MFJ-989D Tuner

turns counter adjustment, meter board shielding…

© 2018 by KV5R. All Rights Reserved. Rev. 2/9/2019.

Introduction

The MFJ-989D Versa Tuner V is MFJ’s largest manual tuner. It is rated at full US legal power, and features a roller inductor, active peak-reading watt-meters with LED lighting, antenna switch, dummy load, and a big 1:1 current balun. So it puts several station accessories in one big box.

MFJ is well-known for making ham radio accessories that are of reasonably good quality and price. They are also known for occasional loose parts and sundry other little mechanical problems. Regular purchasers expect to give things like antenna tuners a good internal inspection and, if needed, adjust and tighten a few things.

Turns Counter Slippage Repair

My new 989D tuner worked fine right out of the box, except for occasional slipping of the turns counter gears. After taking hours to build a tuning chart, I discovered the counter was off, even though I had zeroed it before starting. Bummer.

A while later, the meter circuit started misbehaving, and it took me two tries to get that one fixed.


MFJ-989D Versa-Tuner V.


Roll it fully counter-clockwise (don’t bang the stop) and check the counter. Pushing a paperclip through the little hole will zero it. Unfortunately on mine (and many others, according to the reviews) the plastic gears will slip when spinning the roller inductor, so the counter needed to be zeroed again. Unacceptable! If a counter isn’t reliably repeatable, it’s useless.


The plastic counter is mounted on this L bracket, along with the 9V battery holder.


The bracket was not bent a full 90 degrees, so the counter gear shaft is misaligned and the gears are not fully meshed. They are barely even touching! Can you imagine aligning and tightening those gears and not noticing that?


Pressing the bracket shows where it should be.


The big gear will need to be realigned to the new counter position. Loosen both set-screws using a long 1/16th-inch Allen wrench.


Okay. Take it out.


Remove counter from bracket. Note that one hole is slotted for adjustment. Note also that one screw is missing, as there’s no scuffing on the bracket.


Out to the workshop with it! The framing square shows the problem.


Put it in the vise and give it a couple whacks with drift pin and hammer.


That’s better!


Now let’s put just a bit of grease on the worm gear…


…and the big gear. Caution: some types of grease will melt nylon, so look it up and use the right kind. Silicone and lithium grease should be safe to use. Do NOT use any type petroleum grease, or a synthetic diester grease! Actually, plastic gears do not need lubrication, but I used it just to make them a little quieter and smoother. It did.


Reinstall the counter assembly. Do not tighten the counter-to-bracket mounting screws until the gears are properly meshed. Do not press the gears tightly together or tuning will be rough and bumpy. Just bring the gears together, fully meshed, and tighten the screws. Do not over-tighten! This is fine-thread screws into nylon.


Align the big gear centered on the small one and snug its set-screws. Roll the inductor around and test the feel. Adjust the counter position again if necessary. Wipe off any excess grease that squeezes out of the gears.


While testing the gear meshing, I noticed that the shaft wobbles a little too much. This flexing puts stress on the counter gears on the up-swing of the crank, and spreads the gears a bit on the down-swing. I slipped in a bit of heat-shrink tubing (not shrunk, it’s tight enough) and it took up most of the slop in the shaft bushing.

Extra Views

While it was open, I inspected the internals and make a photo record.


Here’s the dummy load resistor. It’s a 50 Ohm, 25 watt (continuous) non-inductive resistor, 300 watts max at short duration (30 seconds max, 2 minute cool-down). It seems rather silly to put a 300 watt dummy load in a 1500 watt tuner. The manual warns, never transmit into it with an amplifier! Buy an overpriced 1500 watt dummy load, or build a salt-water dummy load (see my Salt Water Dummy Load article).


Here’s the balun. I was very pleased to see it’s the stacked-core, bifilar-wound 1:1 current balun/choke that’s now recommended by various balun experts for ladder line. Yay. Perhaps the days of misusing 4:1 baluns (and the burning up thereof) for ladder line are finally over.


The Air-Core Roller Inductor. Frankly, I’d rather have a good ol’ ceramic-core, but I guess they’re no longer available…


One of the two giant capacitors. After putting it back together, looking at the pictures I noticed that this one needs a bit of adjustment, as the plates are not perfectly centered. And I did, after the next repair.


The Bruene directional coupler for the meters.


The metering board. Several people in the ham forums complained about the stupid placement of the board such that one of the calibration adjustment pots is right under the antenna switch shaft. Apparently they later put holes in the bottom of the case, and the board, so they may accessed from below.


The antenna switch uses two decks in parallel. Looks okay. The leads could be shorter.


A common problem with MFJ cabinets. One would think that after many thousands of such cabinets, they might adjust their metal-working machinery a bit, so that screw holes align and counter brackets are properly bent…

So how does it work?

It does what an antenna tuner should, and after correcting the counter bracket, is very repeatable when using a tuner chart. It has not slipped one bit since I fixed the bracket. I made a table of settings every 25kc across all the bands, and on my 80-meter doublet it tuned everything flat except a portion of 28 MHz, which I never use. A little change in feed line length will usually move a problem spot out of the band.

Later: RF Gets in the Metering Board

This problem didn’t show up at first, because with my 130-foot dipole the transmitter-side capacitor was at around 4 on the dial, and the capacitor plates were far away from the meter board. But after I cut my antenna down to 123 feet, the cap setting was 7-8, and its plates were very close to the board. What happens is that RF gets into the metering circuitry and causes an op-amp on the reflected side to gate full-on. The reflected meter pegs out, even though the power is only about 250 watts and the SWR is flat. Also, the forward meter goes to zero. I don’t know if this is caused by a marginally defective op-amp, or just a cold solder joint. I didn’t want to send it back to MFJ and wait months for warranty service, so I fixed it, finally, by simply shielding the meter board. But first I tried re-routing the cable going from the directional coupler to the meter board.

This is a difficult problem to troubleshoot because it only happens when the tuner is assembled, and it must be disassembled to get to the meter board.

The First Attempt


At about 250 watts,


WHACK! the reflected meter pegs out. I switched to another band where the capacitor tunes at a lower number, and indeed, the problem did not occur, proving that it’s the proximity of the capacitor’s plates to the meter board.


Removing the giant capacitor, I see the cable runs right under it, so I thought maybe re-routing it might cure the problem. Also, I noticed that the leads (white and black wires) run unshielded all the way across the board. UGH!


Remove the antenna switch shaft, then the meter board.


I decided to move the cable to the other edge of the board and shorten its unshielded leads.


There’s a handy ground point there for the shield.


I cut several inches of leads off and re-soldered them to the board.


Then I mounted the board and routed the cable along the side of the box, and taped it down with foil tape, so it wouldn’t be under the capacitor.


put the shaft and bread-slicer back in, solder the input wire and capacitors to it, aaaaaannnnnnd…


WHACK! No change. Wasted time and effort. RF must be getting directly into a component, not by the wires.

The Second Attempt

Discouraged, I put it back together and left it for awhile. Then a couple weeks later I decided to try again. It occurred to me that perhaps shielding the whole board might be effective.


Yes, it would be prettier with folded sheet aluminum, but I didn’t have any (much less a sheet metal brake), so I just used foil tape. I left the paper on the back to insulate it from the components.



It was quite difficult to re-mount the board with the cover in the way. I managed to get one corner under a stand-off, and the opposite corner under the leg of the capacitor. Hopefully, two grounds will be enough.


TA-DAAAAA! It works! There’s 800 with no whack.

MFJ really should shield that meter board… And fix their counter bracket bending machine!

What’s Next?

In the next article, I will attempt to document the building of my latest antenna, the 80 Meter Doublet! I built it super-heavy-duty and put it up to stay. Two continuous runs of #10 wire form the antenna and ~150 feet of home-brew 4-inch ladder line.

73, — KV5R

One thought on “MFJ-989D Tuner
  1. I had the same problem with the reflected wattmeter you did. I put in a ticket with MFJ and never heard back from them. It only happened with high power using balanced line. I thought the RF was getting in to the electronic wattmeter. I put caps across the power leads on the board but that didn’t work. I was glad to see your article. I fix the problem in the same way.

    Thank you very much for putting the your article on the internet.

    73,
    Steve
    K5DJH

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