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G0KYA's Amateur Radio Blog

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Repairing a MFJ-1786 magnetic loop

Posted: 18 Mar 2022 01:33 PM PDT http://g0kya.blogspot.com/2022/03/repairing-mfj-1786-magnetic-loop.html


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.
Steve G0KYA

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