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.

Update


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

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