[ HOME ] [ Linux ] [ HAM Radio ] [ RAS ] [ Networks ] [ PC Hardware ] [ ISDN ] [ Cablemodems ] [ Eval ISPs ] [ Security ] [ Trinity Designs ] [ Who am I ] [ Bookmarks ] [ ChangesLog ] [ Email me ]

MFJ 969 Repair - A MFJ-969 Roller Inductor Antenna Tuner teardown and repair

The MFJ 969 Roller Inductor Antenna tuner shown at the top of the picture above is a modestly priced tuner that promises a lot of features for a minimal cost:

The Problem:

My Alpha Delta DX-CC antenna has a LOUSY match for 75meters (SWR of 25:1) but I have a local weekly Net I like to check into. I've been using the MFJ-969 tuner for probably 1.5yrs with good signal reports running at 100w. Fairly recently, people have been commenting that my signal strength would go from S9+20 to S6 and back. I then started noticing that my FT950 would be flashing "HIGH SWR" though I just tuned for the band on the 969. I then noticed that if I would just move the roller inductor a bit, the SWR issue would go away but only for a few moments and it would come back. The other thing that I realize now on hindsight is that a few months ago, I heard some "sizzling" sounds from the tuner when I was on the net but things were still working so I didn't think much of it. Now I realize what was happening: ARCing.

A fellow HAM on this 75m net mentioned that he too once had a MFJ-969 and also lost the final PAs amplifiers on several HF radios until he figured out what was happening. His tuner was silently going high SWR on him like mine but he didn't have radios that could foldback their power to protect themselves. He specifically recommended that if I liked my radio, to ditch the 969 tuner and get something better made. This HAM has been around a LOT longer than I and the recommendation was a sound one.

The Investigation:

Since the antenna tuner was out of warranty and I was very worried about damaging the radio, I either needed to fix it or replace it. I thus proceeded to systematically tear it apart and I put together this page in the hopes that it will help other HAMs.

Required Tools:

Below is the inside of a MFJ-969 with various comments of what I found:

Here are two pictures of the inductor shortening system when tuning to higher frequencies. The first picture shows the tuner in a low roller-inductor "count" configuration and all coils are enabled (maximum inductance) as the coil contact is lifted up. Also notice the rear post connected to this post is FULLY engaged with copper contact.

Here is the second picture showing the inductor bypass being enabled in a high roller-inductor count (lower inductance). DUMB #1Notice that though the coil contact is engaged (all the way down), the rear post contact is only conducting via the EDGE of the post!! This strikes me as backwards as it would best to have the post fully engauged if the coil contact is also enguaged. You'll also notice that the materials are all mostly aluminum which aren't great conductors. MFJ: -2 points

To open up the roller assembly, I would recommend the following steps:

NOTE: This procedure will take TIME and even more patience. Go slowly but I bet the average HAM won't have any issues with this

When re-assembling the entire tuner, it's KEY you do it exactly in the reverse order as shown above or you'll have nothing but problems. A few notes:

Root Cause and Thoughts: If you didn't notice from all this.. the RF connection in this very elaborate setup is the following:


The MFJ 969 tuner went back together well enough and is once again working without any intermittent SWR issues. The real question is: For how long? Dunno. For now, I plan on running it only at 50watts on 75m. Did using steel wool really do much damage when there was already so much play in the roller to sub-shaft tolerances? I doubt it.

There are other roller inductor tuner manufacturers out there and their designs are both 2x the cost but also seemingly a more simple design. For example, Palstar #1, #2, and #3

I hope this helped you fix your 969 tuner but beware... it's an MFJ and you probably know what those letters stands for. This page only highlights the issues I personally saw. Check out the http://www.eham.com reviews as well. Probably 50% of those reviews are happy with the tuner but the other 50% absolutely hate it. Seems your mileage with this brand really depends on how well it was made that given day. If the technician was hung over, etc. you're out of luck.


Last Updated: 08/24/11
[ HOME ] [ Linux ] [ HAM Radio ] [ RAS ] [ Networks ] [ PC Hardware ] [ ISDN ] [ Cablemodems ] [ Eval ISPs ] [ Security ] [ Trinity Designs ] [ Who am I ] [ Bookmarks ] [ ChangesLog ] [ Email me ]