My MFJ-998 Review and Modification


Jeff SmithVE1ZAC†††† May 1, 2011


I recently invited a solid state amp to join my radio village ( an ALS-1300). Great amp, but I also figured I needed an auto tuner for it. I use some reactive antennas ( windom, Marconi version of it, a few others) besides a Steppir so an auto tuner that can handle the full power is a nice addition. There are amps that have both of these features in one box, notably the SBE amp from Italy ( a very nice package !), but the Ameritron/MFJ combination is quite a bit cheaper.


The amp arrived and is everything advertised. It has a non-embedded CPU control scheme, so it is not so bad to diagnose and repair faults. The fit and finish look fine and the boardmanufacture looks very good quality. You can get an amp for less investment but one that requires manual adjustments on every band. The solid state amps are almost instant on and all have rapid band changing.


The MFJ tuner is really the subject of this article. For the price, it offers a lot. 1500 watt capability, a built in radio interface, ability to route the amp key line through it to keep the amp offline during tune cycles. Mine arrived and I have my comments divided to proís and cons.


Cons first:


Shipping Packaging.. awful. Itís a miracle this thing arrived in any condition. It was dropped in a box with some loose Styrofoam popcorn ( totally inadequate amount) and the rear ceramic antenna post had broken the bag and penetrated the box in several locations ! This is just plain sloppy packaging. I canít begin to imagine what this attention to shipping might be costing MFJ. Compare this to the shipping package for the Ameritron ALS1300 which is extraordinarily good !


Paint.. awful. While I donít expect high end packaging from MFJ products, you would think anybody could paintsimple chassis parts properly. This one should have been rejected and not used.The cover is so bad I am going to give it a rub andquick spray with a black textured paint.. it really needs it. The panel is barely acceptable, but you can see raw aluminium in the meter window, around edges, etc.


Output meter.. borderline useless and is the subject of this improvement.




Board.. looks very well made. Soldering looks good. A thumbs up !.


Function.. the unit performs as advertised. I really wanted to use the radio interface to allow one button tuning, but since I am using this with a high end Icom transceiver, that isnít possible. Icom decided to get rid of their ubiquitous and successful remote tuner interface on the IC 7700 and 7800 for reasons known only to them. I dealt with that problem with two solutions, and you can see a link to another article about it on my website. If the radio interface is offÖ you have to be SURE you have the amp offline and reduce exciter drive before you engage the tuning cycle. I found about 15-20 watts of AM does an excellent job. And, the adaptive memory feature really does aid in returns to the same frequency. When working with the radio interface, it functions as advertised, but I am still learning some of itís foibles. It gets stuck on a memorized position sometime and doesnít think it needs to re Ėadjust, even when I want it to. Not a serious complaint though, and probably just needs a little more familiarity on my part.


Manual.. itís adequate. A bit cheaply printed. But you can download it and print it yourself. Has all the information you would need for any feature. It needs to be read several times to get an idea how the various options work and takes some time to understand which ones you need for your setup. There is no schematic, which is a negative. Turned out the new meter circuit was obvious to interface, but the schematic would be nice to have. MFJ is normally really good about schematics leading me to wonder whatís up with this one. More changes coming ?


Price.. for what this thing costs and delivers in functionality, it is great value. In my case the real complaints come down to horrible painting and an extremely limited meter function.. both items that are easy to fix.


With that, letís get to the meter problem:


This thing comes with the typical cheapy crossed needle MFJ SWR/watt meter. I have had these on MFJ products in the past and found them next to useless for setting up the tuner and monitoring whatís going on with your signal. However, they can be greatly improved by adding a peak hold circuit. Some of the newer MFJ/Ameritron products have this feature, but the 998 does not. And it should.. it is needed. There is a peak display circuit built into the digital display circuitry but itís response time is a bit slow and I often donít see it catching up to reality until a few CW words have left the rig.


I have built some peak hold circuits for these meters in the past, so I simply dug out my notes and made another one on a new board for the 998. It has two circuits, one for the forward and one for reflected meter movements. Since the power is easy to get from the unit, no power sensing battery device is needed. And, there is plenty of room to put the board just behind the meter. Interface is a snap.. you literally insert the board between the existing meter leads and the 998. I checked mine out against an Autek computing wattmeter that is reasonably calibrated with a scope and dummy load. All good now.


Here is the circuit and the board:

Almost any op amp will work. I used a 324 but fet input op amp packages are fine. There is nothing critical about the circuit, it is basically DC. Any kind of construction will work, even dead bug.


The board layout I used:



The finished board


Mine slips into the space behind the meter just right, and I used trim pots allowing easy adjustment with the cover off.


The meter space.. itís easy to get at with cover off.


Here it is in place




Readings with one dit !



Adjustment was easy. I set the hold time up to match the hold time on the Autek meter, which I like using. With the meters in the amp, this 998 meter is just an extra health monitor, and the Autek unit isnít needed at the radio position anymore.


The peak meter circuit is great improvement to the 998 ! I highly recommend it. In fact, you can use the same circuit with any MFJ dual needle meter and it is not a difficult circuit to build.


Results: the peak meter adapter and spray paint touch up to the cover, and this unit is a welcome addition to the shack. Overall, I am pleased with what I have to work with, considering the bargain price.


BTW, just to set the record straight, I am dead set against using the word tuner for these gadgets. They are matching networks to convert a complex impedance at the feed line to something the transmitter (or amplifier PA) is happy to deal with.