3A Rheostat Rebuild, the hard way
Posted 03 October 2006 - 03:24 AM
>think may be "rock wool", rather than fiberglass,
>which I believe is made from steel blast furnace by-product.
>Might be some sulfer in there, enough to corrode the silver
>and destroy the rheostat.
Ken, I think your observation that the "brown fiberglass" used in early AR speakers may not be fiberglass but rather rock wool (a.k.a. mineral wool, slag wool) may be right on the money. For examples: (i) sulfur dioxide and hydrogen sulfide are well known emissions from its manufacture, (ii) there is a French paper that discusses the effects of such insulation in animal lung tissue. It states that sulfur from rock wool is dissolved in the tissue. Original AR x-o wiring was tin plated. As you note, the silver plated brass and bronze in the potentiometer are the weak links. There are a number of Ag-S and Cu-S compounds that can be formed in the presence of moisture--especially when the materials are resistance heated.
If so, the brown insulation may be the root cause; future corrosion might be eliminated by replacing it with yellow fiberglass.
The electrolytic pen sounds like a good idea. Tin or silver would work. Tin plating from a heated (140 F) electroless source (Tinnit) is also a good method. Before plating with either method, one should clean the metals well. Often a sequence of abrasion, followed by detergent and water to remove grease; acetone to remove the detergent; isopropyl alcohol to remove acetone; followed by a long rinse in flowing water should leave a very clean surface.
After plating, one should rinse the pieces for a long time in flowing water to remove any trace of residual plating compounds -- if improperly cleaned the work may later corrode.
Tin oxide and silver oxide are both conductors. The layer from hot-plated Tinnit is easy to solder. If over a very long time, tin oxide becomes too thick, it may have to be roughed before soldering; however, that's not an issue here.
Posted 02 October 2006 - 02:41 AM
I finally found what I wanted from the W Marshall Leach Jr website.
I'm having difficulty downloading a screen print.
I'll be back.
The file is almost 3 mb and is slightly larger than we are allowed to download.
It sure is nice being able to get back into the write-up's after the fact, I just discovered this only a few months ago.
I sent Mark an email requesting a slightly larger download capacity if possible.
Irregardless, the caution is for the liquid used as a psuedo tin plating cold dip.
He advises against using it, as it corrodes solder traces, making it very difficult, if not impossible to solder to, afterwards.
Stay tuned, if I can download the file here I have saved in my computer.
Posted 01 October 2006 - 08:20 PM
I just tried to find the information about copper circuit board coating from the W Marshall Leach Jr web site.
There is so much information there and I wasn't able to find the actual brief referrence quickly.
I felt it more important to mention it here quickly, what I do remember about it.
He had noted that, we should not try to, TIN PLATE", circuit boards with the liquid as it creates a source for serious corrosion.
Often, soldering over un-finished circuit board, pre-tinning, paths is usually just done with solder, but this liquid short cut will cause more problems than it is worth.
This is the only mention I have read about that products use, ever.
Before reading that, I was going to do a short cut and use it.
It is definitely a no go, John, sorry.
De-Oxit or the earlier Cramolin is a definite go though.
Posted 01 October 2006 - 05:07 PM
They also offer gold and rhodium pens. Silver I can cobble together on my own. I am told that gold is used for low level signals. Rhodium is a lot harder than and might not corrode like silver.
I think with some more looking I could find other metals.
I am also thinking of just making some solid silver washers and using a little brass nut and bolt or a copper pop rivet to replace the plated washer and rivet.
Kind of a lot of effort I know, but there are a lot of them out there.
I think the underlying problem may be the insulation, which I think may be "rock wool", rather than fiberglass, which I believe is made from steel blast furnace by-product. Might be some sulfer in there, enough to corrode the silver and destroy the rheostat. Shure is itchy.
Posted 01 October 2006 - 01:20 AM
Posted 30 September 2006 - 10:38 PM
Wise choice; this speaker works wonderfully as designed.
It is my guess from examining Pollack potentiometers in good condition that the wiper is made from phosphor bronze and the contact plate from brass. Both were silverplated when new -- the sliver can be seen after cleaning the surfaces with a pencil eraser. The Nichrome end rivets appear to have been silver plated as well. Plating a layer of silver was something I had considered but never did. A mild etch or electropolish followed by electroplating a thick layer would seem to be reasonable provided the wiper and contact are not corroded beyond use. Should you try it, let us know how it worked. One might also experiment with a tin electroless-plating solution called "Tinnit" that is used for passivating copper circuit boards; tin oxide is also conductive.
Posted 30 September 2006 - 07:21 PM
I had considered several approaches, including bi-amping, since the speakers are designed with that in mind. I think yours is a worthy solution, but I have decided to attempt to restore my pair to usable condition with as little intervention as possible.
What I am going for is a vintage sounding system for vinyl playback.
I want the 3a's to look and sound like very good 40 year old speakers.
I have put new wine in old bottles, but not this time if I can help it.
Posted 30 September 2006 - 06:58 PM
>Working on my "new" pair of alnico woofer 3a's that
>have the normal rheostat problems. I had no problem getting
>the speakers open, removing the rheostats, disassembling the
>rheostats, cleaning them, and getting the whole thing back
>I am not completely satisfied with the results. Big
>improvement, but mid is still scratchy and some dead spots.
I don’t know the answers to any of your questions, but you can see by the number of posts in this forum that those pots are a ... nuisance!
Just wanted you to know that I solved the problems of pots in my AR-3a’s … forever. Let me be clear; I still have to balance the energy split between the woofer and the mid/tweeter, but I do this with volume controls.
That is, I use one amp for the woofers and another amp for the mid/tweeter. Pots have been completely removed from the circuit, even though they are still on the boxes.
My AR-3a’s improved dramatically once I made this change, plus I made an enormous gain in headroom on the amp driving the mids/tweeters.
Ken, I know this is a little off topic, but the reality is low wattage controls like volume controls are far, far easier to maintain than high wattage controls like pots in speakers.
Further, all these pots do is “pad” or absorb power to balance the speakers. When removed, we send to the drivers just the power they need to produce sound … that is, there is less waste and consequently more headroom.
Now, let me clarify. You can only remove the pots when you are bi-amping. The sensitivity of the mid/tweeter increases so much with the pots out there is just no way to balance the speakers with an EQ (any I don't care how good it is). In short, you have to reduce SIGNIFICANTLY the power sent to the mid/tweeter once the pots are out of the circuit.
Ken, I realize this is off topic, but thought you’d be interested in a “long term” solution that simultaneously solves a number of problems.
Posted 30 September 2006 - 06:28 PM
Working on my "new" pair of alnico woofer 3a's that have the normal rheostat problems. I had no problem getting the speakers open, removing the rheostats, disassembling the rheostats, cleaning them, and getting the whole thing back together.
I am not completely satisfied with the results. Big improvement, but mid is still scratchy and some dead spots.
Before I redo that one or start the other, I am looking for some more information about the rheostats and the materials they are made of.
In the center of the ceramic back/coil assembly is a silvery washer shaped contact. It looks like silver plated brass. I didn't want to clean it down to the brass (if that is what it is.) Does anyone know what this contact is made of? Or if it is plated or solid?
Also, the wiper seams silver plated and is stiff, maybe bronze?
Does anyone know if this wiper is solid or plated or what the material is?
I am thinking of having them replated, maybe silver, gold or rhodium.
This is not as crazy as it sounds but knowing the base metal whould help. Any information much appreciated.
Posted 19 October 2006 - 06:04 PM
It turns out that the insulation is rock wool and that is does contain sulfur. So the insulation must be replaced, with fiberglass, so the problem will not reoccur. Then the tarnished parts of the rheostats can be replated and I hope, the problem solved.
I have ordered a silver plating kit, and will be posting more soon.
Posted 06 November 2006 - 01:55 AM
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.
Posted 06 November 2006 - 12:07 AM
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!
Posted 05 November 2006 - 11:26 PM
By the way, Henry had left AR prior to the production version of the AR-2 in about 1957 !
Posted 05 November 2006 - 11:22 PM
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?
Posted 05 November 2006 - 10:40 PM
>Roy, is it possible the rock wool was switched to the yellow
>stuff without your knowledge before you acquired them? Or, are
>you the original owner?
No, I have restored many AR-3a's over the years and these were definitely all original in every way, with no signs of internal tampering. I have only seen the rockwool in the pre-1970 AR-3a that used the alnico, cloth surround woofer. Most AR's I have seen with rockwool in them are pre-1970.
>So, why exactly, did Henry K. switch from pots to switches? I
>recently upgraded an AR 12 (circa 1977).
Henry Kloss was a long time out of AR and busy with KLH, and later Advent, by the time AR moved to switches in the 70's. Interestingly, most KLH speakers back into the 60's, and all of the 70's Advents, used switches. I'm not sure I have ever seen a classic KLH speaker that used anything but switches for attenuation.
In the mid 70's the "AR-3a Improved" and the AR-11 used switches. My guess is that AR knew they had a "pot problem":-).
I have many AR pots out there in restored AR speakers that are functioning quite well without re-plating. I suppose plating could offer extra protection.
Posted 05 November 2006 - 09:52 PM
>You are correct in saying that removal of rock wool is not the
>answer to all our pot problems. IMO the worst, but not all,
>the corrosion is in those cabinets.
>Other gases and vapors including moisture, sulfur dioxide and
>hydrogen sulfide can permeate the surround and woofer cone
>slowly over long times; these materials seal well enough to
>make a loudspeaker, but would never "hold a vacuum."
> If the silverware in a china cabinet turns black in a year or
>two, so will plated parts in the pots.
Roy, is it possible the rock wool was switched to the yellow stuff without your knowledge before you acquired them? Or, are you the original owner?
So, why exactly, did Henry K. switch from pots to switches? I recently upgraded an AR 12 (circa 1977). It's about 10 years older than the 2ax-3a era. It had switches to adjust the mid and tweeter outputs. Did he become aware of the potential for problems? Exactly which model AR was the first to have switches? Kloss seemed to stick with dB switch adjustments for quite some time well into the Advent era.
Quest. 2 or, is it 4 or 5. So why exactly is the plating on the stator rivets and plate so important? Arent the underlying brass contacts okay as well? I thought brass was a pretty good conductor. I don't think the brass rotor was plated either.
Remember, it's all about the music
Carl's Custom Loudspeakers
Carl's Custom Loudspeakers
Posted 05 November 2006 - 08:35 PM
You are correct in saying that removal of rock wool is not the answer to all our pot problems. IMO the worst, but not all, the corrosion is in those cabinets.
Other gases and vapors including moisture, sulfur dioxide and hydrogen sulfide can permeate the surround and woofer cone slowly over long times; these materials seal well enough to make a loudspeaker, but would never "hold a vacuum." If the silverware in a china cabinet turns black in a year or two, so will plated parts in the pots.
Posted 05 November 2006 - 06:36 PM
The stuffing in both speakers is the later shredded yellow fiberglass, not rock wool. Each cabinet had 20 to 21 oz of fiberglass in it.
Note the pot corrosion. I have seen too many pots like these to believe that rock wool removal is the answer to our AR pot issues. The pots in both cabinets are in similar condition.
With that said, I always replace the very nasty rock wool with yellow fiberglass when I run across it in the earlier speakers. It is typically sold today as pipe and hot water heater insulation.
Posted 05 November 2006 - 05:36 PM
>the size of fine beach sand, included in the rock wool, as
>well as a myriad of tiny, broken, breathable and very
>irritating fibers. By weight, both are fairly minor. The
>material varies from fine to coarse, but it seems unlikely
>that it was intended as any thing other than building
>insulation. The color you can see is pretty close to the
>I cannot understand why it is torn in chunks the way it is,
>almost like it was a waste or byproduct salvaged from some
>Uncompressed, it is at least 50% more dense than fiberglass.
>Thanks for your piece on stuffing. I read it with great
Thanks Ken. If you can, please weight what you have and calculate the box stuffing density. I'm interested in how it matches up with the number in my report. Your picture shows exatly what I also used in my study.
I think the pulled apart nature is normal. They used to blow it into houses for insulation.
Remember, it's all about the music
Carl's Custom Loudspeakers
Carl's Custom Loudspeakers
Posted 05 November 2006 - 04:05 AM
I cannot understand why it is torn in chunks the way it is, almost like it was a waste or byproduct salvaged from some other process.
Uncompressed, it is at least 50% more dense than fiberglass.
Thanks for your piece on stuffing. I read it with great interest.
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