XG-10, refurbishing

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Jean Pierre2004-02-23 22:23

I own a pair of D-W XG-10 since 1981; I bought them used, but they were originaly shipped in 79, so those are 22 years old. I had my first troubles with them a few years ago (in 95): the seal of one speaker was unglued; I looked inside, observed some strange bluish powder on the wires near the damaged seal, but thought it was benign. I reglued the seal, obtained SF6 to fill them... but a year later, the corroded wire broke, arcing, and I had to repair again this same speaker.

Later on (in 99), an internal connector on the OTHER speaker broke, and I had to repair it. I did not notice two small holes in the ”bag”, so that I am currently resealing it after having repaired again a broken wire.

My questions:

1. What makes the internal wire connector to corrode and break? When I fix them, I use pure (well, as pure as I can get from a local jewelry) 0.75 mm silver wires. The fix in the first speaker last since 96, so that I have confidence in my fixing technique.

2. I resealed the second speaker after having replaced pieces of corroded connectors yesterday, but since I do not trust my seal (the Mylar\'s quality is doubtful), I operate them in air for now, not wanting to waste expensive SF6. I cranked up the bias, and no arcing occurs, and the sound seems as good as the other one, presumably still operating in SF6. Is the SF6 ***really*** important? Is it possible that operation in air is the cause for the corrosion on the internal wires?

3. Where can I get the .65 mil Mylar to replace the doubtful one on my second speaker?

Many thanks, JPD


Paul Young2004-02-23 22:23

1. The most likely reason for the breaking of the internal wires would be the slow, silent erosion of (first) the insulation and (later) the actual metal core of the wire due to corona discharge (which is an early & not very dramatic stage in the process of arcing). If you have a leaking enclosure, 3 things happen - the SF6 leaves the box, and more moisture (water vapour in the air) gets inside too. When the SF6 is gone and the moisture content of the air in the box rises, the voltage at which a corona discharge (”glowing” ions travelling from one voltage potential to another potential) get lower. When it is low enough, the discharge starts up gently and current starts to flow. This is pretty quiet (you have listen in a very quiet room with you head very near the discharge to hear it) and almost invisible (need completely dark room to see the faint blue glow if it exists). As the micro-amps flow, they strip off molecules of insulation and metal and carry them along for the ride. After many hours of this you can see traces of this migrating material as powder. After a long time - holes, and breaks are visible. Corona discharge always occurs at sharp points - even though they may be too small to see as a point. KEEP SOLDER JOINTS VERY SMOOTH and trim & round all wire ends when feasible if they carry HV.

2. When cranking up the bias and having ”no arcing occur” - be very careful!! It can take the arcing up to 24hrs to occur due to many factors I am not going to mention here. It is far smarter to increase it a bit EACH WEEK and then listen closely for the whole week to make sure that you have not stared to get arcing or corona discharge. You are correct to assume that the lack of SF6 (combined with high bias setting) was the cause of your damage. Regarding the ”need” for SF6 - my opinion is that it is more trouble than it is worth for owners in the field to worry about replacing. It REALLY does allow for higher bias voltages (which translates to higher efficiency) but this is not important if you have 200+ watts per channel available to drive a 2 ohm load. If you are using 25 Watt tube amps - then I guess it would help to have the bias a high a possible. The other advantages it provides are - ability to play to louder max SPL before transient arcing happens, slightly more bass output. Both of these ”improvements” are small (and given that the bass on these units is pretty inaccurate at best, plus they will play very loud without SF6) - I would run then without SF6 unless it was very easy and cheap for you to acquire. ALSO - note that in many climates the summer and winter humidity in houses is very different. I have only every had corona discharge happen in summer where I live - so do not set it in winter and then forget to turn it down & re-test in summer months.

3. I don\'t know where to buy wide 0.65 Mylar in small quantity. If you can get 1.0 Mylar - use it. The difference will only occur above 6KHz and will be small. It will be harder to tear accidentally too! Try contacting companies that do a lot of film treating of office or car windows for sun reflections. They may have some thin and wide Mylar. It won\'t matter if it is tinted or metal coated - it will work fine and may even be much thinner than 0.65 which is not a problem if you are careful.

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