Mission 720 Refurb

[eriksquires]

I used to design PCB's for a living. Based on my experiments, I'd say point to point could be an improvement.

Now when I make PCB's for crossovers I avoid large lands under components. I can still do that and maintain negligible trace resistance. But this is pure theory. I certainly don't think you should spend time on it now. 🙂


Best,


Erik

 

[Mission720]

Update - I have been really enjoying listening to my refurb'ed Mission 720s, and of course pleased that I didn't have to shell out for a complete new set of speakers. As I remember, I bought these in 1979 as a first year uni student (I saved up money from working in the holidays) and they cost me £220 (or might have been £240 - I probably have the original receipt...) from Laskeys in Liverpool. I had originally bought KEF 104ab but didn't like them and Laskeys took them back (in their original boxes) after a week and I bought the Mission 720s instead. I wonder what they would cost nowadays...?

I have drawn the crossover circuit diagram but need to re-draw a little neater and then upload here.

None of the cone surrounds have come unglued and I have been blasting the speakers on occasion to test how good the repairs are.

Now I have one brand new in box, never used, Peerless 832531, 10" Mission 720 Bass driver to sell which I'll probably put on the usual place, with a new replacement foam rubber surround for DIY replacement. This is because although this is a totally new and unused driver with totally intact foam it is a bit 'tacky' and I imagine has degraded over the several years I have owned it.

 

[Audiologue]

Latecomer here... I am wondering whether these 720 Missions could be classed as audiophile speakers? I would be keen to take on a project but just a little unsure at the moment.
To give you an idea of my taste, my ears usually prefer 'monitor' 2-way speakers particularly ones that sound at the most natural on vocals and have strengths in imaging or soundstage. Thanks in advance for your input.

 

[Diabolical Artificer]

I rebuilt my 720's and have been using them for years, I'm well happy with them, but some folk might find them a bit un subtle. No idea if they're audiophile, which is meaningless.

The 720's and the 730's were built using some of the best components around at the time and were IMHO the best speaker's Mission made. I have a pair of 730's upstairs waiting a re build which used the same drivers apart from the tweeters.

Brochure available at Hifi Engine.

Andy.

 

[system7]

I must have been bored today, but I thought I'd try and trace the schematic... and that is hard with just a few photos. 😱

What I got seems wrong. 😕

I think you Mission 720 enthusiasts should check my work. Because, as it stands I could only get a 2kHz crossover to the SEAS tweeter. Rearranging things got a much more plausible 3kHz crossover.

Ah well. We need the original poster, who calls himself Mission720 to look in.

 

[Diabolical Artificer]

Here's the schematic that I traced years ago Steve.

Andy.

 

[system7]

Nice work, Andy. I had found your schematic, as it goes... 😀

Anyhoo, here's where I am at now. Still that 2kHz crossover. 😕

This is like doing jigsaw puzzles. But we know it's not a million miles away from a SEAS 503:
SEAS Kit 503

If we are right so far, there is room for improvement.

 

[Diabolical Artificer]

"Nice work, Andy. I had found your schematic, as it goes..." I wondered if you had, there's not a lot online about these speakers.

When I got mine the tweeters had both blown, so I replaced them with a similar Seas tweeter, not the H107, so I can't really comment on the proper top end frequency response, but as mine are, the top end is excellent. Apart from the Sonatello graph, do you find yours lacking?

I'll run a sweep through them tomorrow and report back.

Andy.

 

[system7]

I don't have this Mission 720. Nearest I have is some ropey old Celestion Ditton 44. They need some work on bass tinsel leads, which I might take on, since Blue Aran stock tinsel.

But I'm always interested in the classic 3-way Steen Duelund filter. So is Troels Gravesen, below. After a while, they all look the same.

This is potentially a better speaker than the SB Acoustics Sonatello. But the guys in that thread don't want to hear about it, so I'm not telling them. There are about 5 glaring design errors. 😀

But as in the Sonatello, I plain don't like what I seem to be seeing with the Mission tweeter filter. I would fix that. You'd be surprised how well you can sim a speaker using different but similar drivers. I came up withbass 4.7mH and 40uF on the optimiser in Boxsim on the purest guesswork. That wasn't far off, was it? 😎

 

[Mission720]

Here's a pic. I made overlaying the x-over components on the PCB layout...

 

[Mission720]

Steve - the choke values were measured out of circuit should should be pretty accurate.

 

[system7]

Good piece of work, my friend. It's yer classic Duelund filter.

I might be tempted to replace 60uF/1mH with 30uF/2mH for higher (dotted) impedance. And that 6uF with 3.9uF. Rough sim, of course, but the essential features. Otherwise nothing to criticise. 😎

BTW, 33uF and 1.8mH would do much the same thing... and more easily obtained. I'd stick to NP electrolytics. They are part of what this speaker does.

 

[Mish 720]

Hoping to piggyback onto this thread as although it's old it's relevant to my issue. I've a similar pair of 720s to the orginal poster needing refoams on one of the 10s and 2 of the 4.25s. Apart from a corroded and broken fuse holder everything else works ( i.e all 6 speakers )and seems orginal. I'm confident doing the foam jobs but would appreciate any advice from any 720 owners. Any suppliers in particular I should go for the foams? What about the crossovers? Anything simple I can check there or should I leave well alone? Any help would be appreciated. Thanks

 

[davephot]

Mission 720 mids

The re-foam kits for the 4.25 Peerless K040MRF Mids arrived from the Netherlands and are perfect. Note, they are of an inverted manufacture and designed to be fitted that way. The original remnants of foam came away failry easily and I used solvent on the parts for the enclosure but not the cone as that's paper.

For the remnants on the paper cone they were removed using finger nails and careful pulling. I'm sure I've taken off a few molecule thickness of paper on the glued edge but can't really be helped.

They cleaned up much better than expected to be honest as I thought the paper cone would be a lot more fragile.

The solvent removed some of the black paint on the surround but as that's covered by the cosmetic bezel I decided not to respray.

Trying without glue first, the re-foam kit sits very snugly and I could see that just a little weight would need to be applied on the surface interface of paper cone and new foam. A round porcelaim coaster did the trick.

PVA glue was used again with a generous application on the surround housing first. The paper cone needed two applications - the first was absorbed quickly and sealed/primed the surface ready for the next application.

After fitting the new foam surround I pressed it down a few times to esnure it had taken before applying the weight.

Pleased with the results cosmetically I hope it sounds good too.

I measure the voice coil resistance and both came up at around 6.25 ohms, so less than the nominal 8 ohms but then again that's with a non-calibrated cheapish meter.

I have tested two of these mids from my Mission 720s and they both read 9.5 ohms

 

[davephot]

What is the Netherlands address for the foam? my two need re-foaming and I cant get this size

 

[Diabolical Artificer]

Here Dave - Peerless KO40-MRF / 821385 midrange > 2x inverted surrounds > no trademark < | eBay

Measuring with a multimeter will give you DC resistance, not AC impedance. DC resistance is usually around 6.5 ohms, 9.5 sounds high. Is there any rubbing of the coil of a burnt smell? It's be worth checking with another meter, but as long as you have continuity, the coil isn't rubbing, you should be ok.

Owners of these speakers should check the capacitors on the Xover for ESR, this is tricky as the Xovers are glued on tight. It takes some gentle prying to get them off.

Andy.

 

[Mission720]

Hoping to piggyback onto this thread as although it's old it's relevant to my issue. I've a similar pair of 720s to the orginal poster needing refoams on one of the 10s and 2 of the 4.25s. Apart from a corroded and broken fuse holder everything else works ( i.e all 6 speakers )and seems orginal. I'm confident doing the foam jobs but would appreciate any advice from any 720 owners. Any suppliers in particular I should go for the foams? What about the crossovers? Anything simple I can check there or should I leave well alone? Any help would be appreciated. Thanks

Hi - jut alerted to your post, sorry for delay in reply. The 10" refoams were easy, just look on EBay for a cheapy. I had to get tthe 4.25-inch from The Netherlands, as per other post.

Any queries, please just ask 🙂

 

[davephot]

Here Dave - Peerless KO40-MRF / 821385 midrange > 2x inverted surrounds > no trademark < | eBay

Measuring with a multimeter will give you DC resistance, not AC impedance. DC resistance is usually around 6.5 ohms, 9.5 sounds high. Is there any rubbing of the coil of a burnt smell? It's be worth checking with another meter, but as long as you have continuity, the coil isn't rubbing, you should be ok.

Owners of these speakers should check the capacitors on the Xover for ESR, this is tricky as the Xovers are glued on tight. It takes some gentle prying to get them off.

Andy.

Thanks very much Andy