MFJ-1708 SDR Problem

Discussion in 'General Technical Questions and Answers' started by F4HSQ, Mar 30, 2018.

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  1. F4HSQ

    F4HSQ Ham Member QRZ Page

    Hi all ,
    I have recently received this item in good conditions , but using it : problems :(

    First I installed and connected it correctly ( ic-706mk2g and SDRplay RSP2 and 12v vdc)

    Reception : when connected , reception falls from S9 to S1 on the SDR.

    Transmission : power input 4 watts , power output 200 mW ! ! !

    This big attenuation is abnormal , and i cannot use it.

    Have you an idea , a solution ?

    Many Thank's
  2. F4HSQ

    F4HSQ Ham Member QRZ Page

    I answer to myself :)
    A precision : tests have been done on VHF fréquencies.
    On HF , it's not catastrophic,but ...
    Conclusion : it's a rig of poor quality , made with an ordinary relay ( not a coaxial relay)
    If you want to destroy your SDR receiver , use it ! :mad:
  3. W5DXP

    W5DXP Ham Member QRZ Page

    I have an RTL-SDR V3 and it works very well. I run 30 watts output for PSK31. I accidentally turned on transmit while the SDR dongle was still connected to the transmission line. I chastised myself for such a blunder (not my first) and expected to have to spring for another $21 SDR dongle. To my complete surprise, the dongle was apparently not damaged and continues to function. I calculate that the dongle survived almost 40 volts on its input.
  4. F4HSQ

    F4HSQ Ham Member QRZ Page

    Thank you Cecil for your answer.
    The problem here is not the resistance of your dongle ( $20) ; my SDR RSP2 Play costs $200 ! ! ! and i want to avoid an accident ! ! !
    The very big problem is that this device has an enormous attenuation in VHF / UHF and is thus absolutely unusable on these frequencies , and that MFJ is silent about this important subject and limitation , that is not honest.
    MFJ quality ? perhaps 20 years ago.
    I emailed MFJ : no answer !
    This rig could be a little more expensive , and built with a real coaxial relay inside, and not with this basic relay without insulation HF
    Conclusion: for the price , do it yourself with radio components and a true coaxial relay !
    73's de F4HSQ
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    There is not much that can go wrong with it. Is the relay working ?

  6. F4HSQ

    F4HSQ Ham Member QRZ Page

    Thank you for your reply.
    The relay is working , but this relay is of very bad quality : it is a simple commutation relay , not a coaxial relay .
    The insertion loss is VERY important up 50 MHz.
    MFJ forgets to specify this important information : unusable on VHF/UHF.
    Other surprising point : 200 W PEP input : I don't dare ! ! ! !
    About schématic : MFJ-1708SDR has a strap , not mentionned .

    Attached Files:

  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    It does look like they made a change to the schematic.


    I seen no info about frequency range.
  8. F4HSQ

    F4HSQ Ham Member QRZ Page

    I asked a question on MFJ reviews website. about 1708SDR.
    Question 1 : frequency range ?
    Question 2 : insertion loss on vhf/uhf ?
    Wait and see ...:)
  9. K8JD

    K8JD Ham Member QRZ Page

    I could not find any mention of usable freq range in the 2 page info sheet !
  10. F4HSQ

    F4HSQ Ham Member QRZ Page

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