Re: Clean and Lubricate Pots in Tek 475


Ed Breya
 

On Fri, Nov 6, 2020 at 10:05 AM, Bill wrote:


I put a little Deoxid into them and tried to find the problem with the
stiffness and the "springback" of these pots with little success. For some
reason I could not locate why they turn ok when apart BUT are stiff when
assembled.
It depends on the particular construction, which may vary a lot between brands. styles, and grades. If you refurbish a pot and it still binds after thorough cleaning and lubing, look at all the possible parts that can deform with age and use. First, make sure the shaft isn't bent, which is especially possible on 1/8" and smaller ones. Next, check the axial end play. It should be able to move in and out at least a few .001" at any rotational position. If not, then the whole thing is just too tight, likely due to any rubber and plastic parts deteriorating or cold-flowing.

The definition and construction of a "sealed" pot varies. Fancier, better grade ones may be more elaborate and include rubber gaskets between parts, and a rubber shaft seal like an o-ring at the bulkhead, or on a neck cut in the shaft. This is what tends to give the "springback" feel, due to deterioration and swelling of the material. If you can find a fresh o-ring equivalent to the original, replacing it should fix that problem. An alternative is to just delete the o-ring - the benefit of the seal isn't that big a deal anymore, in a benign hobby/lab environment. If the o-ring is at the bulkhead, eliminating it will likely also increase the axial play, possibly too much. You can try a thinner o-ring or add appropriate shim washers to tweak it.

If there are no shaft seals, then look at any rubber gasket layers - the rubber gets thinner over time under compression, so the whole assembly gets shorter, reducing axial clearance inside. If there is a shim washer at the bulkhead, you can try a thinner one or deleting it, but be aware the shim is also part of the thrust bearing function, so the pot may run rougher without it. If you can't effectively shorten the rotor assembly, then you can lengthen the body by say making new gaskets with thicker or fresh material, or adding shim washers between sections, around the screws. Again, high grade environmental sealing is not that important anymore.

If there are no rubber parts at all to blame, then cold-flowing of the plastic body parts is the likely cause. Study the construction and look at all the options to either shorten the insides, or lengthen the outsides.

Ed

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