[Collins] Help

[Dr. Gerald N. Johnson]

Weighting an unweighted knob isn't difficult. Forming a new finger tip 
hole from bondo would seem to be difficult. At best Bondo type products 
take several applications to build up thickness, else they warp and 
typically they give you three minutes to mix, apply and smooth. After 
that you smooth by sanding or grinding. You'd want something that would 
polish too otherwise you'd have to try to get a shine by glossy paint.

It might be easier to find some solid black plastic and carve (with a 
lathe) a new finger piece, after using a milling cutter to remove the 
whole finger hole leaving a space for a nearly symmetrical replacement. 
You might want to mount the knob off center in the lathe spinning on the 
axis of the finger hole so you could carve the remains away with some 
shape, not just straight back.

To weight an unweighted knob, you need some lead shot (BBs at a 
Wallyworld sports counter), some slow setting pourable epoxy, and some 
tubing to protect the set screw holes. Probably have to drill from the 
outside and a little bit into the hub at each set screw to take brass or 
plastic tubing. Nice if its a press fit. Then you fill with lead shot 
and anchor it by pouring in the liquid epoxy, though with a slow seeting 
epoxy, it might be better to mix the lead shot into the epoxy and then 
work the combination into the back of the knob. Or likely you could pour 
some very lot temperature melting point metal after protecting the set 
screws. Look at McMaster-Carr for lumps of those metals www.mcmaster.com

I don't know how Arnie Spielbauer got the first one of those knobs made, 
probably made a drawing and sent it to the model shop and waited for it 
to appear a few days. I'd have turned a finger piece and then machined 
part of the original know away to all epoxying the new finger piece into 
place. Or I'd have started with a big enough lump of bakelite to have 
carved the whole thing with lathe, mill, files and polisher.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.

On 1/5/2011 3:17 PM, John Bowers wrote:
> We are building up a 51S-1 from parts we have collected over the last
> couple years. However, we have a weighted main tuning knob that is
> damaged. We have been unable to locate another knob and so are looking
> for info on how to repair the plastic crown part of the knob. The knob
> has the finger tip hole feature and the outside half of that hole
> feature is chipped off and missing. Otherwise it is complete. Our first
> thoughts are to look for something like black bondo used to repair auto
> bodies. Sure would like some advice or to be referred to an expert in
> these matters. This weighted knob has a nice feel to it when compared to
> the unweighted version.
>
> Thanks for you help in advance
>
> John, K9UTK
>
>