Help troubleshooting Model 10b issue

I recently received a pair of 1987 Polk Model 10b speakers. I hooked them up and discovered that the drivers of one speaker plays very low, the sound is "thin". The tweeter sounds fine. I pulled the two driver and tested them - each should about 6.7 ohms. I hooked both drivers to an amplifier leaving the crossover out, played them at low levels and they do produce clean sound even though its very low level and 'thin". I pulled the crossover to find that it was all original. I rebuilt the crossover thinking that might be the problem. I replaced everything on the crossover board except for the small coil soldered to the board and large coil below the board. The rebuilt crossover did not fix the issue, both drivers play sound at a low volume and sound thin just as they did before the crossover rebuild.

It's a pretty simple crossover setup so I am really perplexed about what might be causing the problem. Any suggestions? Could it be one or both coils? Could the speakers be bad?

Answers

  • Gardenstater
    Gardenstater Posts: 2,671
    Just a thought but the drivers could ohm out just fine but the voice coils could be rubbing on the pole piece, which would reduce output. Do you know how to check for VC rubbing and have you checked it?

    Another thing to check would be cabinet leaks by pushing in and holding the Passive Radiator and timing how long it takes the MWs to return (leakdown test).
    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
  • H00kem
    H00kem Posts: 4
    Thanks so much for your thoughts Gardenstater. I just tried gently pushing on the cones now that you brought it to my attention. The cones are very stiff and don't move at all (unlike the speaker that works where they are supple and move easily) so that it probably the problem. I don't know why I didn't think of that! Is there a way to rehab the speaker voice coil or recommendation for locating replacement speakers?
  • Gardenstater
    Gardenstater Posts: 2,671
    edited May 11
    I'd check the model #'s on all the drivers and dates on XO's and drivers and make sure they match, then put in a saved search on eBay and wait patiently for them to come up for sale. If not, there are some kind souls here who repair drivers for forum members, like @Lawdogg, assuming they still have time. There are threads on repairing drivers that you can search for. Lots of members have made their own alignment jigs with PVC pipe and screws or even better materials.


    I think maybe you are supposed to have MW6503 but you need to verify. They went from MW6501 to MW6503 somewhere between 84 and 89 or so. If you need MW6501 you are in luck because there are some on eBay right now but I don't think so, because those would ohm out around 7.75 ohms, not the lower readings you got. MW6503 are supposed to be 6.54 ohms.

    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
  • H00kem
    H00kem Posts: 4
    Thank you George. The drivers are in fact the MW-6503 drivers. I'll watch eBay and reach out to @Lawdogg for help. Best, Kevin
  • JayMX
    JayMX Posts: 382
    Gonna have lots more bass now… 🤘🏻🤘🏻
    Current Collection:
    Monitor 4a (Peerless), Monitor 5b (SL2000), Monitor 7b (Peerless), Modified Monitor 7c’s (194’s), SDA CRS+ (SL2000), RTA12c (194's), SDA SRS 2.3TL, R200's

    Repair pile:
    (1) lonely RTA 11TL to become a center channel... someday...

    Pairs that have passed through:
    Monitor 4b (Peerless), Monitor 5a (Peerless), Monitor 5b (SL1000), Monitor 5b (SL2000) (2x pair), Monitor 7b (Peerless), Monitor 10a (Peerless), Monitor 10b (4x pair), RTA8, RTA8TL, RTA 11T, SDA CRS, SDA 2, SDA 2a, SDA 2b, SDA 1b, SDA 1c, SDA SRS 2 (2x pair), SDA SRS 3.1TL (198’s) (2x pair)

    ...and more to come, it’s a sickness.