MFJ-8100K looking for somone who has built this kit

Discussion in 'Homebrew and Kit Projects' started by STDYIN, Apr 13, 2020.

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    STDYIN QRZ Member

    as ive got some questions
    and guidance

    1-Im missing my L-2 3.3uh and its going to be a month until i get the one ordered :(
    a-can i just jump it? temporarily (probably a bad idea right?)
    b-will the radio still receive on any of the bands without it ?
    2-the Band Switch
    theirs 8 legs, BUT only 7 to be soldered into the board
    as your looking at the knob/front I bent the far right leg up leaving the seven to the left-and soldered it in
    the band switch board fits within the lines on the main board,
    I saw a vid on utube where he did the opposite and bent the far left pin and soldered it up
    his band switch board overhung the main board to the left

    did i solder it in wrong?

    I marked on the board the line they tell you to turn the pot and line up to
    the pot has no hash, seems it can be moved with a small screwdriver at 90 deg inserted either way
    so i done messed it up,
    gently turning it all the way down OR up can someone give me a reference of a starting point
    a-I think but am not sure this is how its supposed to line up with the line under it
    Please correct me if im wrong

    Toroid Torture
    the way they describe wrapping this is so NOT like anything ive seen on utube,
    utube says a full wrap is counted on the inside of the toriod

    I have 7 turns counted by the complete inside wraps, with 1 input/ output half wraps
    is this correct ?

    Thank you for any help
    stay safe

    STDYIN QRZ Member

    so MFJ is sending me my missing knob nut and my L2 and another C5
    he did tell me that i CAN turn it on and try the bands, the only band that wont work is the one on L-2
    so now ive received my sonny walkman headset in the mail, I plug it in
    got through there setting up
    and all i get is a soft hiss,
    changing bands, tuning the dial, no difference, IF i go to touch/move the regeneration adjuster
    it starts crackling and popping, i cannot get it to squeel

    If by using the wrong speaker jack as i probably did with my iphone headset
    it says it can burn something out,

    would that be the IC386 chip?
    my volume is very low, with the volume knob all the way up untill you touch that regen-adjuster then it pops and crackles really loud

    really need some help here,

  3. N7EKU

    N7EKU Ham Member QRZ Page

    Hi John,

    For the toroid winding, the number of turns is the number of times the wire goes through the inside. Your wire goes through the inside nine times so you have one too many turns.

    That not L5 but C5. It's not a pot but a trimmer capacitor. If you get a magnifying glass, you can see the cross inside has one part with cut off corners making that one part look kind of like an arrowhead. That is the pointer. The trimmer cap turns 360 degress, but the adjustment range goes from high to low and back to high in one full turn. The flat side marks one side of the range and the opposite side is the other end of the range. You can turn it either way to go from high to low or vice versa.

    The band switch may be your problem. I can't tell from you pictures, but it could be that the pin you bent up is the main pole of the switch (this pin is connected to each other pin in turn as you rotate the switch. You might have to desolder the switch and test the connections with an ohmmeter or you may be able to check it in place by connecting one meter lead to the pin you bent up and see if it makes a connecter with each pin as your rotate the switch.

    Some headphone jacks only work with stereo headphones, some with mono, and some with both. The manual should tell you what kind it will work with.


    KI4ZUQ and STDYIN like this.

    STDYIN QRZ Member

    THANK YOU Mark, very much
    soon as i get my coffee ill start de soldering and adjusting :)

    STDYIN QRZ Member

    so is the Manual wrong? heres a pic of there Toriod wrap
    not saying your wrong, but more that the manual is wrong as the whole discription is misleading
  6. N7EKU

    N7EKU Ham Member QRZ Page


    Looking at that manual page, I'd say you have it wound correctly. The manual is wrong in how they describe the number of turns -- no one else describes them like this. It is always the number of times the wire goes through the center; they say it's 8, everyone else would say it's 9! Anyway, that inductor is used on all bands, so if they all seem slightly out of range, you could take a turn off later -- no big deal for now.

    With a strong light, you may be able to tell which pin has the pole position without desoldering. You would want to look and follow from the pin to the PCB trace to the wiper connection. The pole wiper should be longer and connect to a ring on the switch trace, while the throw positions should have short wipers.



    STDYIN QRZ Member

    Mark thank you,
    so looking at the back of the band switch with a light shining through it,
    I have the far right pin on the outside circumference of the switch 4 empty holes 5th makes up the first pin headed left
    looking at the opposite side/pin/ that i bent outwards 4 empty holes, 5th makes up the 1st pin headed right
    it shows it going into the outside circumference also
    im not the brightest crayon in the box but my lines are getting so confunded :)

    A-MFJ guys have said my R20 shouldent be crackle and popping or sticking when i try and adjust it,
    there sending me a new one,
    B- they are sending me another 386 chip as i DID stick the wrong headset in the jack, and maybe? toasted it?
    as per whats in the manual "If a mono 1/8 plug is used for any reason, It MUST NOT be pushed all the way in,
    or it will short out the audio.
    I thunk I did done that :(

    as they also told me my volume on loud isnt loud enough as its just a low hiss/seashell with using
    the just yesterday received 3 banded-plug headset thats supposed to be used with this not the 2 banded or 4 banded plugs

    all im getting is a low seashell noise, with no change when i shift the band switch,
    but if i touch that R20 itl send a crackle and pop that would wake up the dead and makes my ears ring

    I did get in close to C5 with a set of magnifying eyeglasses i used to wear when soldering smd chips, and now get it :) thank you

    so as MFJ told me i still should be able to tune "something" in even without the missing L2
    im now on hold until my missing/replacement parts get here

    ill be soldering in the L2 and testing before going to the R20 then onto the IC chip last

    they did say i could send it to them to repair at 17 dollars an hour,
    and im figuring by the time there done laughing, having coffie, and hooking it up to there
    fancy equipment, and fixing + postage both ways
    it id be in it for more than buying 2 new Non-kit versions
    and where's the fun in that

    I again thank you for your kindness in helping me,
    ill be posting again on it as soon as i know more

  8. N7EKU

    N7EKU Ham Member QRZ Page

    Hi John,

    Being stuck inside a lot now I have lots of time to help!

    It's hard to tell from your description of the switch -- it's hard to write about that kind of thing! Maybe you could post a good picture here.

    The LM386 has no output short protection so it may be gone -- it kind of depends on how long it was shorted out I think.

    The inductors are all in series so the first two bands won't work without L2 in place. Bands C, D, and E should work. Make sure you have inductors for L1-4, they do look similar to resistors!



    STDYIN QRZ Member

    yes, the ONLY resistor is in R4 :)
    heres two pics, sorry for the quality, and focus

    top one shows the solder joints pritty good
    second one shows the trace lines good

    4 empty holes on each side (left side covered up my the duhickie :)
    the 1st soldered hole on the left side is the leg i lifted away from the main board as theres only 7 holes to mount in
    notice the band board lines up with the markings (white drawn ) on the main board, hard to see i know
  10. N7EKU

    N7EKU Ham Member QRZ Page


    The pictures ended up not being too helpful because the switch is a bit odd for the appication (looks like a muti-pole switch when they only need one pole) and we can't see inside the switch (on some of them you can). Anyway, looking at the schematic we may be able to do a quick check without desoldering.

    Looking at the schematic, the switch is sitting in position "A". As it is turned, is shorts to ground each of the inductors in turn. In position "A", nothing is shorted. So if you measure across L1 with an ohmmeter, it should read the resistance of L1 (probably just around a couple ohms). Then if you move the switch to position "B" it should short out L1, and the meter should then read zero ohms (or very low as it depends on your meter- short the test leads together to see what you get). As another check, if you put one test lead to ground and the other on L3 it should give a high resistance reading on A or B, but that should go to zero if you move the switch to C, D, or E. Make sure the battery is taken out for these tests.



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