Rheostat conundrum

[Carlspeak]

I opened up a 2ax speaker (Ser. #AX84028) for routine servicing and was surprised to see that one of the rheostats (midrange) had no corrosion while the tweeter rheostat had corrosion which I typically see (pics below). This was a speaker from the original owner. I've never seen this huge a differnce before. The box was stuffed with rock wool FG which has been claimed in posts here to cause the corrosion.

Of note also were the rheostat stems. They were shiny aluminum with small, white plastic nubs on the internal end where the brass rotor sat on.

Has anybody seen this kind of corrosion difference before?

[Mexicomike]

The pots in one of my AR3a s looked just like that when I opened them up - one corroded a lot, the other not so much. The other 3a's pots weren't as bad. None of the pots appeared to have ever been opened before but I can't really be sure of that.

[johnieo]

The pots in one of my AR3a s looked just like that when I opened them up - one corroded a lot, the other not so much.

There are many factors that determine the corrosion rate - the issue is which of those would be different in two closely located pots in the same cabinet in same environment. Factors like humidity, sulfur oxides and so on would be equal. The only thing that comes to mind that could be different is temperature. Reaction rates vary exponentially with temperature. One could imagine a scenario where the tweeter pot was set near its max, and all its current was flowing through a small number of turns in series with the tweeter. This might increase the temperature of that winding and allow reactants to destroy it more quickly than the mid pot, which in that system, might have been set at a higher value of series resistance. Just one idea...

Cheers,

[genek]

Has anybody seen this kind of corrosion difference before?

I haven't opened up a whole lot of speakers, but I have seen similar differences in old TVs and radios where one control was used a lot more than the other; the more used one wore the original coatings and lubricants off the pot windings more than the other controls. Funny thing was, the ones with more visible corrosion often seemed to work better than the ones with less. My theory was that there was more gunk visible inside the case because it was constantly being scraped off the windings by the wiper and it had to settle somewhere, while the less used controls just developed a coating that remained mostly undisturbed.

[Guest mikeg]

I opened up a 2ax speaker (Ser. #AX84028) for routine servicing and was surprised to see that one of the rheostats (midrange) had no corrosion while the tweeter rheostat had corrosion which I typically see (pics below). This was a speaker from the original owner. I've never seen this huge a differnce before. The box was stuffed with rock wool FG which has been claimed in posts here to cause the corrosion.

Of note also were the rheostat stems. They were shiny aluminum with small, white plastic nubs on the internal end where the brass rotor sat on.

Has anybody seen this kind of corrosion difference before?

I saw the same damage on my AR2ax speakers, in fact two sets of

AR2ax's. One alnico set and one non alnico AR2.

I went a different route for fixing mine, I designed a attenuator to replicate

the rheostat setting.

They use metal film resistors for better high frequency responce.