Repairing broken Bourns pots


 

I put some pics here: https://groups.io/g/TekScopes/album?id=261839

Once I opened it up I found a round piece that is linked to the slider of the pot. The axis seems to have broken loose from that piece. Don't know what material that is (aluminium?) and how to fix it back on (which type of glue?).

Also it seems the piece is somehwat bent. So I'm not sure if it won't block the pot when glued back in an uncorrect position. Anybody has a close-up pic of the axis and how it ought to be connected to this piece?

cheers
Martin


rick
 

Decades ago, when these pots were made by Allen Bradley, they made an engineer's kit, a fishing tackle box full of parts that would let you assemble your own mod-pot prototypes. Mod pots were attractive to me as a DIY person because they made the problem of multiple pots with different values/tapers/shafts accessible to a small-quantity user. Of course, A/B marketing hadn't thought of that, and getting parts to replace what you'd used was both difficult and expensive. I was able to replenish mine a few times, but mostly I've taken to finding pots on the surplus market and stashing them for later reconfiguration and use.

What the "kit" got me was a view to how these are assembled, which I will attempt to describe here.

To assemble a single pot, you need the pot body (element), the shaft (axis), the shaft bushing, the front plate that has the anti-rotation tab, and the rear cover. There is a syringe full of shaft lubricant, and you apply a small amount to the shaft where it contacts (penetrate) the bushing, at the end that engages the actuator for the element. Now you have the shaft and bushing assembled, you add the front plate to it, and stack the element on top of that. A/B supplied a wooden block with a hole in it to act as an assembly jig. Now add the rear cover, and then fasten the whole assembly together with 2-56 screws of the correct length. The kit had bags of screws, and the other hardware needed to assemble any pot configuration.

If I can find the manual, I can scan it and post it here. I've never sacrificed a pot element before, so I don't know the intimate details of how the inside of the pot is constructed and if you are able to squirt cleaner, etc, into the front opening, if it can/will reach the resistive element. It seems like drilling a small access hole is the way to fly here, aside from replacing the pot, if you have an element.
In assembly, there was no lubricant introduced between the shaft actuator and the front of the pot element. The front of the pot element looks like the place you'd want to be for injecting cleaner.

I don't know if lubricants are installed at time of manufacture of the element. If so, then when you squirt cleaner into the pot via the cleverly made custom access hole, then you risk the solvent carrier of the cleaner bathing the element in the lubricant. People who douche ordinary pots, via the opening where the terminals emerge, such as the 16mm Alps pots have this happen to them. You can tell because the feel of the pot changes dramatically. Further douching may dilute the lubricant enough to let the pot work again, but it will never again have that viscous damped feel that the Japanese pots were always known for. Yes, this is a desperate situation, and desperate measures are authorized.

The elements are molded into a single blob of plastic, and they don't open unless you smack them with a hammer, which shatters the body and yes, you can now access the element, but the pot is now also broken forever. I've never seen a pot where the resistor element was accessible, such as for the introduction of a cleaner.

Dual pots have a special coupler that goes between the elements that join the wipers together. Concentric pots have a special spacer that goes between the front and rear sections, that builds in the extra clearance needed for the actuator at the end of the inner shaft. Switches require yet some other thing that I can't recall. They really had the whole thing quite figured out. Sadly, it never worked economically for small quantities. You could get them, but the price would make your bank account spasm. Obviously, they didn't want to sell small quantities, say under 100 pieces.

No, I'm not interested in selling parts from my kit. I'll try to find the manual.

--rick chinn