Please note that these models are untested and are based on replacement parts which I acquired.  You may have to adjust this model to get it to work.

A very common problem with used Yaesu FT-857D ham radios is that the Select knob is broken or missing.  The proper replacement parts for it, according to the service manual, are:

RA0269600 Rotary Knob (SEL)

RA0473600 Spacer Knob (SEL)

However, these are a little difficult to obtain and I've only ever been able to buy them from unofficial sources now that the radio itself is discontinued.  These models are my attempt at replicating those parts so that if they do become completely unobtainable, or if you need a temporary repair, they will hopefully be close enough to work.

The knob is composed of two parts.  There is a spacer which goes below the knob and supports the rotary encoder's shaft in the center of the opening in the faceplate.  This keeps side loads from snapping off the rotary encoder's plastic shaft quite as easily.  Then the knob is press fit onto the end of the shaft.  In the real knob, there is a metal insert where the knob contacts the shaft.  With some adjustment and experimentation, this should work without that.  But as I mentioned at the start, these parts are untested, so you may need to tweak the parameters a bit to get it to stick.

If in doubt as to how to assemble these parts, see the Yaesu FT-857D Service Manual Exploded View & Miscellaneous Parts page.