Monday, Oct. 15, 2018 — Today I had the chance to go inside my the MFJ-989B antenna tuner I recently purchased cheaply that needed help. The meter had come dislodged, and the drive belt that connected the roller inductor shaft to the turns counter was broken.
I wasn’t really aware of this, but the MFJ 989 had more than the original, B, C and D versions.
Both the B and C models I have resemble the one in the accompanying photo — the balun is mounted flat on the chassis of the tuner at the rear of the roller inductor.. As a result, the tuner case is very deep; at some point in the run of the “C” model, MFJ mounted the balun vertically at the rear of the chassis. For the “D” model however, MFJ changed the variable caps; they are larger in diameter, but shorter in length; this allowed the tuner to be much shorter, front to back.
You can see the thin rubber drive belt on the white disc near the front of the tuner. I should have taken photos or a video of the process I used to replace the belt because I have read some YouTube comments mentioning how difficult it will be to replace that belt.
Replacing it was simple, really. I had to remove the top and bottom case. I removed the knob on the roller inductor, the removed the four nuts and bolts that secured it to the floor of the chassis. The only other thing I had to remove was the balun, which is mounted to the floor of the chassis with a single nut and bolt.
It was necessary to removed the bolt securing the balon to allow the roller assembly to slide backward far enough for the front end of the shaft to be accessible from inside the tuner. Once the inductor was far enough back, I could slide the belt over the shaft.
After that, the tricky part was keeping the belt on the groove of the drive wheel on the roller inductor shaft while also stretching the belt are enough to go over the smaller shaft that drives the turns counter. After several tries, I finally figured out that I had to put the belt on both while the roller inductor was slide rearward, and then hold the belt in place while the inductor was moved forward to its original place.
Once that was done, I put the case back on and the job was done. I haven’t tried the tuner out yet, maybe tomorrow.
I’ll report back when I have an update.