Sunday, February 14, 2016

Mission 782 repair

Hi everyone.
It seems that I'm having as much bad luck in transportation services, as I'm having good luck in auction items.

I've been looking for a pair of floor standing mission "old school" speakers.
There is a reason for that: old school mission speakers where made-in the UK (not China like later ones), with excellent craftsmanship and even better engineering. 
As newer models have been labeled dull and flat, old models, particularly the ones using fixed vented pole designs for the midrange drivers, where something to look for and then keep with.
The mission engineers managed to match the crossovers to the perfect driver for that type of frequency response. This brilliantly mated combination, produces results comparable to speakers costing 5 to 10 times as much. This, to me, was Mission's trademark... archiving results through efficient engineering, beating "costly materials and drivers" with brain. It has everything to do with instead blindly mixing expensive components and then making the client pay for it, claiming the old "exclusivity" excuse. 
Don't get me wrong; good materials produce better results, it's just that whenever not so good materials with good engineering produce the same results, then the obvious conclusion is that applying the same engineering effort to better materials would give your client a better "ride for the money"... mission did that for some time and I absolutely respect that.

The 782 are a perfect example of what mission used to be:

Back to my history: after a lot of searching and some frustration trying to convince some ebay traders that there was no difference on shipping a pair of speakers 10 miles of 10.000 miles, one very nice lady accepted my challenge and allowed me to bid on a pair of 782.

The 782 are a brilliant designed true 3way speaker. It features a 13cm midrange fixed vented pole ceramic design speaker on top, together with a 2,5cm silk dome tweeter, both enclosed in the top end of the speaker  on a pyramidal shaped box. Then at the bottom, there is a side-firing 16,5cm woofer made from nomex, that enjoys practically 2/3 of the speaker body box for volume and reflecting.

So summing up, after a LONG search, I managed to source a set of some of the best cost/quality speakers known to man.
There was some degree of risk! The reason that make the 782 so exquisite, is that they where produced for a little time... and that was due to a mid-driver production problem! The original drives that came with the 782 where the Keraform 82-LF130/QS. Excellent design, reasonable build quality but a terrible detail. The coil protective plastic sheet, had a glue whose chemical composition reacted with the soldering on the metal, generating extreme corrosion. This would lead to increase in electrical resistance, reducing sound power and quality, with time the oxidation would inflate the once flat soldering and make the coil rub against the magnet walls and producing heat and drag, and ultimately the degradation would just insulate the parts and the driver is dead.

Most people facing this, just decided to trash or sell the speakers. for some time it was possible to buy replacement parts from Keraform, but, in time those would have the same fate.

Obviously, Mission was aware of this and discontinued the series, then instructed the technicians to accept warranty repair claims, and substitute the driver with the Audax HM130Z0 Aerogel units. These where better, more expensive drivers that would perfectly replace the Keraforms without impact in sound quality/image.

So there are some 782 out there that whee trashed, others where sold as repair item, others for parts, others perfectly repaired and updated... and of course, the original ones... few in working order.

I knew this; I accepted that ... and even managed to find a set that was original and in perfect working order. Sure i was expecting some soldering degradation, but I would be able to peel off the plastic, clean the chemicals and re-do the soldering and wrap the coil back with a non acid glue. This, unfortunately never came to be. My couriers, managed to kill the midrange drivers on both speakers. I'm working out the insurance claims at the moment, and apart from some very unpleasant dealing with customer care, the escalation seems to work... so I'll wait until I add to this article the details... either for the best or the worse of it.

While grabbing the speakers I immediately felt a heavy item loose inside one of the speakers. Bad news for sure, confirmed when opening the speaker:
The front protective cover has the pins broken and clearly had suffer a large squashing impact.
A hole was easy to spot where the cone pole would be, and the inner speaker baffle filling was observable.
In panic, I immediately rushed to the next speaker... no good news there either:
The center pole was loose, running around the speaker driver, however no heavy loose part was felt and the mounting point of the cone pole was still in place. but the pole was broken and heavily marked. 

Ok... time to access the total damage on the first speaker:

First I toolk a clamp to remote the broken plastic pins that used to hold the protective front cover. then, using a screwdriver, I gently removed the rubber housings, exposing the Philips screws.

Then I removed the front assembly, which holds the tweeter.

I finally removed the driver assembly and got hold of the destruction:

Whatever hit the driver, pushed the cone pole in, with enough force to rip the magnet from the driver structure, then the weight if the magnet ripped the wire plug wide open. 

This is the dismantled driver.... and, of course before any attempt to fix, a conductivity test was taken...

At least the tweeter was in perfect order

So conclusions: the main drive was many different forms of assassination attempt all at once. Independently, I would not foresee a bright future ahead of this speaker, it is an original Keraform, and the oxidation is way into the soldering.
on the inner side, the bump is the result of extreme oxidation process.

Conclusion: I've made an insurance claim for the transportation, ordered a couple of AUDAX aerogel, and I'm now waiting delivery. Will update this post soon.

So the AUDAX drivers arrived. They are a thing of beauty, but their cast ring is bigger than the Keraforms... a lot bigger. They are also made from aluminum instead of plastic as you would find in the Keraform. In essence, they are a lot better built.

Step 1 -  place the old Keraforms on top of the new AUDAX and use a light colored pencil to template the cutting line on the aluminium .

2- use a dremel tool to cut the excessive aluminium. MAKE SURE you protect the new cone from debris... I just covered it with gum-tape.

3 - Start cutting,

4 - After cutting the metal from the driver, put it next to the wood and template the extra cut... the better you cut the metal, the less you need to adjust the wooden part.

5 - Place the driver and use the drill tool to have new holes for the drivers screws.

6 - Place everything back together and enjoy your brand new UPGRADED Mission 782. These will not go bad of fail...and they sound a lot better than they cost.

Monday, February 1, 2016

Mission 772 speaker repair

I recently brought a pair of Mission 772 speakers. They are old but they are gold. And they sit just perfect together with my set of Mission 782. 

The problem is that age takes the toll on these babies, and the rubber from the speaker outer damper started to degrade.
One of the speakers shown a fatigue fracture... not only not good and with influence over sound quality (when operating and high levels), being this an open center design, this would mean unbalancing and, in time, total driver failure due to the coil rubbing the walls and erode till destruction.

A solution was needed, and I would not let these babies go because of a minor problem. So ebay to the rescue and, soon enough, a repair pack was underway.

Ok then, ready, set, go...
First step is to lay down the speaker and remove the screws.

Then use a small knife to start to unglue/separate the external border of the old bubber/foam damper. BE CAREFUL when pulling as the inner side is still glued in and too much lift will damage the speaker cone.

Then work on the inside, following the same rule: no excessive lift.

Here is an image of the removed cone and the damage on the rubber damper.

It's an excellent time to inspect the main damper spring inside the driver showing in yellow. This one was just perfect.

As materials evolved, so did the design. You can compare the thickness of the rubber vs the replacement foam damper.
This is good news and bad news at the same time: good because less weight and the same tenacity will mean less moving weight and a better response....the bad news is that the heavier rubber would hold the cone I place better during centering and more work for me.

The glue. The glue is another problem:
You want enough blue to hold this in place...but not too much as is will add weight... and just enough to glue fast, but not too fast so you can move and place the damper properly and within geometry.

So the solution I to apply only to the inside, and  apply an almost transparent amount of glue on both the edge of the cone and the edge of the foam damper.

Then you marry the 2 parts together and, while HOLDING the cone with one finger, lift and place the foam on the cone...this will enable the glue to catch some air and react a little faster.

The material will try to counter the glue and the more surface you glue in, the more it will fight back.. SO, my recommendation is to grab gum tape, twist it into a rounded surface that will match the cone surface... and start fixing it in a cross pattern (top, bottom, side, side).
This is the final LOOK :)

Allow it to glue in... time for coffee, and a movie or two. I let the glue set in for half a day... it's not really a contact glue (rather looks like wood glue, actually). So it does take some time to set properly.

50% through... not it's time for the outer part.

The outer part is A LOT simpler... but not linear. 
Glue wise, just slightly lift the foam and pass the glue without worrying too much about the amount of glue... it's a fixed part so no weight to influence motion.
HOWEVER, this is an open cone design.. it is set this way to allow better movement on the cone and consequently a better response, but this means that there is also some fluctuation of the cone and the entire coil assembly. IF that fluctuation means the coil rubs the side of the wall containing the magnet, it will rub the varnish out and then short circuit the coil... and eventually just completely wear it out until it looses continuity.
So after you set the glue on the outside, and before it sets, just move the come up and down, while adjusting the damper outside. You will feel free movement, or a slight vibration, created by the coil rubbing the walls.
Again, you can have gum-tape (this time a simple cross is enough), and adjust until movement is free, I do recommend you bias the setting a bit towards the upper side of the speaker, so that when standing, the small but existing weight will auto-balance.
Again the glue set in... a day should be enough. And then screw the driver back in... connect your speaker and... well it's a speaker, you know what to do with it :)

This is the final look of the 2 speakers:
on the left you have the repaired one, the original (yet to be repaired) is on the right.
The speakers sound better with the new damper. Weight does influence moving parts... inertia, you know?