Damon Hill Posted February 26, 2012 Report Share Posted February 26, 2012 A major amplifier failure compounded by three poor decisions on my part resulted in a severely damaged AR11 woofer (and a small fire in the speaker enclosure!). The woofer will have to be reconed, which I've never attempted before. Are there any recommended sources/services for this woofer, especially rebuild kits, and general techniques? I'm hoping the voice coil can be cleanly removed from the motor structure; it's jammed in pretty solid at the moment.The speakers themselves were rescued from Goodwill for $9 apiece; refoaming and new polypropylene crossover caps, and ignoring cabinet damage, resulted in two fine speakers that have given me a great deal of pleasure. I'd really like to get this speaker repaired, at the most reasonable cost.Still trying to determine the root failure of my Leach amplifier, which had been running for over two decades with no problems.--Damon Link to comment Share on other sites More sharing options...
Tom H. Posted February 26, 2012 Report Share Posted February 26, 2012 If you keep an eye on Ebay, woofers for your speakers do come up on occasion. Seems like the going rate is around 100 each. It might be fun to try to recone though. Link to comment Share on other sites More sharing options...
tysontom Posted February 27, 2012 Report Share Posted February 27, 2012 Damon,Your amplifier probably "met its maker" with an impedance load in the AR-11 that was unacceptably low for its output stages, and the amp overheated, overloaded or something else, causing it to dump dc into the output, eventually causing the voice coil to overheate, warp and jam in place inside the voice-coil gap. The resultant fire or smoke was caused by the coating on the voice coil itself igniting due to the extreme heat build-up. This is not an uncommon problem with some older amplifiers or receivers that do not blocking capacitors, transformer outputs or fuses/circuit breakers to prevent dc offset from getting into the output. It might be possible that you have been driving the speaker excessively hard at bass frequencies, eventually overheating the woofer voice coil, but the woofer would have bottomed hard and begun to make distorted sound. The fact that the coil is lodged in the voice-coil gap is a tell-tale indication of high amounts of dc voltage and current were present in the amp's output stage going into that speaker, causing the coil to stand still and get extremely hot and simply warp in the gap, thus causing it to smoke and lodge in place. Getting that coil out of the gap would not be easy without causing damage to the gap and pole piece, so your best bet would be to find a new AR 12-inch woofer and start all over. Your first line of action, however, should be to measure the output of the amplifier at the speaker-output terminals to determine if you have high amounts of dc voltage in the output. There should only be a few millivolts of "dc offset" at the output at any time. It is also likely that if the amp is still working, it possibly has the full rail voltage dc present at the output on the bad channel, but the output stage for that channel may have suffered when the speakers received the damaging output. Basically, you should not connect that output to any speaker until you are confident that the output is functioning properly. Good luck! --Tom Tyson Link to comment Share on other sites More sharing options...
Damon Hill Posted February 28, 2012 Author Report Share Posted February 28, 2012 The amplifier channel is blown; two shorted output transistors snd a couple of passive parts. It's been reliably driving the AR11s since I refurbished them well over a decade ago. I have all the spare parts and test equipment to correctly fix and test the amplifier--it's the woofer that'll present the real challenge and major expense. Link to comment Share on other sites More sharing options...
RoyC Posted February 28, 2012 Report Share Posted February 28, 2012 Damon,I have some spare woofers. Send me a PM if I may be of assistance.With an initial cost of $9 a piece you are still way ahead of the game. :-)Roy Link to comment Share on other sites More sharing options...
tysontom Posted February 28, 2012 Report Share Posted February 28, 2012 Damon,The Leach "kit" amplifier design is supposedly a good, high-power design, but it has been known to oscillate and so forth; therefore, the best advice to you would be to put fuses in series with the output of each channel. Mount the fuses close to the speaker terminals themselves, and use the dual-element 1-1/4 amp Fusetron FNM slow-blow fuses, if you can locate them. You would need to also locate the Fusetron 4421 open-style fuse holder for this larger-size fuse; do not ever use the inline fuse holder for this purpose. Alternatively, you could also use the smaller Buss or Fusetron glass fuses. A 1-1/4 amp fast-blow standard fuse will certainly work, but it will blow with short-term transients, whereas the slow-blow variety will pass short-term high-current peaks without blowing. The woofer can handle much more current-handling capability than the tweeters, but to protect the tweeters you should not go too much higher than the recommended size. The fuse will instantly blow if the amp puts high-current dc in the output -- the very thing you need to do to protect your woofers.--Tom Tyson Link to comment Share on other sites More sharing options...
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