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As mentioned on more than one occasion, all six (6) pairs of my Koss CM/1030 speakers have been professionally refurbished here in Ottawa, at Speakermart.
Also as I've previously posted, and as visitors to my main HT had noted, something was more than a bit "off" with one of those pairs - which occupied the lower half of the side surround "stacks." The top end was down significantly. I've known this for some time, but given how much physical effort it takes to take a pair out of service in my main HT, I chose to let it go.
But it kept bugging me so a while back I had my son-in-law help me swap that pair for my "spare" pair, and the misbehaving ones were dropped off at Speakermart. I just picked them up today.
The big issue was that, since the original refurb (>10 years ago), at which time all of the crossover resistors were changed, all of the caps (Koss original) had drifted out of tolerance (some by a considerable amount). So they were all changed. Here's a before (blue caps) and after (black and orange caps) shot of one of the crossovers. The astute and knowledgeable Koss fans amongst you will immediately recognize them as the "old" version of the Koss CM/1030's crossover: the "new" version as a much more "orderly" layout / appearance. (And as I've said before, this is one of the more complex commercial passive crossovers ever used.)
Additionally, Bill (Speakermart owner) asked me to bring in a couple of my spare tweeters. He had plotted the response curve for the original two and they were somewhat different. After testing the two I brought in he was able to select two of them that were near-perfect matches.
When I went to pick them up today, Bill had set them up for some listening and I must say that they sounded fantastic; ie. exactly how a like-new pair of Koss CM/1030 should sound.
This pair will now become my spare pair. What the exercise has alerted me to is that, it is possible for caps that tested ok during the initial refurb to subsequently drift significantly out of tolerance within a fairly short period of time (in terms of overall speaker lifetime.) I suspect that the performance change is not linear with time as they approach end of time, but rather it is exponential.
What this also means (and part of the reason why I'm posting this thread) is that my advice to all Koss CM owners is that, if you are having a crossover refurb done anyhow (almost always to replace all of the original resistors) replace all of the original caps while you are at it - even if they still test ok. The additional cost of new caps is not that much ($170 for the pair of speakers), and the incremental labour to replace them is minor when everything is done at the same time. Much less than bringing them in a second time, like I just did. (Some of the previous refurbs did involve cap replacement, but only about half the time.)
I'll also be wary of changes in sound. I just went through each and every installed speaker, compare them to each other, and right now they all sound identical (at least within the capabilities of my hearing.)
Jeff
Also as I've previously posted, and as visitors to my main HT had noted, something was more than a bit "off" with one of those pairs - which occupied the lower half of the side surround "stacks." The top end was down significantly. I've known this for some time, but given how much physical effort it takes to take a pair out of service in my main HT, I chose to let it go.
But it kept bugging me so a while back I had my son-in-law help me swap that pair for my "spare" pair, and the misbehaving ones were dropped off at Speakermart. I just picked them up today.
The big issue was that, since the original refurb (>10 years ago), at which time all of the crossover resistors were changed, all of the caps (Koss original) had drifted out of tolerance (some by a considerable amount). So they were all changed. Here's a before (blue caps) and after (black and orange caps) shot of one of the crossovers. The astute and knowledgeable Koss fans amongst you will immediately recognize them as the "old" version of the Koss CM/1030's crossover: the "new" version as a much more "orderly" layout / appearance. (And as I've said before, this is one of the more complex commercial passive crossovers ever used.)
Additionally, Bill (Speakermart owner) asked me to bring in a couple of my spare tweeters. He had plotted the response curve for the original two and they were somewhat different. After testing the two I brought in he was able to select two of them that were near-perfect matches.
When I went to pick them up today, Bill had set them up for some listening and I must say that they sounded fantastic; ie. exactly how a like-new pair of Koss CM/1030 should sound.
This pair will now become my spare pair. What the exercise has alerted me to is that, it is possible for caps that tested ok during the initial refurb to subsequently drift significantly out of tolerance within a fairly short period of time (in terms of overall speaker lifetime.) I suspect that the performance change is not linear with time as they approach end of time, but rather it is exponential.
What this also means (and part of the reason why I'm posting this thread) is that my advice to all Koss CM owners is that, if you are having a crossover refurb done anyhow (almost always to replace all of the original resistors) replace all of the original caps while you are at it - even if they still test ok. The additional cost of new caps is not that much ($170 for the pair of speakers), and the incremental labour to replace them is minor when everything is done at the same time. Much less than bringing them in a second time, like I just did. (Some of the previous refurbs did involve cap replacement, but only about half the time.)
I'll also be wary of changes in sound. I just went through each and every installed speaker, compare them to each other, and right now they all sound identical (at least within the capabilities of my hearing.)
Jeff
