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3A Rheostat Rebuild, the hard way


kenyonbm

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It seems possible to me that the "yellow fiberglass" could contain sulfur as well or it could just be another brand/form of rock wool. All of this stuff was intended as a building insulating material, not an electrical component. It would have been made in whole or in part, from the cheapest feedstock available. Sulfur content was not controlled, I speculate. And few thing are cheaper than slag.

I was astounded when, while visiting my brother at school in Pittsburg, PA, I looked through the phone book and found multiple companies under the heading "slag" and dross and ash and fly ash and cinders and soot, ect. These things apparently could be purchased by the barge load.

Or the sulfur source could be another component, perhaps the capacitors, but I don't think so.

John, would you be up for testing a few more samples?

Ken

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I have noticed that on average, 1960's "rock wool" versions seem to have more corrosion on the pots than their 1970's counterparts. Having said that, I have been working on a 1968 AR-3a with the Alnico woofer and the "rock wool." This one, by far and away had the best pots of any AR speaker I have worked on to date. They were perfect !

By the way, Henry had left AR prior to the production version of the AR-2 in about 1957 !

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>John, would you be up for testing a few more samples?

Yes, conditionally. Mr. Chandler has an undergad student who is in need of a project for his scanning electron microscope course next semester. We gave him about 9 fiber samples already. Please include information about their source and they will be included. The condition is that this course does not start until January so results will not be forthcoming quickly. His students learn to mount and analyze materials; insulating fibers that charge and blow about in the electron beam are a challenge for a learner!

cheers,

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I must say to all, that this has been a lively and unpredictable discussion. I have learned from each of you, in various ways and had a lot of fun.

I would like to refocus on the original topic, the rebuilding of those problematic rheostats.

The repair of the original rheostats by pen replating is a viable procedure, cost effective and practical. I am sure that any of you who care to try will have similar results.

The advantage of this approach is authenticity, simplicity, cost effectiveness and directness. All other restoration techniques require some compromise on these matters.

In my mind the only question that remains is the replacement of the original filling material, in the name of removing the source of the underlying corrosion. If this can be done while retaining the integrity of the brilliant original design and sound we all admire, Problem Solved.

I have made some measurements of my speakers with 20oz of fiberglass replacement stuffing and am disappointed with my results. I will be retesting and posting my results when I feel confident in my data.

I will repeat that pen replating is a viable option for rheostat restoration.

Ken

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