In The Loop! – Repairing a MFJ-1788

Over the last few years I've seen various musings about magnetic loop antennas and their supposed efficiency and performance. I had even toyed with the idea of building one considering the eye-watering prices commercial ones are sold for! Sadly that idea had been added to the big pile of 'to-do' projects.

Earlier this year I spotted a posting on social media by a member of my club who had brought a MFJ-1788 second hand who was having issues with it not working. I had offered some advice on its repair and glibly offered to take it off his hands should he want to dispose of it. A few months later I got a message asking if I was still interested? I certainly was at the price he wanted.

I collect it at the club meeting a fortnight ago still with a 5ft pole still attached as a bonus! Thankfully it fitted in the car (just).

Loop as purchased, with pvc tape covering loop

Control Box

I downloaded the manual and schematic from the MFJ website and I saw there were conspicuous warnings about using an 'isolated' power supply both in the manual and on the back of the controller, with ominous warnings of damage if you didn't. Most shack supplies have the negative/black pin connected to the chassis/ground.

The controller had come to me supplied with a standard fused lead, you know the ones that come to connect your ATU/SWR meter lamp to the shack supply? Mmmm.. my spider sense was tingling!

I pulled out a small double-insulated 12V plug-in supply from my collection (you can spot them as they often have a plastic earth pin) and with nothing else connected powered up the controller. The meter lamp came on and some of the LEDS briefly flickered and heard a few clicks from the internal buzzer then nothing. Pressing buttons did nothing and then I caught the unmistakable scent of burning electronics!

Opening the box up on the bench quickly spotted the source, the regular had well and truly smoked, but since nothing had been connected the short must have been in the controller itself (and it was not the dodgy wires that look like they had been victims to a wayward soldering iron)

Inspecting the rest of the board it was clear that it had been repaired (badly) once before, the two main transistors/FETs (Q1/Q5) used to control the motor clearly showed signs of being replaced (misaligned and with tell-tale scorching from an hot air gun)

a SMD diode had been swapped and one of the 'fine tune' switches had been replaced, its removal had obviously been problematic taking with it some of the through hole copper and adjacent tracks which had been lifted/damaged.


Before opening up the box I had expected to see 'through hole' components and DIL logic ICs not for it to be all surface mounted. Undaunted I went through the parts list and decided to get replacements for all the semiconductor parts not knowing at this stage what I would have to replace. Getting two of each device in some cases five or ten of each due to minimum order quantity, the staggering cost was £8 ($11) including postage!

While waiting for the parts to arrive I decided to check out the actual antenna head. I removed the plastic covering and attempted to extract it from the pole..

First issue I had was the mounting. One bolt had seized, the nut inserts are held in quite soft aluminium so not surprisingly the insert came out out the mounting when trying to remove it. Using grips to hold the insert the bolt still refused to turn and in the end it sheared off with very little force! Obviously quality fittings these! 

The variable capacitor was suffering a common problem due to poor quality control, the fins had gone out of alignment. Each fin is a separate assembly held on a shaft with a nut to compress them together. It is a simple fix, just realigned and tightened the bolt. The limit switches and motor connection were okay (another common problem) Applying a voltage to the coax connector the motor turned fine, reversing the voltage reversed the direction of turn, the vanes going fully in and fully out before the limit switches activated.

The components arrived prompty and so I got the hot air gun out and set about repairing the board.

I started off replacing the regulator (and the damaged power track) and Q1/Q5 and we had the flickering of life. Pressing the AUTO TUNE button the LED lit and there was 9V across the antenna connector, pressing the other AUTO TUNE button had -9V across. This is why you need an isolated power supply since it reverses the voltage to control direction. The radio doesn't see any of this voltage due to the bias-t arrangement. However if the supply shared the shack ground you would short out the supply, but the current would go via all those delicate electronics! Which is what I think had happened.

It soon became clear that most devices had suffered damage and so I ended up replacing the regulator 78L12, Q1 (SN7002 FET), Q2 (MMBT3906), Q5 (MMBT3906) all the logic ICs CMOS 4001, 4011 and 4066 and the LM324 Quad op-amp chip. I also gave the board a good clean since was covered in grime and flux residue.

Once I was happy the controller was reassembled and connected to my FT-857D on minimum power and the loop antenna propped up against the side of the shack. Success! I was able to successful tune it as per the instructions on 40m and 10m, the two extremes of operation and could here the telltale rise in receive noise as it became resonant.

I have yet to put the loop up in situ for a proper evaluation but have refurbished an old TV rotator to mount it on. I have fitted some nice new quality bolts. The black insulating tape round the loop has been removed, preferring the silver look myself.

Waiting to be put on the pole

Shiny once again

Assuming it does performs and will find out this weekend, I have paid a total of £65 ($90) (£50 for the initial purchase the rest for replacement parts) which is an absolute bargain as to buy new it is expensive.. very expensive!

One of the main supplier in the UK, have it on sale for £699 ($940) but this includes their engineers inspecting and rebuilding each one before shipping! (Can only be due to poor build quality control and warranty claims) the RRP seems to be around £570 ($770)

It does amaze me how much this units costs new. While the loop construction is generally good, the mounting fittings appear to be poor and the issues with the capacitor and quality control are widely reported.

The components in the controller are not expensive, the switches, meter, box are the usual MFJ fare and the design is quite old (the PCB has copyright 1998 on the silk screen) the auto-tune process requires the radio to be putting RF into a mismatched load for up to a minute and even with low power and SWR protection this isn't good for the radio PA.

Perhaps most surprising is given the risk of damage due to incorrect use is why firstly the loop isn't supplied with a power supply anyway as they only cost a pittance and secondary why their isn't any protection built in? I intend fitting a simple 100mA fuse to offer some protection should a problem occurs.  

If the loop performs it may be another project to build a better controller, there are a few designs out there on the internet using Arduino and DDS devices to create auto-tuners. Has that pile of potential projects just grows taller?

73 for now.. and promise to post a bit more regularly 
Andrew Garratt, MØNRD, is a regular contributor to and writes from East Midlands, England. Contact him at [email protected].

15 Responses to “In The Loop! – Repairing a MFJ-1788”

  • Harry K4BAD:

    Thanks for detailed report.Absolutely jaw dropping!

  • Mike N5MAJ:

    Great article, and a good find on your part!

  • Bryan M0IHY:

    Well done on your bargain Andrew, I’ve just purchased a 1786X (10MHz – 30MHz) at a cost of just over £531, the power supply had to be purchased after realizing it didn’t come as part of the kit at a further cost of over £21 including postage.

    Great article, you must let us know how you get on with it.

  • Dallas; KD9BIF:

    Great article, and we’ll described! This of course strengthens my resolve to not buy much of anything MFJ. The only piece I have is a used 259 analyzer, and it needed a little attention inside to correct some poor original wiring and solder work. I’m very interested in an active loop antenna, so report back with any further work and/or developments!


    Dallas KD9BIF

  • Larry WB8LBZ:

    Any thoughts of upgrading the antenna with a vacuum variable?

    73, Larry WB8LBZ

  • Richard,K0RDS:

    Great job on rebuilding the unit. Hope it will perform for you good! I know new one’s are way to much to invest in. Again great article. 73, Richard, K0RDS

  • walt n5eqy:

    I purchased the large loop tuner from MFJ. Then purchased a long 1 inch roll of copper tubing to make the loop. Cut the loop to specified length, mounted it to the controller/tuner box. Total failure. Sent the controller back, got refund and bought a large capacitor and a reduction gear with isolation mounts. Bingo, works great, but I only use a couple watts so as not to fry whats left of my ancient brain.

  • Mike VE3WDM:

    I have the 1788 loop for the past 5 years and being in a condo it has severed me very well. I did have a few minor issues when it arrived from MFJ but nothing I could not fix myself. You post was excellent with how you were able to trouble shoot and get the loop functional once again. Thanks for the detailed post and do post again with how the loop preforms for you.

  • Edward McCann AG6CX:

    Great article, an accurate reflection of my experience with various MFJ kit. I await operational results.

    But more importantly, the notion of “isolated” power supplies should be core to your next article. On the Elecraft Reflecttor recently there raged such a set of opinions on paint scraping and fixing the black wire to chassis in an Astron Power Supply that the site moderator shut down the discussion, with effectively strong opinions on both sides of the question of grounding or not grounding the negative side of the 12 volt supply to the chassis.

    How about expanding your comments on isolated vs non-isolated power supplies, with some practical advice on the matter?


  • Elwood Downey, WB0OEW:

    FYI, my loop tuner is documented at The Arduimo code is for Electraft radios but it only uses a few commands which could be changed for a different protocol.

  • David WB4ONA:

    Yet another article about how bad MFJ’s quality still is – after all these years. Why won’t they learn? Because there are enough Hams out there that keep paying too much for the MFJ garbage? Probably, I can’t come up with another reason.

  • AI6XS:

    Well done nice piece of work

  • Rob KB6CUN:

    Does it come with coax, and if so, length? thanks.

  • James Graham KK6UFE:

    I’m concerned that the mast connection bolts screw directly into a metal plate which is welded to the antenna radiator itself. This puts the antenna mast, tripod, etc. into direct contact with the antenna. Won’t this affect performance of the antenna, and cause the mast to affect the ability of the loop to radiate? Thanks!

  • Bill wa2wl:

    how did you repair the broken bolt? I have the same problem.

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