Crossover Gurus: rehab advice for mangled Monitor 4s?
On3s&Z3r0s
Posts: 1,013
I recently came into possession of a pair of old Monitor 4s, the kind with a Peerless Tweeter, MW6500, and a binding cup with a fuse and posts instead of the spring clips used in later 4s. To say that they've seen better days is an epic understatement. When I first hooked them up I got very weak sound from the tweeters and nothing at all from the MWs. One MW had a shifted magnet and was frozen, but the other appeared fine. But in making that diagnosis, I discovered a much bigger issue. At some point someone "modded" the crossovers and just butchered them.
The problem is that these are older (1982) than the earliest schematics available here, and the crossovers don't look like anything I've ever seen before. There are no resistors at all, just two inductors and a 5.8uF cap. Based on the condition of the hot glue, it looks like these components are all original, but obviously some of the electrical connections have been changed. The leads to the fuse were cut, but I have no way of knowing if there were other parts that were removed. Interestingly, the 5.8uF cap has one leg crimped into the terminal for the negative lead. I've never seen anything like this in any other Polk crossover, but it appears to be original and untouched by the modder. Same thing with one of the small inductor leads but in the positive terminal. That at least looks a bit more normal.
I've included a picture of the crossover (warning: some viewers may find this image disturbing :eek:) and my attempt at a schematic of the crossover as it's currently assembled (pardon my limited skills in this area). I'm hoping one of the guys who actually understands crossovers could take a look and see if this beast looks like something that could actually work as a crossover. If not, would anyone be willing to hazard a guess as to what the original crossover might have looked like so I could try to get them into some kind of working condition?
At this point, I'm thinking these things are beyond repair (the cabinet that contained the frozed MW is shot also), but I'd like to figure out if the tweeters and unfrozen MW6500 are still viable. Any help would be greatly appreciated... even if it only ends up being an academic exercise. Thanks, as always!
The problem is that these are older (1982) than the earliest schematics available here, and the crossovers don't look like anything I've ever seen before. There are no resistors at all, just two inductors and a 5.8uF cap. Based on the condition of the hot glue, it looks like these components are all original, but obviously some of the electrical connections have been changed. The leads to the fuse were cut, but I have no way of knowing if there were other parts that were removed. Interestingly, the 5.8uF cap has one leg crimped into the terminal for the negative lead. I've never seen anything like this in any other Polk crossover, but it appears to be original and untouched by the modder. Same thing with one of the small inductor leads but in the positive terminal. That at least looks a bit more normal.
I've included a picture of the crossover (warning: some viewers may find this image disturbing :eek:) and my attempt at a schematic of the crossover as it's currently assembled (pardon my limited skills in this area). I'm hoping one of the guys who actually understands crossovers could take a look and see if this beast looks like something that could actually work as a crossover. If not, would anyone be willing to hazard a guess as to what the original crossover might have looked like so I could try to get them into some kind of working condition?
At this point, I'm thinking these things are beyond repair (the cabinet that contained the frozed MW is shot also), but I'd like to figure out if the tweeters and unfrozen MW6500 are still viable. Any help would be greatly appreciated... even if it only ends up being an academic exercise. Thanks, as always!
Post edited by On3s&Z3r0s on
Comments
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I have not seen the factory X/O so I cant tell what he wired (if anything) wrong , but if he wired just a inductor to the tweeter and tried it he killed the tweeters
I would get any close value capicitor and wire that from a stereo to a tweeter to test them .
Looks like a 12 db slope on one of the two drivers (cap & coil) on the mw.
If the drivers are dead the cabinet is bad grills and logos are bad I would have to say (like you said) its junk ,
I suggest looking on ebay for some x/o pictures of the monitor 4 see if that helps . -
The schem. in post #1 is of a series connected crossover which is why it looks unconventional vs the typical far more common parallel config.I guess Polk dabbled with series designs in the early days.
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I have not seen the factory X/O so I cant tell what he wired (if anything) wrong , but if he wired just a inductor to the tweeter and tried it he killed the tweeters
I would get any close value capicitor and wire that from a stereo to a tweeter to test them .
Looks like a 12 db slope on one of the two drivers (cap & coil) on the mw.
If the drivers are dead the cabinet is bad grills and logos are bad I would have to say (like you said) its junk ,
I suggest looking on ebay for some x/o pictures of the monitor 4 see if that helps .
I've done lots of googling and haven't come up with much that looks close. The closest has been an old M5Jr XO circa 1986 that was built similarly (everything soldered point-to-point and taped together in the center of the cup). But even that one appeared to use more common components (smaller small inductor than what I've got, 12uF cap, and a resistor). The pic was too lo-rez to see all the connections, but I'm guessing it followed the more normal schematics. I'll attach the pic I found below.The schem. in post #1 is of a series connected crossover which is why it looks unconventional vs the typical far more common parallel config.I guess Polk dabbled with series designs in the early days.
Cool, so it sounds like this is a somewhat valid crossover design? Like I said I did get some sound out of each speaker, it just all seemed to be coming from the tweeter and pretty faint. I don't know if that's characteristic of a tweeter that's blown or just bad soldering/badly corroded hookup wire, etc.
I think maybe I'll try to clean everything up but keep the XO the same electrically... see if I can get the MW working and make them sound at all like a speaker. Thanks for the help! -
Yes and some such as the late Bud Fried consider series networks superior to parallels.However it's not used much these days except for maybe a few DIY'ers.Heres some further reading on the differences between series and parallel networks. http://sound.westhost.com/parallel-series.htmOn3s&Z3r0s wrote: », so it sounds like this is a somewhat valid crossover design? ! -
Interesting article, thanks. After having gotten everything cleaned up and put back together the sound is better... the non-frozen MW works now, but even still it doesn't sound right to me. It's totally anemic in the low-end, even for a bookshelf. I'm sure something could still be wrong, but if this is even close to the way it's supposed to sound I can see why Polk abandoned this XO design. It's my first time hearing Peerless tweeters and they are definitely interesting... very twangy. I'm curious what they would sound like in a well-functioning speaker.
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On3s&Z3r0s wrote: »Interesting article, thanks. After having gotten everything cleaned up and put back together the sound is better... the non-frozen MW works now, but even still it doesn't sound right to me. It's totally anemic in the low-end, even for a bookshelf. I'm sure something could still be wrong, but if this is even close to the way it's supposed to sound I can see why Polk abandoned this XO design. It's my first time hearing Peerless tweeters and they are definitely interesting... very twangy. I'm curious what they would sound like in a well-functioning speaker.
As a experament just use another XO , I suggest ebay try the monitor 5 xo , try and find something w/the same size mid and tweeter , I use the moniter 5 (not the JR model and they sound real good) .
Never know you may have great sounding stuff just needs to be tinkered with .
But I have had a few polk models and all sounded good (cs & ls series) -
Scott,
Sorry for the delay in posting these, I was caught up in work stuff for the last couple of days and couldn't break free...
Just my opinion, these (my) speakers seem to be on-par with my other vintage Polk's in terms of tweeter performance and bass response... in fact. I would rate the Peerless tweeter equal to or higher than the RDO-194's I've swapped into my Monitor 5's and 5jr+'s. I'm listening to them now back-to-back playing "The Nine Worlds" by Achillea. Bass response is also very good in my opinion for a bookshelf speaker.
Anyway, here are the pics as you requested. It didn't appear that these speakers were ever touched or modded in any way from original....
Polk Monitor 4
Polk Monitor 5jr+
Polk Monitor 5B
Polk SDA 2B Studio -
Hey Starbuck, thanks for posting those pics! I really appreciate the time you took to do that. Yours and mine are definitely the same beast, and other than removing the fuse from the circuit the person who modded the ones I've got left them electrically the same. Unless the connections to the drivers in mine were changed at all polarity-wise (and I don't have any reason to think they were) the schematic I'll attach should be how they came from Polk.
I'm sure there are lots of reasons why mine don't sound right. The previous owners were definitely not gentle with them. The tweeters do sound good, but overall the one working speaker just doesn't sound right. The MW6500 in it looks fine, but I think I might try putting an MW6503 in its place and see if that makes a difference. I considered trying a different XO as 20hz suggests. If I can find something cheap that would have been used with the Peerless I might give it a go just out of curiosity. Since one of the cabinets on these 4's is unrepairable it really doesn't matter much what I ultimately get the one speaker sounding like. -
On3s&Z3r0s wrote: »Hey Starbuck, thanks for posting those pics! I really appreciate the time you took to do that.
np, I'm happy to contribute what I can to the collective knowledge contained on this board.... it was interesting to learn that these early 4's were a surprising departure from the 'norm' in terms of speaker design employed by Polk over the years... makes 'em that much more unique in my opinion... and.. good luck with your project!
StevePolk Monitor 4
Polk Monitor 5jr+
Polk Monitor 5B
Polk SDA 2B Studio -
I wonder what the Ohm rating is on those. I would almost think it would be in the 2ohm range.
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Knowing that the crossover was right I couldn't wait... retrieved a spare MW 6503 and gave it a try. No question that a bad MW was the problem. I tried to do an excursion test on the old MW 6500 with a battery and got some faint static but little movement. The magnet definitely shifted. I should have figured it was that all along, but the other one was totally obvious.
With the 6503 in there the one that works sounds great! If I get ambitious I might try to rehab the XO in the busted cabinet just to get a better idea of what the Peerless sound like in stereo. Too bad these got kicked around so much. They definitely do seem like a nice little speaker and a very interesting historical departure from the norm as far as the crossover goes.
Regarding the impedance I don't know enough to know how they figure that. I'm pretty sure the 6500/6503 is nominally 8 ohms. I don't know what the Peerless is, but I think the later tweeters (SL 2000) are 8 ohms also. -
I would almost think it would be in the 2ohm range.
Nah it would be somewhat higher.Polk would not produce a speaker with that punishingly low of an impedance since they would most likey be mated to stereo receivers and int.amps.which in those days (and even many modern units) would prefer seeing something closer to an 8 ohm load.
