I purchased this nice little 100 Watt CW or SSB PEP class antenna tuner at a swap meet as a gift to a friend, to use with his Yaesu FT-450. If he inherits my DX-40, it will likely be fine with that, or a barefoot DX-60 too. But I doubt it will handle a 100 Watt AM rig like the DX-100. Damage to the plastic coil may result with full duty cycle full carrier 100 Watt AM. The capacitors may arc. The internal tuner in the FT-450 will not match more than a 3:1. This manual tuner extends the range to work with an end fed wire, G5RV, open wire fed doublet, or the band edges of a standard 80 meter dipole.
MFJ likely would have advertised this tuner as a 300 Watt unit. Some similarities to the Johnson Matchbox JR 275 watt tuner can be drawn, and perhaps I am being too conservative in my speculation on power rating. Later versions of the Dentron unit shown here may be built for more power.
As I received this tuner, the left hand capacitor was wired as a sort of "voltage divider" similar to what is used on the output of the Johnson Matchbox JR. This does not match the schematics on the website listed below. I modified the one I had to match the later version of schematics he shows.
Schematic and Manual (2 pages). Thanks to W4DEX for the info:
Disconnect the wire shown by the pointer (wooden barbecue skewer) from the ground lug near the front panel and attach it so that both of the stators (stationary plates) on the left hand capacitor are connected together. This new wire does not attach to the bottom of the coil, it just is a splice, up in the air over the lug that supports the coil. Sorry if this photo is confusing, but there is not much room in this compact tuner. (IMPORTANT: LEAVE THE EXISTING WIRE FROM THE COIL SWITCH ATTACHED TO THE GROUND LUG. Just disconnect the wire to the capacitor.) This increases the total capacitance available, making for better range of matching, especially on 160 meters. Note that this modification is actually the way Dentron did it in later production models of this tuner. It matches the schematic shown above. Tuning in this configuration does not seem to be touchy, and it makes a good match on all bands 160, 80, and 40.