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Re: Slightly OT - Bourns trimmer pot failure

 

The silver was a very thin film that was applied to the substrate before the resistive element was printed on top of the substrate. The the terminals were crimped over the other end of the silver film. I saw a lot of failed foreign made versions back in the '80s where the silver had turned into black flakes that curled up and opened the pots. Just like the oxides on good silverware damage the surface with sulphates and other contaminants in the air.

See if you can uncrimp the terminal on one ind of the bad pot. Then you can see how much remains where it wasn't exposed to contaminants. It's possible that the board cleaning process started the failure mode, and it took years to finally fail.


Michael A. Terrell

-----Original Message-----
From: Adrian <Adrian@...>

Hi Michael,

You know, I've just been looking at that and you could be right, but I
*really* can't tell how those connections were made!

Comparing it with the apparently identical one on the old board I would
say there is a bit missing entirely! Without taking the second one apart
I can't be 100% sure, but it looks to have a fairly solid silvery/gold
film extending to the pcb pins that are staked through the paxolin with
what could be a varnish of some sort applied over the top. There is not
the faintest trace of anything like that on the failed part, nor the
slightest trace of any residue whatsoever, it looks like pristine
paxolin substrate with an intact resistive film applied to it and no
means of connection at all - very, very odd!

If it was open circuit all along why did things only go berserk when I
turned the pot? Floaty HF at several kV with assorted capacitors around
went unstable when screwdriver approached perhaps? Not convinced!

I have some Piher parts on the way that look like they'll fit nicely and
we'll see what happens when they arrive.

Best,

Adrian
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