[size=large]If your TR IV is anything like the other BH machines I have owned, you need to remove the batteries, then unscrew the 4 screws on the faceplate...the whole faceplate should slide out then if you grab a knob and gently pull...watch the battery leads...push them through if you have to...be VERY careful of the ribbon cables and wire leads...If they pull loose, and you do not know where they go, you have a problem on your hands...They are also very fragile...Once the face is loose from the control box, note the wires on the switch to be replaced...Mark them somehow (masking tape and perm. marker work well). Mark both the wire, and it's position on the switch (better yet, snap a good close-up picture with a digital camera, but still mark everything as a backup plan), then unsolder the wires, and take the whole switch to your local Radio Shack, or electronics parts supply store (look up in Yellow pages)...That way you know you have a match. Go home, and reverse all these instructions...Not hard if you have the slightest Electronics and soldering skills.
P.S. I know it is tempting to tweak those pots on the main board...DON'T, unless you really want your machine to go back to the factory to be re-tuned (like mine has to)...lol
Good Luck,[/size]