Tweaking the Mission 762
A friend recently presented me with a pair of these floor standing speakers from about 20 years ago. They worked surprisingly well, although the treble was a little low. I put to one side the damaged grilles. The 762’s do not look impressive with a small radius foam surround on the 8 in. paper coned woofer and a 19mm tweeter in Mission’s trademark inverted tweeter configuration. However this combination was used on the first decent speaker I built using a KEF B200 woofer and T27 tweeter.
Incidentally not a lot of people know why Missions tend to be the other way up, but there are 2 good technical reasons. One is that the reference axis which is drawn perpendicular to a line drawn through the voice coils is sloping upwards. The other is that the polar response of a third order crossover is better that way. So the 762 can be a floorstander since its best output is drected upwards. If you do not believe me have a look at this (3rd order B for Butterworth)
http://musicanddesign.speakerdesign.net/Power.html
I measured them using the trusty IMP MLSSA system and found the treble was indeed low. They were roughly level to 2KHz when there was a 3dB step down, level again to 10KHz followed by a 5dB step down, then flat to 20KHz.
There appeared to be a mild breakup at 2.2K on the woofer. The raw response of the woofer was 3dB down at 2KHz and 8dB down at 5K when the response fell like a stone. The raw tweeter response did not extend very low either. So a bit of a puzzle how this works so well. The answer is phase tracking. Between 2K and 4K the phase tracking is superb. If one reverses the intended phase of the tweeter there is a broad trough indicating that both drivers are contributing to the output when the phases are as intended.
The -3db point for the tweeter crossover is 4K. The crossover consisted of a series 1 ohm resistor. a 5uF capacitor and an inductor with the tweeter across the inductor. I removed the 1 ohm resistor to raise the tweeter general level. I changed the 5uF to a polyester type. I measured a nominal 4.7uF polyester as being 4.5uF and put a 0.47uF across it to make 5uF because I suspected the original 5uF was a bipolar electrolytic which do not age well. Actually in this case it made no difference except for my peace of mind and hopefully even longer term quality.
To raise the upper frequencies I added a zobel network across the tweeter consisting of a 1.5uF and 10ohm resistor. I tried fitting the grille which gave 2dB diffraction humps so its best to run without the grilles.
Incidentally the woofer crossover consists of a 1.7 mH choke, which presumably gives baffle step compensation as well as suppressing a 800 Hz peak in the woofer response.
Update: The foam surround on one of the speakers has broken so I had a quick look to see if there was a substitute driver available. There were 3 which are listed as “generic” replacements; the Monacor SPW205 which while otherwise suitable has a too high Qts which would give boomy bass. The Monacor SPH210 is not the same physical form as the original. The other one, the Visaton WS20E is listed in CPC’s catalogue as having a response only up to 2KHz. However I checked on another website and found this response
Since it is available for a reasonable price I will try this and if it has too high a Qts I will use a negative resistance amplifier to lower the Qts which means going active that will allow crossover tweaks.
Another option is a surround kit available from goodhifi.com on E-bay. On the Mission it is slightly more difficult to fix as the surround is glued to the back of the cone rather than the front. They list a foam and also a rubber surround. The reason foam is used despite its long term fragility is that it is lossy and can absorb some break-up energy giving a flatter response. However it is probably better to go for the rubber surround as it will last longer.
Update1: Received the WS20E’s from CPC Due to the different mechanical configuration the “mounting ears” are spaced about 4mm above the recess so I used 6mm nuts with 4mm thickness as spacers to mount the units.
The sensitivity is lower , the WS20E having about 3dB less sensitivity than the old drive unit. However the frequency response is good and if anything has a slightly more extended response than the previous unit. To compensate for the sensitivity the tweeter needed to be padded down with an L pad consisting of a 2.2 ohm series resistor and a 18 ohm parallel resistor between the LC network and the tweeter. The zobel network mentioned previously can now be removed. To give a flat response the tweeter capacitor was changed from 5uF to 2.7uF
Update 2
I found I got an even better response by putting a 4.7uF capacitor across the woofer, thus making a second order filter and by reversing the phase of the tweeter. Gave a crossover of 4.5kHz
Interesting article about the development of the Mission 770 and indeed the Mission brand:
Consort3's Blog 
Funny, you mentioned the Kef 200 woofer (SP1014). I replaced the rotten Mission 762 woofer and mounted the Kef instead. They are somewhat larger but you can still use the screw holes. I used tape that has rubber foam on one side to make sure the woofer is mounted air-tight. You should try this, without altering the crossover.
The result is wonderful – bright, lots of detail, nice vocals and sound stage. I eased back on the treble control. The sound is similar to the B&W DM4 which is a standard to me for speakers that size. The Kef woofer is of such high quality that it makes the Mission woofer look very sub-standard with its paper cone. I need to sort out the bass however. It’s on the boomy side. Do you have any suggestions.
I happen to have a B200 so I will try this. The resonant frequency of the Kef is lower and will sound boomy One possibility is to put a 500uF bipolar electrolytic in series to restrict the bass.
Use extra damping material. I used the thick padding of a Kef Chorale (which is closed box). Narrow the port hole. Allow some time to have the Kef woofer get used (I know that sounds ridiculous) to its new housing. You may have to adjust the treble a bit.
If you are searching for help in the selection of the correct type of foam rubber for any project, can I suggest that this site may be worth a look. They have extensive knowledge of the area and are always willing to help.
The above comment seems to contain a hot link which does not work, but it reminds me that the original foam sealing gasket on the back of the woofer was broken in the dimantling process. So I sealed the new driver using self adhesive foam sealing strip sold for sealing draughty doors.
After have given it some thought and auditing, using the Kef woofer made no sense to be honest. Primarily because of the material it is made of: bextrene. Sold the Mission woofers. Mounted the full Kef Corelli kit and worked on further bracing and stuffing the 762 cabinet up to the point that it works like a critically damped unit. That worked. Sold it 2 years ago, and returned to my former love, the B&W DM4. IMO the best of all medium-sized vintage speakers (height 45-55 cm).