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Post by markymark on Dec 3, 2016 20:19:20 GMT
My issue is output in one speaker, of a pair of PMC Twenty.21, developed a fuzzy, crackling distortion. Before this event, I noticed a general drop in output from that speaker, but did not notice the distortion at first, but did notice the reduced output enough to move that speaker a little closer to my listening position. I figured it was the amp, an Exposure DR-4 that has never been serviced.
I attempted to isolate the problem by switching speakers in the same system (L to R), and also swapping speakers from another system both ways. The one 20.21 exhibited the same issue on both systems, while different speakers on the original system sounded fine -- not the DR-4 in the system powering the 21s.
So I purchased another tweeter. The exact same PMC tweeter, but new, that is in Twenty.21. Same problem! Lower output in the speaker and distortion in the tweeter. Both tweeters sound identical in either system. BAD!
Any ideas?
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Post by MartinT on Dec 3, 2016 22:32:17 GMT
I would try to access the rear panel and crossover. It's either a very poor connection or a failed component, which you may see as a burned out resistor or obviously blown capacitor. Check solder joints and spade connector crimping. Failing that, the entire path to the drivers need to be checked as well as each crossover component individually.
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Post by Greg on Dec 3, 2016 23:39:05 GMT
Yep, get your multi-meter out and check everything from beginning to end. Do this and you'll get your solution.
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Post by dsjr on Dec 4, 2016 10:55:13 GMT
Aren't these things still under warranty?
The bass units are not expensive I gather and so it's likely they won't be as durable as more heavily built drive units. PMC do make the crossovers well, but a party or similar heavy session could cause thermal stress to the coils and caps therein as well as the main drivers.
I'd replace the bass driver. There's rarely much coming out of the tweeter at most times on its own, so if these are disconnected, what's left in the midrange should tell you where the issues are.
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Post by ChrisB on Dec 4, 2016 11:08:50 GMT
Looking at a photo of them on the PMC website, it seems they're biwireable, so have you tried feeding one driver at a time to see which arm of the crossover the problem is on?
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Post by markymark on Dec 4, 2016 12:19:50 GMT
Thanks to everyone for the replies. I should have been clearer in the OP, in that it's only the tweeter distorting. However, the output from the speaker, which includes the woofer, seems lower to me. The lowered output was gradual before the distorted tweeter appeared. I'd estimate something like a 10% reduction in output in that speaker.
I contacted PMC first, and because I'm the second owner they wouldn't cover anything. I assumed the warranty was transferrable. Apparently it's not. Their quoted price for a replacement tweeter was over $200 USD. I contacted SEAS, who said they can replace it with an OEM tweeter of the same specs, but without the mesh grill, for $50. That's a pretty high markup. So I looked around online, found and purchases a PMC tweeter for a little more than that, and it's the only one I was able to find after an hour of searching online. I figured I'd better grab it, convinced it was the tweeter since I'd swapped things around and those tests pointed to that conclusion.
I'll install the tweeter I bought into the other cabinet to make sure it works properly, and the tweeter I think is blown as well. That should clear up whether it's in the x-over. I did look at the x-over but didn't remove it from the box. It tilts outward. One thing I didn't know was that the clips on these wire ends have to orient in a particular direction (smooth side towards the cone) or there's no sound at all. Never seen that before, but I'm not a speaker technician!
I have more time today so I'll do these tests, dust off the multi-meter and report back. Thanks again.
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Post by markymark on Dec 4, 2016 12:26:50 GMT
Looking at a photo of them on the PMC website, it seems they're biwireable, so have you tried feeding one driver at a time to see which arm of the crossover the problem is on? Yes, I did that at the end. Both new and original tweeter distorted with just the HF leads connected in bi-wire configuration. I have to move the speaker to a different system to do this test. The mid-bass unit didn't sound distorted, but the output seemed lower. Could be my imagination. |
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Post by markymark on Dec 4, 2016 12:51:05 GMT
This is the board (web image).  |
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Post by markymark on Dec 4, 2016 15:11:31 GMT
I finished swapping what I thought was the bad tweeter and the one I bought into the known good PMC box, and both tweeters sound great. So it's in the other box somewhere.
I tested for continuity on both x-over boards, and everything connected to both boards has continuity except for one of the caps, but it's the same on both boards (the good one too), so something in the wiring is preventing that cap from showing continuity. I'm basically an electronics moron.
One bit of weirdness is that during the swapping of tweeters in and out of the known bad box, I sometimes get no sound and other times the static-distortion. I thought it was the orientation of the connectors, but that obviously wasn't the problem. It's acting like a short somewhere, as in a wire. Can't imagine it's something as simple as a connector wire shorting?
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Post by zippy on Dec 4, 2016 15:32:51 GMT
I presume that somewhere along the way you swapped over the cables to make sure they weren't the cause ?
Crackling noises sound to me just as likely to be a bad connection in a cable or plug.
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Post by markymark on Dec 4, 2016 16:00:45 GMT
I presume that somewhere along the way you swapped over the cables to make sure they weren't the cause ? Crackling noises sound to me just as likely to be a bad connection in a cable or plug. Do you mean the internal cables? I would have done that but it's a tight transmission line box, and the cables are folded around and through it with a lot of dampening material. I did run a continuity check on the internal cables from the x-over to the driver connectors and it tested positive. I eliminated amp to speaker cables first off. |
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Post by Stratmangler on Dec 4, 2016 16:40:47 GMT
Is there a dry joint on the crossover of the offending speaker?
Most recent equipment is put together using lead free solder, and lead free solder can have issues with cracks in the joints.
It might be worth running a hot soldering iron over the crossover joints.
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Post by markymark on Dec 4, 2016 17:23:27 GMT
Found it.
In the picture above, the nut at H4 was just loose enough to short. I found it by hooking up a junk receiver with the crossover hanging out of the cabinet. I began putting pressure on the speaker cable crimp connectors at the board and the sound would cut in and out, but it wasn't right somehow. That's when I realized the board was slightly flexing when I did that. I reached around and turned each brass speaker post from the back until one spun freely on the inside -- the post under H4.
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Post by MartinT on Dec 4, 2016 20:10:43 GMT
Well done!
By the way, capacitors are not supposed to measure with continuity - they block DC. You need more specialist testers for them, but they often show a slow build-up to infinite resistance which indicates that they are charging from the meter's power.
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Post by ChrisB on Dec 4, 2016 23:34:02 GMT
Well done for tracking it down.
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Post by Deleted on Dec 5, 2016 13:13:15 GMT
O the joys of PCB Crossovers  |
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Post by markymark on Dec 5, 2016 19:58:59 GMT
O the joys of PCB Crossovers I can see your point after what I went through! There's a nylon bushing - like a short section of PVC pipe - between the board and the back of the plastic plate for each of the binding posts. And two are shorter to angle the board in alignment with the trapezoidal shape of the cabinet. What's odd is that the binding posts from the outside can feel tight but the way the post/nut/lock washer is implemented on the inside you wouldn't know there was a problem -- it moves independently from the brass post. I'm not sure I would have found it if not for both in and out pieces being loose. |
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Post by Greg on Dec 5, 2016 21:58:34 GMT
Considering how easy it is to extract the crossover attached to the rear of the input socket panel which should be a standard size, I would be tempted to remove it and replace with a directly wired to speaker socket panel. Then remove the components from the printed circuit board, hard wiring them into an external enclosure. Space them out as crossover components need a bit of space around them, especially the two inductors which should also be mounted at 90 degrees to each other. You could improve the components although those Solen caps are pretty good for crossovers. Replacing the white ceramic coffin resistors with non-inductive Mills resistors would bring improvement to transparency, but I recon just getting the network out of the box will get a better sound as a result. I am a firm believer in keeping crossovers external from the cabinet. Doing it to my speakers made a large difference. I've done this with several pairs of speakers over the years and in every case it was beneficial to sound quality.
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Post by Deleted on Dec 6, 2016 4:49:48 GMT
I always hard wire my crossovers. PCB' crossover boards are a sound barrier imho. But i guess are a necessary evil where manufacture costs are concerned. I always used Mills 'MRA12' Resistors/ Ansar 'Supersound' poly props.
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Post by MartinT on Dec 6, 2016 8:15:28 GMT
I hard-wired my replacement Gale crossovers and used Mills resistors and Ansar Supersounds, too. Was very pleased with the results.
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