Monitor 7 sound unbalanced

smibe

Hi everyone, I'm a new member to the forum. Signed up after I got a pair of second hand Polk Audio Monitor 7s.

I am very impressed by the sound, are running them from 90s Sony pro logic receiver rated at 120wpc with an old Mirage 10 inch sub to fill in the low end and they sound great on both music and movies.

Only issue is, as I've gotten used to their sound, I am becoming more and more convinced the output isn't balanced. The seller I bought them from said he owned them since new and at one point managed to blow the midrange driver on one of the speakers, which he had replaced by the local polk distributor. However, I feel that this speaker is quieter than the other one. It has less presence I guess, when sitting in the sweet spot, equally apart from both speakers, the channel that wasn't blown seems to have most of the music coming through it. I have swapped the speakers around to different channels of the amp, to make sure it isn't the amplifier, and the issue follows the speaker.

So what are the probable issues here? From what I've read it may be that the tweeter was also damaged, and it's output has been reduced as compared to the other speaker. I remember reading the magnetic fluid could solidify.

Also, perhaps it could be the crossover network was also damaged?

Finally, when replacing a midrange driver, is it better practice to replace both at the same time so they are "matched"?

I have no expertise at all what it comes to soldering, what to look for when taking apart a speaker, etc., so any advice is greatly appreciated.

I'd also just like to add how much I appreciate this forum as a source of info, and Polk for having easily accessible specs on their vintage speakers. Was very helpful when seeing if it was worth it buying these speakers, and it definitely was, despite this issue I am currently having.

BTW the speakers I have are the Series IIs with the SL2500 tweeter.

[Deleted User]

Hello smibe,
Welcome to Polk's forum! If the speakers were mine I believe I would be suspicious of the repaired one. It might be a good idea to begin by removing the 6.5" driver and look at the part number found on the label. Let us know what you find.
Regards, Ken

smibe

Kenneth Swauger wrote: »

Hello smibe,
Welcome to Polk's forum! If the speakers were mine I believe I would be suspicious of the repaired one. It might be a good idea to begin by removing the 6.5" driver and look at the part number found on the label. Let us know what you find.
Regards, Ken

Hey just opened them up, was a bit nervewracking to be honest, first time I'd ever taken out a driver.

I checked both speakers and what the seller told me seems legit. The repaired speaker with the obviously newer driver is model MW6502 and it was manufactured in May 2001, whereas the original speakers uses the same driver but it's dated to April 1991, so it's about ten years older, yet that speaker sounds louder.

I guess as a process of elimination I should check the crossover/tweeters next? Not sure what I am looking for there. Would it be best to just replace the crossover with newer capacitors and purchase the RDO 198s from Polk and see if that solves the problem? I am planning to do that eventually anyway, once I have some spare cash (poor uni student here).

Thanks again for your help.

[Deleted User]

Hello,
That is the correct driver, so one possibility eliminated. Usually if the crossover network is damaged you can see parts that look stressed. Is it possible to pull out the crossover and take a photo?
Regards, Ken

smibe

Kenneth Swauger wrote: »

Hello,
That is the correct driver, so one possibility eliminated. Usually if the crossover network is damaged you can see parts that look stressed. Is it possible to pull out the crossover and take a photo?
Regards, Ken

Hi Ken I just looked at the crossover before I saw your reply so I've screwed it back in already.

I don't have a digital camera handy on me but it looked ok I guess? What sort of stress should I be looking for.

Only thing I noticed looking closely was that there was some slight brown discolouration to the larger, yellow tube, along the length of it and on the metal ends. Is that a capacitor? (showing my ignorance here). And one of the smaller ones also had some darkened, black parts on the metal ends, where the other... two I think had normal coloured metal ends.

Would that count as stress? Or is it more noticeable than that generally? Like maybe a physical bulging of the tube?

Again, thank you Kenneth for your help and the quick replies.

[Deleted User]

Hi smibe,
No problem, everyone's glad to help another vintage Polk owner. The kind of discolouration you've described is what to look for, signs that too much heat was generated somewhere in the circuit. The Monitor 7 series II crossover is pretty simple, so getting things sorted out will be no problem. The best diagnostic approach would be to exchange the two crossovers (just remove the tweeters and two active 6.5" drivers and exchange them from one cabinet to the other) and see if the lessened sound stays with the suspect crossover.
Regards, Ken

smibe

Well it's good to know we've narrowed it down fairly quickly, and that I'm not just hearing things.

I might just look at the other crossover now and see if it appears to be discoloured as well. Not sure if I'm up to swapping over the drivers tonight, it's getting pretty late in New Zealand. But I may try that tomorrow and see how I get on.

If it is the crossovers I assume I just need to buy new capacitors that match the original specs. I've already downloaded a crossover schematic so guess it should be fairly straightforward, but do you have any tips or links to a how-to-guide? I haven't soldered before so this will all be new to me. Also, is there a certain brand or type of capacitor that works well with these Polks?

Cheers.

drumminman

Hi Smibe, welcome to CP.

+1 on the approach KS described to diagnose the problem with your M7's. From your description it sounds like you have some fried caps and maybe resistors.

Replacing the caps with high quality film caps and the resistors with something like Mills will yield an impressive improvement in sound quality. There are different levels of quality for caps depending on your budget, though you generally get what you pay for.

Search the forum and you'll find lots of threads on rebuilding Xovers. Even some on how to solder.

As Kenneth said, we're happy to help a fellow vintage Polk owner. Don't hesitate to post your questions.

jimbo1421

Before you order parts, get the capacitor and resistor values directly from parts on the crossover. Polk sometimes made undocumented changes.

Soldering is easy. Learn how here: http://mightyohm.com/blog/2011/04/soldering-is-easy-comic-book/

Of all the vintage Monitors, Matt Polk liked the 7s best. So do I.

Jim

smibe

Thanks for the advice drumminman, I'll have a search of the forums and see what I can find.

Any ideas on what sort of budget I'd be looking at to replace caps/resistors, in both speakers, with mid-range components?

I am on a tight budget, but I don't want to go too cheap as I'll probably end up doing it again later when I have more money to play with.

smibe

Thanks for the heads up Jim. I'll make sure I do that.

Even with the unbalanced output of these speakers they still sound very very good. I don't own or have much experience with other vintage speakers, other than a pair of KEF Concord IIIs I recently bought for my study and a pair of Technics Linear Phase Sb-F2s which I borrowed from my Uncle.

I have noticed that the KEFs tweeter sounded slightly sharper in comparison to the Monitor 7s, but I guess that's probably due to the limitations of the burnt out crossover or the SL2500 tweeter, or even simply a matter of taste.

I am very happy I got into the Polk Audio world, and I am now looking forward to finding out how much better these speakers can sound once I've upgraded them a little. The thing that stands out the most for me is the soundstage, even with the unbalanced output(which to be fair is only a slight difference to my ears), I don't feel like I really a 5.1 system when stereo sounds so lifelike watching tv/movies.

kidglove13

I too have the same issues with the yellow caps in my 7's crossovers
The ends of the yellow caps are brownish, looks like they started to leak or something
I ordered new caps clarity caps for the highs and dayton caps for the lows
I have not replaced them yet

drumminman

Jim makes an excellent point - the schematics are useful, but have a look at what's in the speaker as well.

The schematic can be found here and is the one labelled "m7s rev2". Don't know what your budget is but I'll throw this out there: go with sonicap gen 1 caps, and mills resistors, which can be found here: http://www.soniccraft.com/products.htm

According to the schematic each speaker requires:
1 each 8uf cap $17.93 usd
1 each 12uf cap $22.50 usd
1 each 27uf cap $40.04 usd

1 each 2.7ohm resistor $4.95 usd

Total $85.42 per speaker. I've modded three different speakers (one SDA, one RTA 12C from the monitor line and a Def Tech) using these components and been very satisfied. However, you can go cheaper, and others will chime in with other good recommendations.

Just my experience.

kidglove13

I am no expert but I know that there are only 2 caps per each of my 7's crossovers
1- 12uf
1-34uf
per crossover

stevepe

kidglove13 wrote: »

I am no expert but I know that there are only 2 caps per each of my 7's crossovers
1- 12uf
1-34uf
per crossover

My 7's are the same, but get your values from yours. If you don't need to go for first class right now on account of money, (like me), may I suggest you order some stuff from Parts Express. Install 5% dayton poly caps for the tweeter and new electrolytics for the midrange. I used two 17uf's to get 34uf for down below.

Do you have a multi-meter? Desolder and pull free a leg of your resistors to test the values. Leave them for now if they're OK, and upgrade later when the money is there. This will get you balanced if the caps are causing the problem, and will sound better than 25 year old stock caps. Get practice doing this, improve the sound, and get confidence for an upgrade later if and when it suits you. I think were talking $15 for both.

Steve

westmassguy

kidglove13 wrote: »

I am no expert but I know that there are only 2 caps per each of my 7's crossovers
1- 12uf
1-34uf
per crossover

The original and revised Mon 7s (with SL2000 tweets) have a single 12uf cap. The revision 2 Mon 7s with the SL2500s have two caps in the high pass. The 12uf like the others, plus an 8uf in parallel with the resistor. Similar to the "TL" mod.

rromeo923

I was having a similar problem the other day and it turned out that one of my patch cords was faulty. Have you tried swapping them?