Monitor 7b purchase now Mod time

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  • Gardenstater
    Gardenstater Posts: 4,945
    edited August 2024
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    One thing to keep in mind with these fuseblocks is, the way Polk wired them, there is steel in the signal path. I'm not an electrical engineer but I know that Danny Richie is always saying no ferromagnetic material in the signal path. He points out that the sandcast resistors have steel end caps and he points out how many (expensive even) speakers people send in to him have steel in the binding posts, sometimes only the nuts but other times more than that. It probably has to do with self inductance or something due to a stronger magnetic field.

    In this case it is the rivet that holds the brass or phosphor bronze spring contacts to the plastic body. In order to hide the wires Polk spot drilled the heads and soldered the wires to that. When I redid mine I didn't like the steel but wasn't aware of Danny Richie. I kept the Polk configuration.

    One option if you are willing to have the wires show a little bit on the outside, is to solder to the end tabs. Or, if you don't mind the inconvenience the fuse could be moved to the inside. After all
    you only have 4 screws to remove to check it.
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
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  • F1nut
    F1nut Posts: 52,406
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    Fuse block from 7B.
    rhagzs3zx3hy.jpg

    Do yourself a favor, eliminate and bypass it altogether.
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  • Sumbrada69
    Sumbrada69 Posts: 41
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    I understand the fuse block issue. Such small amount of steel could effect things I guess, but having the protection to the tweeters especially with the very limited availability for replacements overrides the very slight possible induction issue. Replacing the wires with good and good solder (Cardas) will be my first plan. Can be changed later. Bringing the wires out and directly to the clips isolates them better but now you have to seal the wires for air leaks (can be done). I do not want to put them inside as visual inspection will be hampered. Geesh the binding posts I'm getting even have steel but plated parts. I know much better ones may not have any steel but I am not spending that much at this time. Law of Diminishing Returns! Got to figure out what replacement or mods gives you the best Bang for Buck as your electronics source, available funds, and "aging ears" make more difference than very slight possible inductance change.
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  • Gardenstater
    Gardenstater Posts: 4,945
    edited August 2024
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    Sumbrada69 wrote: »
    I understand the fuse block issue. Such small amount of steel could effect things I guess, but having the protection to the tweeters especially with the very limited availability for replacements overrides the very slight possible induction issue. Replacing the wires with good and good solder (Cardas) will be my first plan. Can be changed later. Bringing the wires out and directly to the clips isolates them better but now you have to seal the wires for air leaks (can be done). I do not want to put them inside as visual inspection will be hampered. Geesh the binding posts I'm getting even have steel but plated parts. I know much better ones may not have any steel but I am not spending that much at this time. Law of Diminishing Returns! Got to figure out what replacement or mods gives you the best Bang for Buck as your electronics source, available funds, and "aging ears" make more difference than very slight possible inductance change.

    Up to you :smile: . If I did it again I might still keep the fuse but only because I am using vintage equipment, but I believe what I would do is mount the fuse inside with the single aluminum rivet that is in the center going from inside to outside. This would allow me to solder the Cardas Litz wires that I used to the spring fuse holder tabs and there would be no steel in the signal path any longer. It has been a few years since I rewired my fuseblock and PCB etc. and I haven't checked the fuses once since there hasn't been any signal outage (knock on wood).

    It isn't the easiest job to desolder (can't let the plastic get too hot) the old wires and solder new ones to the rivet heads but it is totally doable. Easier if you don't use Litz like I did.

    Oh, and you will find everything is gooped up pretty good inside with hot melt glue which does a great job of sealing up anything like wires passing through and aluminum rivet and would work fine for the two wire holes if you decided to keep yours external but solder wires to the tabs. I used a very small hobbyist chisel to remove it easily enough.

    bbdzm4yoozj8.jpg

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    2ox97ai2dkcq.jpg



    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
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  • Sumbrada69
    Sumbrada69 Posts: 41
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    Well everyone, I have to admit a real big screw up. Any suggestions very much welcome. Got my 7bs I purchased and got burned without any recourse. They are 7bs cabinets, crossovers, peerless tweeters, and appear to be correct PRs. But they are not MW6500s, instead are Dayton Audio midwoofers which are 8 ohm. Sound I replace the mids with MW6500s if I can find some or other paths. I will be doing my 10s with all the suggestions and thank you!
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  • Gardenstater
    Gardenstater Posts: 4,945
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    That's a bummer. I see a couple on eBay. One silver basket and one black basket MW6500. Last week there was 4 silver baskets but 3 sold. I would stick with MW6500 myself and wouldn't want to experiment with substitutes.
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
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  • F1nut
    F1nut Posts: 52,406
    edited August 2024
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    Sumbrada69 wrote: »
    Well everyone, I have to admit a real big screw up. Any suggestions very much welcome. Got my 7bs I purchased and got burned without any recourse. They are 7bs cabinets, crossovers, peerless tweeters, and appear to be correct PRs. But they are not MW6500s, instead are Dayton Audio midwoofers which are 8 ohm. Sound I replace the mids with MW6500s if I can find some or other paths. I will be doing my 10s with all the suggestions and thank you!

    There are a bunch on eBay right now. Also, the vintage Polk MW6503 is a close match to the 6500.

    MW 6500
    Fs: 31.0000 Hz (Free air resonance)
    Vas: 2.6486 ft^3 (Compliance volume)
    Qts: 0.2190 Total Q
    Qms: 1.3000 Mechanical Q
    Qes: 0.2640 Electrical Q
    Sd: 0.0130 M^2 (Piston area)
    Xmax: 3.1800 mm (Cone excursion, linear)
    Revc: 6.5000 ohms (DC resistance)
    Znom: 8.0000 ohms (Nominal impedance)
    Pmax: 35.0000 watts maximum rated power


    MW 6503
    Fs: 29.8600 Hz (Free air resonance)
    Vas: 3.0017 ft^3 (Compliance volume)
    Qts: 0.3100 Total Q
    Qms: 1.7680 Mechanical Q
    Qes: 0.3760 Electrical Q
    Sd: 0.0130 M^2 (Piston area)
    Xmax: 3.1800 mm (Cone excursion, linear)
    Revc: 6.5400 ohms (DC resistance)
    Znom: 8.0000 ohms (Nominal impedance)
    Pmax: 35.0000 watts maximum rated power
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  • joebass3
    joebass3 Posts: 311
    edited August 2024
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    I'd also check Marketplace. For the price of 2 ebay MW6500's you might just find a set of complete speakers that have what you need. Hell might even find a set of Monitor 10's and end up with a couple of spare MW6500's.

    Not sure where you're located but here is a single 5a. I think (not 100% sure) that it has a 6500 midwoofer. Bonus is a spare Peerless! Price is right. https://www.facebook.com/marketplace/item/476816991661797/
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  • Sumbrada69
    Sumbrada69 Posts: 41
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    I found 2 mw6500s from same seller (Ebay). Visually they appear good. They measured the resistances of the 2. One is 6.9 ohms the other is 6.7 ohms. Is this ok? I have no idea. They said they know how to and will pack them correctly. They have a very high rating on Ebay.
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  • Gardenstater
    Gardenstater Posts: 4,945
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    Sumbrada69 wrote: »
    I found 2 mw6500s from same seller (Ebay). Visually they appear good. They measured the resistances of the 2. One is 6.9 ohms the other is 6.7 ohms. Is this ok? I have no idea. They said they know how to and will pack them correctly. They have a very high rating on Ebay.

    I wouldn't sweat that. We have no idea how accurate their meter is or whether it has a zero reading with the leads connected to each other or whether the terminals are free of oxidation. If people measure with the needle probes instead of alligator clips, the readings are less consistent.

    Make sure to tell the seller that you want them packed with plenty of cushioning material and to check them for any signs of voice coil rubbing before shipping.
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
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  • Sumbrada69
    Sumbrada69 Posts: 41
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    I sent them a note today after getting the values to check for magnet shift and gave them the procedure. We will see. Thanks
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  • F1nut
    F1nut Posts: 52,406
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    Sumbrada69 wrote: »
    I found 2 mw6500s from same seller (Ebay). Visually they appear good. They measured the resistances of the 2. One is 6.9 ohms the other is 6.7 ohms. Is this ok? I have no idea. They said they know how to and will pack them correctly. They have a very high rating on Ebay.

    I saw those, excellent seller and the measurements are good.
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  • Sumbrada69
    Sumbrada69 Posts: 41
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    Well, did my questions to the seller got latest reply tested well, and in between messages sold both on me this late am. ****! Purchased just now a different one blind to get at least 1. Hope I did not get burned.
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  • Gardenstater
    Gardenstater Posts: 4,945
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    That's unfortunate, especially since it is nice to get ones that are from same production dates. These are getting snatched up quickly it seems. Anyways, you are one down and one to go, hopefully!
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
    · Share on Facebook
  • Sumbrada69
    Sumbrada69 Posts: 41
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    A MW6500 is on order true condition unknown but still need 1 more. In the event a second working is a long term find what can or should I use for replacements? MW6503 or MW6510 (2) to match? I know they are not exact replacements but will they be that different? There is also the new replacements from https://www.midwestspeakerrepair.com/product/mw-audio-mw-6503-8-6-5-inch-polk-audio-copy-woofer/ I am searching EBay and Marketplace for speakers (5,7,10s) and some are available but distance is great (4-7hrs away) with a possibility they could be bad. I have 10As but stealing from them makes them unfunctional.
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  • F1nut
    F1nut Posts: 52,406
    edited August 2024
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    As I posted above, the MW6503 is closest to the MW6500.

    The Midwest is a one model fits all that fits none.
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  • joebass3
    joebass3 Posts: 311
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    Post your location so others can keep an eye out on FB Marketplace for you. I've picked up several sets for cheap but I'm in the Baltimore area. Make sense they would be all over Marketplace near me!
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  • Sumbrada69
    Sumbrada69 Posts: 41
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    I'm in Binghamton,NY 13903. Right at the corner of the boot.
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  • Gardenstater
    Gardenstater Posts: 4,945
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    I saw a 10A on eBay in NJ but it had the SL1000 tweeters. Should be 4 MW6500s though. Cabinet joints looked funky.
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
    · Share on Facebook
  • joebass3
    joebass3 Posts: 311
    edited August 2024
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    Single MW6503
    https://www.ebay.com/itm/156336391720?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=_F5CygBZTaS&sssrc=4429486&ssuid=kLEP-O5WQHm&var=&widget_ver=artemis&media=COPY

    Or a pair with dented dust caps. Perfect candidate for Rob's phase plugs!
    https://www.ebay.com/itm/176515838139?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=Xvx5ZFIuRG2&sssrc=4429486&ssuid=kLEP-O5WQHm&var=&widget_ver=artemis&media=COPY
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  • joebass3
    joebass3 Posts: 311
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    This is close to you if it's still available.
    https://www.facebook.com/marketplace/item/480585704916091/
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  • plastic_avatar
    plastic_avatar Posts: 747
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    Side notes to OP:

    - Read between the responses and make informed decisions.

    - If you decide to do a bucket of mods all at once, given the results from many in the forum *and* the documented (albeit subjective) improvements of each one cumulatively, it'll more than likely work out great (just like it has for others).

    It's strange that 7Bs turned out to be the prevalent speaker project for so many people lately! It's also *really* cool.

    I plan to do 'all the things' to mine, but that's how I am.

    Upgraded crossovers and properly working stock drivers are great on their own. Everything else is gravy for the goose.

    (I love gravy, so...)
    The Thrifty Setups in Mah House Big thrifty stereo in the basement w/ my custom SDA-1C (built with help from kind forum members) * Beautiful 1966 MCM GE console upgraded w/ Bluetooth, Dual turntable, and Paradigm speakers in family room * Swanky 1980 Realistic system and great TEAC eq with dancing colored lights in the living room * custom 5B on a system for my Dad * Ye Olde college stereo in the garage
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  • skipshot12
    skipshot12 Posts: 2,156
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    Don’t forget the gravy…..
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  • Sumbrada69
    Sumbrada69 Posts: 41
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    I received the MW6500 from EBay and the cone is froze. Even with excessive pressure the cone barely moves. Definitely not free moving like mine in the 10As. Got burned again. Trying to deal with him, but have a feeling EBay will get involved. Next, I go see and probably get a pr. of 10As semi local from Marketplace. Pics look good, had him do the Cone Push test which he did via video. 3 were quiet, 1 just slight sound. Drivers all look original. This project is getting expensive.
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  • F1nut
    F1nut Posts: 52,406
    edited August 2024
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    Sorry to hear that.

    I'm not sure if he is still doing this, but lawdogg was offering to repair frozen Polk drivers. You might try reaching out to him.

    This may be of interest to you.
    https://forum.polkaudio.com/discussion/197152/diagnosing-fixing-shifted-polk-mw-drivers
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  • Sumbrada69
    Sumbrada69 Posts: 41
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    Thanks what a detailed write up. I might try it myself if I get the 10s.
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  • Sumbrada69
    Sumbrada69 Posts: 41
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    Just got another set of 10As. 2 MWs are good and 2 are very minor scratchy. They are in better condition than my 10As. So I am going to take the 2 good MWs and test the 7s unmodded. Then comes the fun for the 7s! Then if I get the other MWs (have 2 in my 10s also little scratchy) fixed will do a more major mod to the 10s. My wallet is getting much lighter.
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  • Sumbrada69
    Sumbrada69 Posts: 41
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    Lawdogg agreed to fix some MW6500s I have from my 10As and the 10As I just got. I put 2 good MW6500s in the 7s and am listening to them now fully stock and aged what a difference from my 10s. Got my parts in so mod time. Thanks everryone for your help, suggestions. I will take pics.
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  • Sumbrada69
    Sumbrada69 Posts: 41
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    Hello all, working on the mods, crossovers first. I am thinking on using Armacell foam tape for the component spacers like the grey foam that was on the original boards. Comments, suggestions? Also, as the Caps are much larger that the originals I plan on stacking them using zip tie to hold them. Is it ok to drill new holes in the boards to get straighter leads? There will be quite a bit of lead exposed, heat shrink tubing or not? On the drivers when dynamatting, Just strips on the vertical supports or all over metal? I have seen pics of both ways. Thanks
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  • Gardenstater
    Gardenstater Posts: 4,945
    edited September 2024
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    Sumbrada69 wrote: »
    Hello all, working on the mods, crossovers first. I am thinking on using Armacell foam tape for the component spacers like the grey foam that was on the original boards. Comments, suggestions? Also, as the Caps are much larger that the originals I plan on stacking them using zip tie to hold them. Is it ok to drill new holes in the boards to get straighter leads? There will be quite a bit of lead exposed, heat shrink tubing or not? On the drivers when dynamatting, Just strips on the vertical supports or all over metal? I have seen pics of both ways. Thanks

    If I was going to stack capacitors without a valley to support them, I'd probably make some spacers out of 1/8" aircraft birch plywood (Midwest or other supplier) that is radiused to fit the caps, maybe with that Armacell also. Then for the cable ties I'd use a large enough size for strength and use a diamond point to make holes in the PCB, that are rectangular for the ties. I get the cheapo diamond points for a Dremel from Harbor Freight.

    I don't recall what caps you are using....ClarityCaps? Anyways, I used Sonicaps and didn't have to stack. I left about 1/8" air gap underneath the resistors. I was able to use the existing holes for the leads but drilling new ones is fine. I put some heatshrink on which is purely OCD :

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    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
    · Share on Facebook