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N7DCC - burned, is it possible to fix?

Discussion in 'General Technical Questions and Answers' started by SP6F, Sep 21, 2021.

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  1. SP6F

    SP6F Ham Member QRZ Page


    I've burned my tuner during last SOTA activation, because of hurry and short circuit of wires from battery. It was short, but I saw a spark.

    For lucky, I can power it on and it reacts, but on a screen there are so small values of PWD (I use 818 in AM mode - it is 2W). It looks like this: (PTT 3,4 times with AM during movie).

    Is there anybody who had similar problem and solved it?

    Thank you for any advice
    73 de SP6F
  2. UT7UX

    UT7UX Ham Member QRZ Page

    You seem have Chinese built, right? I've got similar w/o internal battery (I don't need it). Luckily mine still works so I have no experience with similar fault. I only have to say I disappointed with its power handling: with 100W it overheats almost instantly and drifts (50W from my 50W TRX seems to be OK).

    If the tuner powers on I'd suppose the controller is powered normally as well as LED TV. So you seem have to check the trace from directional coupler to controller's ADC - forward and reflected power is all the controller knows. If the coupler works normally and voltage goes to the controller then it could be like the controller's ADC input(s) has been killed with battery accident so you have to swap the controller with a new one and flash it with the appropriate firmware. Worse case scenario: buy new kit for parts.
    Original N7DDC circuit could help you a lot as Chinese one is almost identical to N7DDC's.
  3. SP6F

    SP6F Ham Member QRZ Page

    Yes, Chinese one, without battery inside. Simple 7x7. My tuner works fine before incident. Of course, buy new one is the one of solution, but I'd like to fix. I think it should be possible and simple.
  4. UT7UX

    UT7UX Ham Member QRZ Page

    Then again, if the controller seems to be working (at least powers on and displays smth on the screen) but incorrect (or zero) forward/reflected power is displayed you seem have to check that part of circuit from directional coupler to the controller. If the appropriate voltage is present at controller's pins that probably means controller's ADC is dead and entire controller has to be swapped with a new one. If no appropriate voltage is present at controller's pins that probably mean some parts (most likely diodes) are faulty.

    Look at image I've found. The image is not clear but it looks like diodes are Schottky BAT41 or 1N5711. I'd check them.


    Do relays click? When powering on and off the tuner usually clicks with relays. Is HF path through the tuner correct? If a relay is dead you may get true reading of such low power at least at measurement point.
  5. SP6F

    SP6F Ham Member QRZ Page

    Hello again,

    Yes, relays click when tuner is powering on and off. What is more, my friend last night fix his tuner which had the same issue by replacing diodes to bat85 (he didn't have another). Probably you have right. But right now I have different question.

    Which diode I should to buy, bat41 or 1n5711 because they have different voltage and amperage.

    bat41: 70V i 15mA
    1n5711: 100V i 100mA

    I'm curios if on schema it means that you can take two the same diodes bat41, 1n57 or D1 should be bat41 and D2 should be 1n5711?
  6. UT7UX

    UT7UX Ham Member QRZ Page

    These diodes act similar to envelope detector. So any diode will work somehow. Schottky are preferred due lower voltage drop i.e. higher sensitivity or in other words ability to decode (rectify) lower voltages.

    Don't worry about voltage and amperage as directional coupler acts as a step-down transformer loaded with R1 and R2 respectively thus neither high HF voltage nor high HF current will be applied to diodes. Maximum rectified voltage must never exceed maximum input voltage on controller's pins (usually 5.0V or 3.3.V depending on controller) that is much lower than 70V for BAT41.

    I'd suppose the diodes should be equal i.e. should have similar current-voltage curve. Both BAT41 or both 1N5711. Still it'll work with mixed diodes but probably less precise: while software should compensate diode's current-voltage curve non-linearity to represent precise figures of forward and reflected power still I'm not sure it is done for any particular diode type so I'm not sure you have to care a lot about diodes similarity.

    Anyway, the lower is measured power the less precise figures will be due, again, current-voltage curve non-linearity that may be noticeable at lowest voltages and has almost no effect at higher voltages, and due low number of bits used by ADC to digitize low voltage.
  7. SP6F

    SP6F Ham Member QRZ Page

    Hello again,

    Replacing diodes fixed the tuner! Thanks a lot! I chose 1N5711 instead of bat41 (they were inside before accident).

    73 de SP6F
    UT7UX likes this.
  8. K8JD

    K8JD Ham Member QRZ Page

    ANOTHER reason I will be sticking by my old reliable manual tuners !!!!

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