Friday 18 March 2022

Repairing a MFJ-1786 magnetic loop

I just wired up my MFJ-1786 magnetic loop, which hasn't been used for years. Unfortunately, it looked like the loop controller had gone U/S. 

The meter lit up, but pressing the loop buttons had virtually no effect. The amber lights flicked slightly, but they were not so bright as I remember them, and the motor didn't turn. I got hold of a schematic, but it is way more complex than I thought it would be. I first changed the only electrolytic capacitor in the unit as it was getting quite old, but it made no difference. 

I then stripped the antenna and made sure it was all OK. Connecting a 9V battery made the motor turn, so I sprayed the motor with grease and put it all back together. Turning to the controller again I first changed Q1, but that made no difference. Then I changed Q5, but nope! So I moved on to the U5 78L12 voltage regulator, which is fiddly job as it is an 8pin SMD. 

And low and behold it works! I just need to fix the power meter, which displays reflected power, but not forward, but that can wait until another day. Then finally I can mount it outside for final testing. I've attached the MFJ-1786 schematic as this was quite hard to track down - might come in handy for someone in the future.  Click on the image above to see a larger version.


Well, it wasn't a diode that was stopping the meter from working, just a poor connection. 

So I now have some spare diodes for the future! I've uploaded a higher resolution version of the schematic too.

Steve G0KYA


  1. As of today, the remote controller of my 1788X no longer controls slowly up or down, the red up LED is constantly lit - fast still works.
    Thank you very much for the wiring diagram.
    73 Juergen DL4JR

  2. Hello All

    Likewise thanks for sharing the circuit diagram.
    My controller works on slow but not fast!
    On slow the motor sees about 5v, but on fast it only sees about 2v.
    However if I disconnect the antenna I get 9v on the antenna output terminals???
    This sound like it should be about fight for faster turning but I don’t know why it drops to 2v when the controller is connected.
    Any advise would be appreciated!!

  3. Hello Juergen

    I have the same problem with Fast tune….Slow works fine….did you have any success in finding the problem?