cds
02-22-2004, 10:25 PM
In the past few months I’ve had 4 SL3000’s die. The failures were characterized either by no output or some serious distortion. After repairing them and checking the ones that were “OK”, I started wondering if there is a materials aging issue with the drivers or some other common cause for failure. What follows are my observations and thoughts on the reasons for the failures. Hopefully this may help others who have had the drivers mysteriously quit.
Before I get started, let me state that I do play the SDA’s loud at times using a Carver A-760X. Hard not to do. I have done this for years and have never noticed the tweeter protection device activating so I did not have a concern that they were approaching electrical meltdown. My only “problem” is that during large bass events I have to watch the volume to prevent the 6.5” drivers from bottoming…. not a good thing.
Initially I suspected some of the tweeter materials were showing signs of age or crossover caps had become leaky allowing too much LF energy to reach the tweeters. Now after evaluating the failures I also believe sound pressure from inside the cabinet is another likely scenario.
During repairs I found either the voice coil open or the voice coil detached from the dome. Initially one would think the open coil would be an electrical failure and the other to be mechanical. However, I now believe both are mechanical in origin.
The detachment of the coil from the dome appears to be an adhesive failure. The open coil failures in all 3 cases looked like mechanical work hardening of the wire rather than electrical burnout. My conclusion is based on examination of the break using 60x magnification.
After repairing them, I decided to perform a basic test to evaluate the repairs. Applying an input from 1 – 20KHz in 1KHz steps, I measured the SPL about 6” from the dome. Besides measuring the SPL at each point, I also fed the SPL meter into an oscilloscope to monitor for distortion. Crude yes, but significantly better than by ear alone.
I compared the performance of the repaired units and found them to be within 2 dB of each other over the full frequency range. When I checked the ones I thought were OK, I was surprised to see the output 6-8 dB lower than the repaired units with noticeable distortion at various frequencies. Taking these apart I found no problems like the others, so I did a little maintenance (cleanup and adjustment) and voila, they now performed on par with the repaired units.
If my suspicion is correct both types of failures are due to excessive dome movement. In one case the wire fractured and in the other, the glue separated from the dome. My thought is that excessive low frequency energy is reaching the domes. Either 1) there are leaky caps in the crossover allowing excessive low frequencies to reach the tweeters but not enough current to trip the protection device, or 2) air pressure inside the cabinet finding its way through the rear of the tweeter and moving the dome.
There is a gasket in the tweeter under the dome assembly that should prevent this. It looks like time has started to take its toll on these gaskets and they have taken compression set. While still usable, the mounting plate screws have to be torqued appropriately to make the air seal. If I keep having this problem I may eventually make a new gasket since I doubt this part is available. It looks to be made out of the same material used for the gasket to mount the drivers to the cabinet. In performing the repairs, I noticed that some of the screws were not quite snug. Perhaps they were tight at time zero, but 15 years later? Who knows?
Note: If you elect to check the screws on your drivers, alternate the tightening sequence to keep the voice coil centered and don’t tighten them excessively.
I’m planning on replacing the crossover caps when I get a chance just to be on the safe side. I have also been thinking of building a sealed enclosure behind the tweeters to isolate them from the internal pressure of the cabinet, thereby making the gasket less critical.
Any thoughts?
One last thing. I’m assuming my repairs have restored the 3000’s but I have nothing to reference them to. I have ordered one of the new 3000 equivalents from Polk just to see how it compares. I’ll post an update after I’ve had a chance to test is against the repaired versions.
I am also interested in comparing original 3000’s to the repaired versions. If anyone has spares sitting around and would like to loan them, let me know.
Cheers
Before I get started, let me state that I do play the SDA’s loud at times using a Carver A-760X. Hard not to do. I have done this for years and have never noticed the tweeter protection device activating so I did not have a concern that they were approaching electrical meltdown. My only “problem” is that during large bass events I have to watch the volume to prevent the 6.5” drivers from bottoming…. not a good thing.
Initially I suspected some of the tweeter materials were showing signs of age or crossover caps had become leaky allowing too much LF energy to reach the tweeters. Now after evaluating the failures I also believe sound pressure from inside the cabinet is another likely scenario.
During repairs I found either the voice coil open or the voice coil detached from the dome. Initially one would think the open coil would be an electrical failure and the other to be mechanical. However, I now believe both are mechanical in origin.
The detachment of the coil from the dome appears to be an adhesive failure. The open coil failures in all 3 cases looked like mechanical work hardening of the wire rather than electrical burnout. My conclusion is based on examination of the break using 60x magnification.
After repairing them, I decided to perform a basic test to evaluate the repairs. Applying an input from 1 – 20KHz in 1KHz steps, I measured the SPL about 6” from the dome. Besides measuring the SPL at each point, I also fed the SPL meter into an oscilloscope to monitor for distortion. Crude yes, but significantly better than by ear alone.
I compared the performance of the repaired units and found them to be within 2 dB of each other over the full frequency range. When I checked the ones I thought were OK, I was surprised to see the output 6-8 dB lower than the repaired units with noticeable distortion at various frequencies. Taking these apart I found no problems like the others, so I did a little maintenance (cleanup and adjustment) and voila, they now performed on par with the repaired units.
If my suspicion is correct both types of failures are due to excessive dome movement. In one case the wire fractured and in the other, the glue separated from the dome. My thought is that excessive low frequency energy is reaching the domes. Either 1) there are leaky caps in the crossover allowing excessive low frequencies to reach the tweeters but not enough current to trip the protection device, or 2) air pressure inside the cabinet finding its way through the rear of the tweeter and moving the dome.
There is a gasket in the tweeter under the dome assembly that should prevent this. It looks like time has started to take its toll on these gaskets and they have taken compression set. While still usable, the mounting plate screws have to be torqued appropriately to make the air seal. If I keep having this problem I may eventually make a new gasket since I doubt this part is available. It looks to be made out of the same material used for the gasket to mount the drivers to the cabinet. In performing the repairs, I noticed that some of the screws were not quite snug. Perhaps they were tight at time zero, but 15 years later? Who knows?
Note: If you elect to check the screws on your drivers, alternate the tightening sequence to keep the voice coil centered and don’t tighten them excessively.
I’m planning on replacing the crossover caps when I get a chance just to be on the safe side. I have also been thinking of building a sealed enclosure behind the tweeters to isolate them from the internal pressure of the cabinet, thereby making the gasket less critical.
Any thoughts?
One last thing. I’m assuming my repairs have restored the 3000’s but I have nothing to reference them to. I have ordered one of the new 3000 equivalents from Polk just to see how it compares. I’ll post an update after I’ve had a chance to test is against the repaired versions.
I am also interested in comparing original 3000’s to the repaired versions. If anyone has spares sitting around and would like to loan them, let me know.
Cheers