SDA 2B rebuild

I recently picked up a pair of 2B studios and am (slowly) starting to work on brining them back to life. A few of the posters in the Vintage Monitor and SDA Speaker Deals thread were interested in the details of how they turned out, so here you go...

I've wanted a set of SDA's for a long time and finally came across these on CL. The seller disclosed that one of them didn't work, but accepted a low enough price that I took a chance on them. It turns out there's another thread on the forum from the previous owner attempting to fix the problem, but I didn't know that at the time. Unfortunately, they're going to need a bit more than I was hoping for.

rwvafh023zqw.jpg

The cosmetic condition is pretty nice. The cabinets have some minor scratches on top and a few small chips at the rear edges. The grills are very good. The PR's are both good.

I was not so lucky on the drivers. I'll start with the left speaker (the one that works): Everything seems fine except the mw6511 has been replaced by a 6500.

The right speaker has suffered something catastrophic. The thread from the previous owner showed the polyswitch completely destroyed. They attempted to replace it along with the 12uF cap, but they must not have owned a meter, because it was a waste of time. In addition to the polyswitch, the 2.7Ohm resistor and all three drivers are shot. They all read open. The inductors seem to have survived. There are no visible signs of damage, and they all measure at the expected DCR.

So it looks like replacing pretty much every driver in the set in addition to new caps and resistors for the crossovers (I expected that).
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Comments

  • I have some questions about drivers. So far I've managed to pick up a pair of 6503's and I'm planning to get the RDO-198's, but the 6511's don't seem to be readily available. There was a pair on ebay recently, but I wasn't quick enough.

    Are 6511's hard to come by?
    If so, are there any good alternatives?

    It looks like Midwest recommends the reproduction 6502 as a replacement. There are plenty of used original 6502's for sale. Anyone tried them as a substitute for the 6511?


    Here's something interesting I found while examining the damaged speaker. The sticker on the blown 6503 came loose from the magnet and I noticed it was actually two stickers stacked together. Holding it up to a light shows that the sticker underneath is for a 6502. My first thought was that a shady seller had re-labeled it, but the 6502 label is blank, no stamps or writing. The blue text in the photo is the label underneath. The red text and all of the writing and stamps are on the top.
    et09rgy09tv8.jpg

  • F1nut
    F1nut Posts: 46,296
    Hold out for some 6511's. The ones Midwest offer are a one size fits all, which is definitely not the case.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."


    President of Club Polk

  • VSAT88
    VSAT88 Posts: 1,062
    bigmontana wrote: »
    I have some questions about drivers. So far I've managed to pick up a pair of 6503's and I'm planning to get the RDO-198's, but the 6511's don't seem to be readily available. There was a pair on ebay recently, but I wasn't quick enough.

    Are 6511's hard to come by?
    If so, are there any good alternatives?

    It looks like Midwest recommends the reproduction 6502 as a replacement. There are plenty of used original 6502's for sale. Anyone tried them as a substitute for the 6511?


    Here's something interesting I found while examining the damaged speaker. The sticker on the blown 6503 came loose from the magnet and I noticed it was actually two stickers stacked together. Holding it up to a light shows that the sticker underneath is for a 6502. My first thought was that a shady seller had re-labeled it, but the 6502 label is blank, no stamps or writing. The blue text in the photo is the label underneath. The red text and all of the writing and stamps are on the top.
    et09rgy09tv8.jpg

    Looks like you got the top secret subliminal message Polk Audio hidden CD there ! COOL !!! Play it backwards !!!!!
    Where in Alabama ?
  • I'm finally making some progress on these. I have almost all of the parts I need and have started tearing them down and working on the cabinets and crossovers.

    I picked up some replacement MW's on ebay and ordered new RD0198's and crossover components. The only item left is dynamat for the driver baskets which should be delivered this week.

    I've got the terminal cups and crossovers done, and am currently in the middle of giving the cabinets a thorough cleaning. Next up will be installing the hurricane nuts and re-assembling.
  • Here's a photo of the finished terminal cup:
    axhbj3iyilxl.jpg

    I turned a plug out of delrin to fill in the hole from the pin/blade connector and then used it to mount the new binding post. I think the look is a great match and blends into the rest of the black plastic. It's also removable if I want to change to a different connector in the future.
    d2ujlq654cbn.jpg

    The binding posts are Pomoma 3770's. I usually use banana plugs, so they'll work fine for me, but I'm not sure I would recommend them to others. The cross hole turned out to be very small, so anyone using bare wire would have a hard time getting it to fit. The knobs are also smaller than most, so probably more difficult to torque down on spades or wires. On the plus side, the smaller knob does make it easier to get your thumb or finger in between the posts when turning them. They're gold plated tellurium copper, pretty well made, and are fairly compact (don't stick out past the back of the speaker). I think they do a nice job of mimicking the factory look.

    The Pomona's use a D shaped mounting hole that's a larger diameter than the stock posts, so I machined a little punch tool to make the holes in the plastic. You feed the screw through the original hole and then tighten it to punch the D shape. It worked surprisingly well.
    tjk1pj14pdfq.jpg
    14w108fv1pk0.jpg
  • Before & after photo of the crossover rebuild:
    j1gdqpuzgru6.jpg

    I used Claritycap PX series for the low pass and CSA series for the high pass and TL mod. There's juuuuust enough room to attach the big 40uF cap, but the other three fit beautifully! The resistors are Jantzen Superes.
  • F1nut
    F1nut Posts: 46,296
    I love your punch tool.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."


    President of Club Polk

  • Gardenstater
    Gardenstater Posts: 2,026
    edited December 2020
    I remember wondering in the other owner's thread if they had received a lightning hit or some other source of current surge. Sorry you got stuck with them but they seem to have found the right owner for sure! That label thing is, as Sherlock Holmes would say, highly irregular. I can only ASSume that someone at the Polk factory put the wrong label on it initially.

    Yes very ingenious and crafty that D Punch you made and I like the Delrin plug. The short Cardas binding posts I used would have benefitted greatly if installed with a similar punch (would have to be 2 flats version) because it would have allowed the elimination of their mounting block and then there would've been room inside for double nuts. I think it worked out OK with a single nut but would've preferred two, with the ring terminal sandwiched between them.

    So you ended up adding a 40uF cap, in addition to the 5.8uF for the TL mod? That can't be right so I must not be totally getting what you did.....
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Crown D150 amp
    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
  • F1nut wrote: »
    I love your punch tool.

    Thanks!

    I wasn't quite sure how the plastic was going to behave, but figured I could always drill them out if it didn't turn out well. I really wanted to eliminate that mounting block as Gardenstater described. The punch left a little bit chipping of the material along the flat edge on the back side, but it seems to be plenty solid enough to keep the post from turning.


    Sorry...when I said "three caps", I was just referring to the ones located on the left side of the big inductors in the photo. Only one of them was added for the TL mod (the one on the far left edge of the board). The others are just replacements for the original caps. I replaced the original resistor and added one extra resistor in place of the polyswitch.
  • I was wondering if there would be any problem with the receptacle of the punch not being D-shaped but clearly it worked out ok for you. That binding cup plastic is thick!

    No, it was my confusion. The original 40uF is so small that it is hidden on the other side of the inductor in the "before" view. :)
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Crown D150 amp
    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
  • So you ended up adding a 40uF cap, in addition to the 5.8uF for the TL mod? That can't be right so I must not be totally getting what you did.....
    Ah, I see the confusion now! The big 40uF is a replacement for one of the original caps on the board. On the un-modified crossover in the photo, the original 40uF cap is hidden behind the inductor just below where it says "P2" on the board. If you look closely, you can see one of its leads protruding past the inductor and going into the board at the very right edge.
  • VSAT88
    VSAT88 Posts: 1,062
    SDA 2B crossovers are sneaky like that.
    Where in Alabama ?
  • F1nut
    F1nut Posts: 46,296
    2BTL, Cardas posts mounted directly. Just takes a little ingenuity B)
    wzyrtcr10ca0.jpg
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."


    President of Club Polk

  • F1nut
    F1nut Posts: 46,296
    SDA SRS 2.3TL
    x7o0vpevt6sc.jpg
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."


    President of Club Polk

  • Are those the CCGR?
    If I ever do another pair, those would be a good option. I've never seen them with the red and black markings, so I'm assuming you painted them yourself. Looks good.
  • F1nut
    F1nut Posts: 46,296
    bigmontana wrote: »
    Are those the CCGR?
    If I ever do another pair, those would be a good option.

    Yes.
    I've never seen them with the red and black markings, so I'm assuming you painted them yourself. Looks good.

    The red and black markings are pinstripes used for models.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."


    President of Club Polk

  • What is the purpose of this plug in the rear of the cabinet?
    It's a plastic Christmas tree style plug pressed in from the exterior.

    Is it a vent to equalize pressure due to atmospheric changes? Or a hole that's used for something on other models?

    fj8zyef5wi02.jpg
  • F1nut
    F1nut Posts: 46,296
    Hole used for something on older model.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."


    President of Club Polk

  • F1nut wrote: »
    Hole used for something on older model.
    Ah, thank you!


    I made some good progress over the long holiday weekend. The cabinets are as clean as I can get them...not bad for 30 year old speakers. The grills were in good condition to start with, but a little cleaning took the few dirty spots right out of them. They look great!

    I installed the hurricane nuts, and added dynamat to the driver baskets. It didn't seem to make a huge difference on the MW's, but the effect on the passive radiator was amazing. That thing rang like a bell before, but barely makes a thud now.

    I made new gaskets for the tweeters and one of the passive radiators since the originals were a bit torn up. I installed the drivers and noticed one of the MW's appears to have a leaking gasket too, so I'm going to try to salvage one off of a blown driver to replace it.

    I have to say I'm not impressed with the new design of the faceplate on the RD0198's. I don't like using flat head screws in plastic to begin with, and these don't have any support directly under the screw. The faceplate starts to bend as you tighten down the screws. I didn't like the idea of leaving them loose to avoid deforming the plastic, so I turned some bushings to fill in the gap. They should still allow a little bit of compression around the rim of the faceplate and then bottom out before the plastic bends too much.
  • F1nut
    F1nut Posts: 46,296
    We use a #10 washer glued to the backside of each tweeter bezel hole.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."


    President of Club Polk

  • audioluvr
    audioluvr Posts: 3,720
    xschop wrote: »
    The permanent fix....

    5ifddmmzybre.jpg

    Who sells those???
    Cambridge Audio 851N Streamer/DAC
    Belles 21A Pre modded with Mundorf Supreme caps
    B&K M200 Sonata monoblocks refreshed and upgraded
    Belles 350A Reference modded with Mundorf Supreme caps
    Polk SDA 1C's modded / 1000Va Dreadnaught
    Wireworld Silver Eclipse IC's and speaker cables


    There is about a 5% genetic difference between apes and men …but that difference is the difference between throwing your own poo when you are annoyed …and Einstein, Shakespeare and Miss January.

    by Dr. Sardonicus
  • bigmontana
    bigmontana Posts: 28
    Those tweeter plates look nice. That's a bit more than my typical machining projects, but maybe someday.


    I reassembled the speakers and gave them a listen to maker sure there were no issues. Everything sounded fine, so I pulled the MW's and glued the magnets tonight.
    I had bought a tube of Power Grab to use for this, but it turned out to be as thick as peanut butter. I'm not sure if this is normal or not, but it definitely wasn't what I was expecting. I ended up using this Loctite epoxy instead: Loctite Instant Mix
    It's a dual syringe type with a static mixer that makes it easy to apply on the inner edge of the magnet. No issues with dripping or stringing either.
  • After gluing the magnets and putting everything together, I've given them some break in time and some listening. Overall, they sound very good...but not quite right. :/

    The tweeters don't seem to match up well with the mid woofers and it makes vocals sound weak. I have triple-checked every connection from binding post to driver and everything is correct according to the schematic (white wire is on the wide terminal of the RDO198). I even tried reversing the tweeter phase just to make sure, and that did not help.

    I have also tried some different values of resistor in place of the polyswitch. I thought .33 ohm made them sound kind of "hissy". Going up to .56 ohm made the vocals much more natural, but sounded as if the singer had taken a step or two back from the microphone. I have the typical .5 ohm installed at the moment, which ends up being kind of a compromise. Instruments sound fantastic and the soundstage is huge, but vocals are still a little weak and tend to make every singer sound slightly nasally.

    I've taken some crude measurements and it looks like the output is a little low around 1000 to 1300Hz. Measuring the tweeter independently, it appears to be behaving as expected, tapering off below 2000Hz. When measuring the MW6503, it looks like the crossover starts dropping off around 1200Hz. This seems about right based on the component values in the crossover, but it appears that the output of the MW6503 gets a little weak above approximately 800Hz, before the crossover slope kicks in.

    I have a couple spare 6503's and I did find that part of my problem was that the one I happened to install in the speaker I was measuring was the worst of the bunch. It had a big dip in it's output at 1100Hz. I measured them all and each one showed the same reduction in output starting around 800Hz to some degree. I swapped out the one I had originally installed for the one that measured the best. That helped slightly, but definitely did not fix the problem.

    Has anyone encountered something like this before? Any suggestions to try?
  • Gardenstater
    Gardenstater Posts: 2,026
    edited January 28
    Were the measurements taken with the MW and tweeter still mounted in the speaker baffle? Could you do a freq. response measurement with whitenoise with the mic positioned about a foot in front of each speaker, midway between the tweeter and the MW's? I use my android phone and an app called Advanced Spectrum Analyzer PRO as I don't have a calibrated USB microphone or REW. There is very little smoothing or averaging with this app and the graph is constantly changing but you pause it and take a screenshot. Edit to add: These are with the BH5 mod behind the MW's and I don't recall if you said you have done this yet or not.......anyway it made vocals so much better in my speakers!

    L Speaker:

    agkpboemm6ip.png

    R Speaker:

    jc7v5d30geld.png

    If I hold the mic about 4 or 5 in. from the MW's in each speaker there is a huge trough around 2kHz because the tweeter isn't picked up very well by the mic in this position but (of course) that's not at all representative of the overall freq. response.

    umrkeyrmsghp.png


    Post edited by Gardenstater on
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Crown D150 amp
    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
  • Thanks for posting those Gardenstater! I'm in pretty much the same situation with no calibrated microphone. I was using an app on an ipad, but it produced a much rougher looking plot than yours. I'll have to try out a few others and see if I can find one that's a bit easier to read.

    To answer your questions...
    Yes, individual MW and tweeter measurements were done while mounted in the baffle with the other unplugged from the crossover.
    No, I have not added BH5.
  • Gardenstater
    Gardenstater Posts: 2,026
    edited January 28
    By the way my speakers have MW6500's which are somewhat close in parameters to the MW6503 but of course we have different crossovers.

    BH5 is a must! Can't recommend it too highly. Perhaps you could get some advice on how much to apply in your speakers, and where.

    It would be interesting to see the overall response you are getting. Why don't you try this app, which seems at least as nice as the one I am using:

    Spectrum Analyzer for iOS

    https://onyx3.com/SpectrumAnalyzer/

    wbx0qcx2j124.jpg
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Crown D150 amp
    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
  • I think the recommended BH5 treatment for the 2B's is a 6" square directly behind each driver. How would you describe the change in the sound before/after you added it?

    I'll set up and try to get a decent measurement with the app over the weekend.
  • F1nut
    F1nut Posts: 46,296
    edited January 29
    A 4 inch square is more than enough.

    Put away your stupid measurement gear and use your ears.
    Political Correctness'.........defined

    "A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."


    President of Club Polk