Monitor 7c suddenly out of Phase?

Dmac99 · January 2020

I have a pair of 7c's and I am the original owner. I have used these on and off for years and really like them. I just brought them out and hooked them up to my main system and they did not sound good. Thin bass, imaging was off. They sounded out of phase, so I reversed the wires on one speaker and they sound great again. I checked all of my wiring and it is correct, opened up the speakers and verified that black wires were going to both red terminals on the woofers. This has never been the case with these before and I am a little confused. I have never had any work done on them or replaced any drivers. Has anyone else ever had this experience?

Tony M · January 2020

Did the wires get switched at the amp end?

The older I got, the more it's happened.

F1nut · January 2020

Perhaps you connected the speaker cables incorrectly at the receiver end.

Dmac99 · January 2020

Thanks, I checked the wires at the Amplifier and they are correct. I just brought these out today and hooked them up in my main system where everything was working fine with my main speakers. no other changes were made.

F1nut · January 2020

In that case I'd have to say you had the wires reversed at the speaker, but didn't realize it.

Dmac99 · January 2020

I have checked at both ends and all reds are going to red and all blacks are going to black. same as my main speakers that were in place. I even removed the drivers to ensure they were hooked up the same way in each speaker. Wondering if an out of spec cap or resistor could reverse polarity? This is bizzare.

Dmac99 · January 2020

I think that I found the problem. No idea how the terminals got reversed, I did not even know that they could come off. Someone (kids) must have fooled around with them when they were in the attic these last few years. I will assume that the colors should be on the same side. Whew!

Tony M · January 2020

GOOD!

That was going to be my next question for it happened to me also. I bought a pair of SDA-CRSs that had the switched knobs. I didn't know red is supposed to be on the left for both speakers at the time. That was a trip! But the good guys here on the forum helped me find out what was wrong.

I think red is supposed to be on the left...

delkal · January 2020

On my Monitor 7s the plastic color coded caps just screw off. Nothing to hold them on. I can see someone playing around and getting them mixed up.

Dmac99 · January 2020

Thanks, i checked my 5 jrs and both have red on the left. The 7Cs are hooked up and sounding great now. Thanks for the suggestions everyone!

Tony M · January 2020

My SDA-CRSs knobs unscrewed till they came off too.

Gardenstater · January 2020

My 7B's have red on the right.

westmassguy · January 2020

Gardenstater wrote: »

My 7B's have red on the right.

That's wrong

Gardenstater · January 2020

@westmassguy do you have a link to the crossover schematic so I could pull mine out and verify. I'm the original owner so I'd be a bit embarrassed to have got them reversed but anything is possible over the years since 1979.

mhardy6647 · January 2020

OK, I offer this suggestion with trepidation

but I am gonna offer it anyway.
STRICTLY AS-IS and FWIW; no warranty express or implied.
I ALSO don't GUARANTEE this will work for a Monitor 7 -- so take this with a big grain of salt.

It is straightforward to test the polarity of a driver (woofer, e.g.) with a 'battery' (i.e., a single 1.5 volt cell...like an AA or C "battery"). A crossover won't pass DC per se, but the transient generated at the instant when a DC circuit is completed may make it through a crossover, at least to the woofer.

This is a testable hypothesis, so I just tested it with a Polk "Monitor Series" loudspeaker.

My Monitor 7As are boxed up and in the basement, but there is a pair of newer ("newer" being, of course, relative) Monitor 5s here. I believe these to be "Monitor 5B" , but I am not 100% clear on that! They look like this. The s/n of the one I tested is 5 90873.

DSC_6916a by Mark Hardy, on Flickr

Remove the grille so that the drivers are visible. Take a piece of zip cord or speaker cable and connect to the 'red' and 'black' input terminals on the speaker to to be tested. Take the other end of the "negative" speaker lead (i.e., the one connected to the black terminal) and hold it to the negative ('bottom') end (electrode) of a 1.5 volt cell. While watching the 'midwoofer', touch the "positive" speaker lead (i.e., the one connected to the speaker's red terminal) to the 'top' (positive) electrode/button on the cell. You should hear a scratchy /static-y sound the instant the circuit's completed, and the woofer cone should move when the circuit is completed. If the speaker's terminal polarity is as expected, the woofer cone will move out. If it moves in, the polarity is reversed from the expected.

There are myriad caveats for any given loudspeaker, but the aforementioned Monitor 5 family member demonstrated the 'expected' polarity with this test -- and I would fully expect a Monitor 7 family member to do likewise.

The use of a low voltage, relative low energy 'battery' (cell) should make this a safe test* -- but current will be flowing through (at least) the woofer voice coil, generating heat... so the test should be brief indeed. It only takes an instant to perform the test... but, again, I assume no responsibility for any mishaps!

HTH.

__________________
* I, e.g., routinely test 'as found' drivers including tweeters thus -- although a DMM is indubitably better/safer (e.g., for testing expensive and very delicate tweeters for "voice coil continuity"). A DMM won't answer the polarity question, though.

mhardy6647 · January 2020

For completeness, I will also mention that there is an inexpensive (again, it's all relative), simple tool designed to determine polarity unambiguously. I have one of these gizmos because I deal with very vintage Altec and JBL drivers, and those companies, over the years, had interesting (and even variable) polarity labeling conventions for their drivers!

For a hundred bucks, one can always know, for sure, the polarity of any driver (or speaker system). This gizmo also can be used to test cables (i.e., the wiring of the terminations) -- so it is danged handy for folks who have to worry about such things.

https://www.galaxyaudio.com/products/cpts

This is designed for 'pro' use, so adaptor cables for typical home hifi (unbalanced "RCA" connections) need to be procured or fabricated, but that's not a big impediment

pitdogg2 · January 2020

Gardenstater wrote: »

@westmassguy do you have a link to the crossover schematic so I could pull mine out and verify. I'm the original owner so I'd be a bit embarrassed to have got them reversed but anything is possible over the years since 1979.

@westmassguy Dave is correct

https://us.v-cdn.net/5021930/uploads/attachments/4/6/7/2/8/18774.pdf

Gardenstater · January 2020

pitdogg2 wrote: »

Gardenstater wrote: »

@westmassguy do you have a link to the crossover schematic so I could pull mine out and verify. I'm the original owner so I'd be a bit embarrassed to have got them reversed but anything is possible over the years since 1979.

@westmassguy Dave is correct

https://us.v-cdn.net/5021930/uploads/attachments/4/6/7/2/8/18774.pdf

Thank you! That's not my exact schematic BUT I doubt they would've made any major polarity changes....SO.....it looks like all I have to do is pop out my MW6500 mid bass driver and check resistance between the black binding post and the (-) terminal on the driver, that is assuming that the inductor L2 would have a measurable DC resistance with my meter. May be too small to measure without a milliohm meter? Hmm.

Alternative would be to test at the tweeter (-) to the black assuming I can tell which is which with the midbass driver removed. I've never removed the tweeters but I guess I could do it without damage.

Thanks.

Gardenstater · January 2020

@mhardy6647 Thanks for that. I got nervous just hearing the crackle sound when I tested the DCR of my MW6500s. I may try what you suggest if I can't visibly or electrically trace the polarities by using the schematic or direct observation of the crossover.

mhardy6647 · January 2020

I am sensitive to your sensitivity, believe me!

pitdogg2 · January 2020

@Gardenstater you could go yourself to the vintage speaker page and find all you need. Polarity never changed and Dave is still correct.

https://forum.polkaudio.com/discussion/38755/polk-audio-speaker-wiring-schematics-amp-more-all-models-except-sda#latest

Gardenstater · January 2020

@pitdogg2 @westmassguy Thank you. You guys were right. I traced the circuitry and indeed red (+) binding post cap is supposed to be on the left. I may have confused myself a long time ago when I did a crossover mod recommended by my audio store where they had me put a silver mica capacitor (don't remember the value) in parallel with each of the other capacitors. I was curious or even skeptical so I actually installed them with an A/B switch so that I could compare on the fly Mod/No mod. Can't say I could ever hear the difference! When I put things back together I may have said well black wires should go to the black binding post or something like that. It still sounded good but I guess I was 180deg out of phase with the sub (which I never had at that time long ago), although I have the sub on 0deg phase shift. Sounding maybe a little more coherent now! None of those schematics are early enough for my crossover which is for the Peerless tweeters and the MW6500. A couple differences I noticed relative to the earliest one I could find there is that I have a 2.7Ohm, 5W resistor instead of the 2.0 and I have an additional 4.5 Ohm, 5W resistor, one side of which goes to the little inductor (I couldn't verify my value w/o LCR meter) and the other side goes to the black (-) binding post and the 34 uF electrolytic cap.

My hokey A/B switch installation:

I wish I could find the correct schematic for my Peerless + MW6500 crossover. I guess this is the closest we have on the site:

https://us.v-cdn.net/5021930/uploads/attachments/4/6/7/2/8/18772.pdf

tonyp063 · January 2020

You appear to have the same crossover as I do in mine.
Here you go.

Gardenstater · January 2020

@tonyp063 Thank you!! Perhaps I got the resistors mixed up and I'll have to double check. I *thought* the 2.7 Ohm and 4.5 were reversed from above but I probably got my eyes crossed since they are right next to each other on the circuit board! Sucks being fallible lol. I see one problem where I have 250V2AMS fuses which I'm assuming are 2.0 amp, but don't know whether fast or slow! I do remember blowing the fuses once or twice and in my youthful foolishness may have upped the value! That was way back when I had a Technics SA500 55W per channel receiver which probably was going into clipping or such.

As an interesting aside, my speaker boxes definitely say Model 7B and my handwritten receipt from the audio store says 7B (paid $350 pr) but the sticker on the crossover says 7, no A, B, or C. Bought 'em in 1979.

pitdogg2 · January 2020

When we rebuild the XO with better film caps that 750pf mica goes bye bye. My understanding was it was to make the Mylar caps sound better... Better than what I have no clue lol

Gardenstater · January 2020

@pitdogg2 Thanks I honestly NEVER heard any difference. Speakers still sound so good I can't imagine them sounding better with a rebuild. I know curiosity will get the better of me and I will have to find out though.

pitdogg2 · January 2020

Trust me with better polypropylene caps your sack will be blown back.
Take your pick Sonicaps or Clarity and new wire wound non inductive resisters = BLISS

Gardenstater · January 2020

@pitdogg2 What effect would I get by replacing the big inductor with a Copper foil inductor like a Jantzen or Erse? Worth it?

westmassguy · January 2020

Gardenstater wrote: »

@pitdogg2 What effect would I get by replacing the big inductor with a Copper foil inductor like a Jantzen or Erse? Worth it?

There's nothing inherently wrong with the original Inductors. Foil would be my last choice, due to the way they're made. The lacquer or synthetic film they apply to the copper is easily injured during the manufacturing process. Oxidation spreads very easily over time.

Gardenstater · January 2020

westmassguy wrote: »

Gardenstater wrote: »

@pitdogg2 What effect would I get by replacing the big inductor with a Copper foil inductor like a Jantzen or Erse? Worth it?

There's nothing inherently wrong with the original Inductors. Foil would be my last choice, due to the way they're made. The lacquer or synthetic film they apply to the copper is easily injured during the manufacturing process. Oxidation spreads very easily over time.

I think the Jantzens are impregnated with wax and use a paper dielectric. The Erse Foil Q's use BOPP (biaxially oriented polypropylene) dielectric and the whole inductor is encased in a polymer which is quite difficult to remove but seems to prevent oxidation. I was thinking perhaps the lower DCR compared to the stock 1.55mH magnet wire inductor would be one of the advantages? I don't have a milliohm meter but I measured about 0.7 Ohms and a 16 gauge Erse Foil Q of 1.60 mH (which I guess I'd have to adjust by removing some foil yikes) is 0.524 Ohms and has 4N copper and eliminates any skin effect due to the 0.003" thick foil. It was just a thought. Thanks for your comments.

VSAT88 · January 2020

I can confirm that a small piece of tissue paper can be used on top of a tweeter with a power source (I.E. 9v battery) to check tweeter polarity and yes, the red is on the left, the black is on the right viewing the speakers from the back. Same as my SDA 2b's.

Monitor 7c suddenly out of Phase?

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