text: Older Mission Drivers - DIY • Canuck Audio Mart Hifi and Audio Forum
  Older Mission Drivers
  Post subject: Older Mission Drivers
  Posted: Tue Jul 24, 2018 4:58 am
  So I picked up an older set of Mission 78ds rear surrounds for my folks and unfortunately all four mid drivers and one of the four tweeters appear to be dead. For the mids I was thinking of replacing them with the 4" mids that Dayton makes. They obviously won't be perfect but this is honestly for whoosh noises for surround sound only so I'm not being as picky as usual and don't want to break the bank. For the tweeter though it will be a lot harder to find a drop-in replacement as I'm only replacing one. It's a 101989B tweeter, 1" soft dome and looks like it was in most of the 78 series.
  So the question is, does anyone know of any Canadian suppliers that used to carry Mission that might have an old tweeter sitting in their shop on a shelf somewhere.
  Post subject: Re: Older Mission Drivers
  Posted: Tue Jul 24, 2018 5:27 am
  You may want 2 tweeters, they contain ferrofluid, if 1 is shot the other will not be far behind.
  Based on age, any used ones may suffer the same issues in the near future.
  Mission.jpg [ 5.91 KiB | Viewed 560 times ]
  Yep that's the exact unit. It's not an ideal situation and finding one seems to be tricky let alone a pair (the ideal)
  audax driver possibly. Likely. Made in France.
  Dome tweeter Audax TM025C1, 8 ohm,
  If these speakers are for "whoosh noises for surround sound only", do they really need to have working tweeters? Is it even worth the expense?
  If you are concerned about balance, just disconnect the working tweeter and see how it works. I doubt that you would be missing that much.
  Posted: Tue Jul 24, 2018 9:02 am
  Honestly I've considered leaving it, it's one of those "if they're out there for 20 or 30 bucks I'll get one" things. If they're prohibitively expensive my left ear will hear a bit more woosh and little less high end.
  Posted: Tue Jul 24, 2018 1:38 pm
  https://www.ebay.co.uk/p/single-Mission ... 3057868608
  maybe good enough....
  Posted: Tue Jul 24, 2018 4:41 pm
  So, for the tweeter, it seems like at some point one of the terminals must have lost it's connection between the fine voice coil filament and the tab on the terminal, reflowed the solder and it's back to life and measuring close to the correct 6 ohms, it is....very unusual and very satisfying.
  As for the mid bass drivers, looks like one per cabinet is cooked, and since the crossover is designed in such a way that they run the two mids in series that's why they all seemed dead. Still going to replace all four as only one of the living ones is showing the proper 8 ohms. Fingers crossed this is fixable. That's the majority of the enjoyment of the tinkering side of things anyway.
  zeagan wrote:
  Don't assume that a driver rated at 8 ohms is actually going to measure 8 ohms. When you use an electrical tester, you are measuring the electrical resistance at 0 Hz, not the electrical IMPEDANCE within the driver's frequency range (which can vary significantly with frequency). As a couple of examples of this, look at the specs of 2 typical 8 ohm mid-bass drivers and their DC resistance.
  Nominal Diameter3-1/2"
  Power Handling (RMS)30 Watts
  Power Handling (max)60 Watts
  Frequency Response100 to 20,000 Hz
  Sensitivity83.5 dB 1W/1m
  Voice Coil Diameter0.75"
  Resonant Frequency (Fs)125 Hz
  Mechanical Q (Qms)2.7
  Electromagnetic Q (Qes)1.33
  Total Q (Qts)0.89
  Compliance Equivalent Volume (Vas)0.04 ft.³
  Mechanical Compliance of Suspension (Cms)0.67 mm/N
  BL Product (BL)3.01 Tm
  Diaphragm Mass Inc. Airload (Mms)2.43g
  Maximum Linear Excursion (Xmax)2.55 mm
  Surface Area of Cone (Sd)36.3 cm²
  HiVi B4N 4" Aluminum Round Frame Midbass
  Nominal Diameter4"
  Power Handling (RMS)25 Watts
  Power Handling (max)50 Watts
  Frequency Response60 to 4,000 Hz
  Sensitivity85 dB 2.83V/1m
  Resonant Frequency (Fs)56 Hz
  DC Resistance (Re)6.5 ohms
  Mechanical Q (Qms)3.91
  Electromagnetic Q (Qes)0.63
  Total Q (Qts)0.52
  Compliance Equivalent Volume (Vas)0.16 ft.³
  Mechanical Compliance of Suspension (Cms)1.11 mm/N
  BL Product (BL)5.1 Tm
  Diaphragm Mass Inc. Airload (Mms)6.8g
  Maximum Linear Excursion (Xmax)3.2 mm
  Surface Area of Cone (Sd)53 cm²
  Finding out the electrical parameters of your existing drivers should be your first priority before replacing anything.
  That's a very good point, I definitely noticed that when verifying the fix on the tweeter. They were all listed as an 8 ohm unit and all measured 6.6 which now makes much more sense. Looks like a little more digging and a lot of hoping it's similar to the dayton audio drivers as they're the only reasonably priced, reasonable shipping option I have found so far with regards to dimensions.