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

    I'm trying to to connect a MFJ 1708 SDR to a 450D, plan to use a SDRPlay

    Yaesu 450D Pin #2 of the 10-pin DIN jack labeled "LINEAR" it pulls to ground on transmit and can provide up to 400mA of switching current

    Is this correct wiring:
    Yaesu 10-pin DIN Linear plug
    Pin#1 (+13.8 V Out) to the center of the RCA plug on the 1708 SDR and Pin#2 (ground) to the shield of the RCA plug

    I'm helping a friend and want to be sure

    Thank you
  2. AC2QH

    AC2QH Ham Member QRZ Page

    NO. I do not think you should take the +13V line from pin 1 of the LINEAR jack.

    Pin 2 is an open collector and I would connect that to the PTT input jack (center pin of the RCA jack) and connect the ground on the RCA to pin 3 - the rig's ground. The voltage to power the relay is supplied from the power supply connected to the MFJ 1708.

    BTW the same connections are available on the TUNER jack which is an 8 pin DIN plug which might be easier to find than the special 10 pin on the LINEAR jack.

    I have been thinking about using a relay to disconnect a separate antenna and ground it on transmit for using a SDR dongle with my FT450D. So my plan is to take pin 1 +13V and send it to a small relay coil and connect the other end of the relay coil to pin 2 and use the open collector switch to energize the relay.

    I hope this is helpful. Let me know how it works.
  3. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    The MFJ 1708 is still on back order, will let you know

    Thank you
  4. AC2QH

    AC2QH Ham Member QRZ Page

    I opted to buy a MFJ1708 and it came today. I used pin 2 on the Yaesu FT450D "Tuner" 8 pin DIN connected to the RCA center pin on the MFJ switch (labeled PTT) and pin 3 on the Yaesu to the RCA ground. Works fine. I am using it to disconnect a separate receive antenna so I am leaving the "Radio" transmitter connection not connected. I am using the SDRplay 2 pro version. BTW I found an old Apple printer cable in my junk box that has the 8 pin DIN connectors on both ends and I cut it in half to make the PTT cable.

    Here is the Yaesu schematic showing the pin outs. The top connector in the print is the "Tuner" jack. Pin 2 on the 8 pin also goes to pin 2 on the 10pin. They get pulled to ground when in transmit. You could also get +13VDC from pin 1 if you want to use the rig to power the MFJ1708 switch. The FT450D has a 0.5A polyfuse in the circuit for protection.

    Screen shot 2019-03-08 at 10.27.33 AM.png
  5. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    Thank you for the wiring help, we have not received our MFJ1708 yet but this will help

    Did you order yours from MFJ
  6. AC2QH

    AC2QH Ham Member QRZ Page

    Yes, direct from MFJ
  7. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    Mute Audio on a MFJ1708SDR

    How do you mute a remote speaker when transmitting
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    For what radio ?

    That may matter a bit.

    Have fun.
  9. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    Yaesu 450D
    We have it switching Transmit / Receive
    It looks like you use the aux connection
  10. KA9JLM

    KA9JLM Ham Member QRZ Page


    Are you trying to mute the audio on the 450D or the SDR ?

    Is your radio modified for the SDR IF tap ?

    It sounds like you want to mute the SDR, But that may not work using the radio to mute it.

    The MFJ has / did have a dry pair relay that you can use to disconnect the SDR audio output. What is the version of your MFJ ?

    Have Fun, It can be done. :)
    Last edited: Mar 16, 2019

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