MFJ 1708 SDR T/R Switch and Yaesu 450D

Discussion in 'Software Defined Radio (SDR)' started by KF5NAJ, Dec 13, 2018.

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  1. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    We are trying to mute the sdr and the model is MFJ 1708B-SDR with the AUX contact
    The AUX contact did not work very well still got lots of noise thru the speaker

    How much current draw will the AUX contacts handle, I want to put a small 12v relay to break both the audio and ground to the speaker

    With a little more effort the could of made the MFJ1708B SDR a lot easier to use
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    It can handle enough to drive another small relay coil.

    Just disconnecting the RF does not give enough isolation. As you have noticed.

    A think the idea was just to keep from smoking the front end, For that it is fine.

    If you do want to disconnect the speaker, It may be a good idea to use a load resistor to protect the audio output. Some things do not like to see a wide open. Depends on the AF output design.

    Good Luck. Have Fun.
  3. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    SDR Play Speaker Problem Solved
    We are using a MFJ-1708BSDR, to solve the problem of feed back thru the speakers
    Using the AUX connection on the MFJ-1708BSDR to control a small 2 pole 12v relay to open the audio and speaker ground on transmit works great. You have to break both speaker and ground, breaking one does not seem to work. We are using RF Sense to control the MFJ-1708BSDR, you have to adjust the delay so you do not get slight audio at the end of the conversion or if you are a slow talker and it switches back to receive in the middle of talking.

    * Not sure how much current the AUX connection can handle or the 12v relay requirement so I used a small relay.
    * Getting the relay 12v from power supply not from the radio
    KA9JLM likes this.
  4. KF5NAJ

    KF5NAJ Ham Member QRZ Page


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