796E.
          Result:
                    RA3701 -122.7dBm/-131.4dBm (RF amp off/on)
                    RA6790/GM -116.4dBm, RA6793A -118.3dBm
                    RF-590A -122.2dBm

Overall AF characteristic by Filters
     1. Racal receivers, if used symmetrical SSB filter, automatically sets it's BFO to +-1.8kHz apart from 455kHz.
     2. If you want to change to narrower BW filter, it gets worsened at lower audio response.
     3. I tried 2.3kHz BW replacing 3.2kHz BW filter, and resulted to shift its lower audio response to 500 to 600Hz
     4. It was made known that 3.2kHz and 2.8kHz BW symmetrical filters were only viable and that 2.3kHz BW is not fitted.
     5. Finally, I fixed filters to be 2.1kHz Collins MFs(USB,LSB) for SSB receiving.
  I think as crowded Amateur Radio communication, uses narrower bandwidth is useable like 2.1 or 2.4kHz BW.
  It's audio response are 300 to 2400Hz is readable. 

RF input: 28MHz. AF line output: -6dB point (Hz)

Impression

        1. 100channel memory is highly valuable.
        2. Necessary to change AGC time constant (Long) 3.75S to 1.5S.
        3 .Extended lower frequency range down to 500Hz!
        4. This RA6793A is fairly good, but RA3701 is far better in
           A. Sensitivity
           B. Multiple filters selectable for SSB reception.

Modification done

        1. Changed to external reference of 1MHz.
        2. Replaced both display LCDs.
        3. Converted AGC time constant. (Slow: 1.5S, Mid: 1S)
        4. Changed 3.2kHz symmetrical filter to 2.1kHz Collins Mechanical SSB filters (USB,LSB)
          Parts number: LSB 526-9939-010, USB 526-9897-010
           * In case 2.1kHz Collins Mechanical SSB filter, insert USB filter to FL1 location.
            In order to avoid inverse SSB.
        2.1kHz Collins Mechanical filters for SSB.

     MF and its loaded capacitors.
        From the manual of HARRIS RF-590
              3.2kHz       270pF LSB/USB
              0.3kHz       180pF
                 1kHz       110pF
              3.2kHz        360pF AM
              6.8kHz        750pF
               16kHz       1200pF

  Inside of 455kHz filters
　　all filters photo courtesy of JA2AEV
        inside of 16kHz filter, using LC circuit

        inside of 400Hz CW filter, using 6 crystals

        inside of SSB filter, using 16 crystals!

Inside of Kokusai 0.3kHz Mechanical filter.
Kokusai MF 455-03AZ

From Collins mechanical filter HP
Inside view of seven-resonator torsional mechanical filter.

 Links
eHam RA6793 review       

If you need manual of RA6793A   look "BAMA" site

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