Classic Equipment Cracked / Mushy Knob Fix

From the Ten-Tec Reflector August 17, 2005

I've noticed for some time that some of the switches on my TenTec gear seem to be, well, mushy. The Mode Switch and the Band Switch are especially prone to this.

It turns out it's the knob, not the switch, and the problem is that if you overtighten the setscrew, the inner circle of the knob cracks and doesn't maintain a firm grip on the shaft. Take one of those knobs off and look at the underside and you'll see what I mean. There's a circular gap between the inner circle which fits the shaft and the outer circle which is where the body of the knob begins. (Seems like a poor design, given the function of the knob.)

I thought that if I filled in that gap in the knob, it would make the knob solid and less likely to crack when the setscrew is tightened. So I tried some J-B WELD as filler, and it worked. First, I had to restore the inner ring to its original position. This was done with a flat wooden toothpick, but other items will work also. Just don't plan on re-using this spacer because it'll become a permanent part of the repair. Mix about a 1 inch stripe of Part A and Part B of the J-B WELD together, and carefully fill in the gap. The J-B WELD has the consistency of toothpaste, so you'll have to work it down into the gap. I used another flat toothpick to do this. Wipe off any stray epoxy before it sets. The package recommends a 15 hr cure time, and I'd recommend following this recommendation. The last thing you want is to apply any stress to the epoxy before it's fully cured. By the way, I chose J.B. Weld because it seems to be readily available at any autoparts store, and also because the description on the package states, in part, "The city of Dallas, Texas repaired a cracked Caterpillar engine block with J-B WELD and saved $4,000.00 plus 30 days down time." If it's good enough for that, it should be good enough for plastic knobs.

So, if you have this "mushy knob" problem, give J-B WELD a try. It works for me.

ed - k9ew

August 17, 2005

Ed, I used the JB Weld about two years ago on a Corsair II bandswitch knob that did the Ten Tec split. I only had one good knob left so I filled the split knob just like you did with JB Weld. It has had two years of hard use and has not split again.

I called Ten Tec and told them that they needed to tell anyone that bought a replacement knob to fill it with JB Weld before putting it on the rig. Seems the JB Weld acts as a metal sleeve does on a well designed knob. I fill all Ten Tec high stress knobs with the stuff and recommend that others do as well. If you have a Ten Tec rig your knobs will split. It is just a matter of time due to a bad, cheap design. By the way I still have my new backup knob, which has been filled, for future use, if needed.

I have never had to replace the repaired split knob.