Introduction
For a long while, my main set of speakers have been a pair of Mission 751s that I purchased in England in 1993 while I was there on a business trip. Unfortunately, while the boxes are in near perfect condition, the time and tropical climate have not been kind to the drivers. The tweeters were the first to go, with the coating disappearing from the domes and the domes themselves developing kinks that resulted in increased distortion at higher frequencies.  Then the rubber surrounds for the bass drivers started to go (which was a bit unusual and unexpected). 

So, a project that was geared toward just replacing the tweeters turned out to be a full driver and cross-over replacement, with the only thing being kept being the cabinets. So officially these are no longer Mission 751s, but a new speaker design using the Mission 751 cabinets.

Driver Choices
It turns out that the choice of drivers available to me was extremely limited, if I didn't want to make any visible modifications to the Mission 751 cabinets.  The only bass driver that I could find that both physically fit and had t/s parameters that suggested that they would work decently well in the Mission 751 cabinets were the Dayton Audio DC130A-8 drivers.  Luckily these drivers are also pretty cheap (available for about US$20 each when I purchased them).  As for the tweeters, the best match for the Mission 751's OEM tweeter faceplate that I could find turned out to be the Peerless OC25SC65-04 1" dome tweeter, available from Parts Express for about US$8 each.  This means that the total driver replacement cost turned out to be US$56, not too bad for a pair of speakers that cost me around US$500 when purchased new.  Are there better drivers available?  Sure, but they would have required visible modification of the cabinets, which I wanted to avoid.

Driver Installation
Replacing the Mission 751's OEM driver was pretty simple to do - just remove the bolts holding the metal bracket over the OEM driver, replace the OEM driver with the new bass driver and then replace the metal bracket and bolts. I was planning to remove the paper bracket around the surround of the the DC130A-8 drivers to allow the bracket to mount a bit more flush to the front baffle, but decided not to in the end, and I'm glad that I chose not to, as I think the change would have very little difference. The tweeter on the other hand took a little more work to install.  I removed the OEM tweeter faceplate from the tweeter, remove the metal protective mesh and basically hot-glued the new tweeter into place (it's a small neo-magnet based tweeter, so the hotglue would be more than enough to hold it in place).  I applied hotglue liberally along the sides if the tweeter as well to ensure that there would be no air leaks around the tweeter when the OEM tweeter faceplate was reinstalled.

Crossover design
I took the same approach designing a new x-over for the chosen drivers as I've done in the past - I measured both the impedance and frequency response curves for the drivers, measured the z-axis offset for the tweeters, and then used XSim to come up with a suitable x-over for them.  Originally I planned to try and match the original response of the Mission 751s as closely as possible, but subsequently decided to tweak the response a little bit to remove a little of the "sizzle" at the top end of the response curve.  Illustrated below is the current version of the x-over that I'm using at the moment. The parts cost worked out to about US$50, bringing the cost of the whole exercise to US$100.

What's that 0.12mH inductor doing in the bypass leg of the woofer circuit? Well, that's there to add a "notch" filter that trims the DC130A's response at higher frequencies, making it a better match for the Peerless tweeter through and above the x-over region.

Another note: while the x-over diagram shows a common ground, the Mission 751 is configured to support biamping, so in the actual build I kept the ground path for the tweeter separate from the ground path for the woofer.  

Results
I think that the results I've achieved seem to be quite successful. And the rebuilt speakers look like they came from the factory that way.

Further Notes:
The grille for these speakers really does a number on the frequency response (see chart below). When I have a chance, I'm going to look into seeing of the grilles can be modified (possibly by adding a felt panel) to reduce or eliminate their impact on the response curve.

After I started rebuilding these speakers, my attention was drawn to another tweeter that just might fit - the Dayton Audio ND25FN-4. Those look like a copy of the Peerless tweeter that I used, except that they also include a heatsink.I think if I was going to do another rebuild, I'd probably try those instead, to see if they fit.

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