My Mission 720 speakers have been with me for 36 years, the foam has rotted on the 10-inch bass driver and the 4.25-inch mid-range is shot too.
I had replaced the 10-inch drivers some 15 years ago with genuine Mission units and even had one extra spare, which I still have BNIB. These replacements have rotted out too and of course there's no way to get new Peerless drivers for either bass or mid.
The original SEAS tweeters look ok and I'll check for open circuit at some stage.
In the meantime, after a fruitless search for suitable replacements and encouragement from this site I decided the look at refurbishing!
The first challenge was to get a 10-inch re-foaming kit and I took a chance on the usual place and bought 4 for £4.39!
Attached are photos I've taken of the refurb from the physical aspect. I've yet to fire up as I'm waiting for foam kits to arrive for the 4.25-inch mids as they seem to be particularly special and are coming from the Netherlands.
The only difficulty was in ensuring the foam stuck down all the way round on the cone since it tried to buckle up in a couple of places. PVA was used, which I had let dry out a little to thicken and I had to re-check and press carefully down until the tack of the glue finally held it in place.
The cones move smoothly with no rubbing.
I'll post more pics when I get the new foam for the mid-range and of course let you know how they sound. I'll check the crossover components as I have an LCR meter and do a visual too.
I had replaced the 10-inch drivers some 15 years ago with genuine Mission units and even had one extra spare, which I still have BNIB. These replacements have rotted out too and of course there's no way to get new Peerless drivers for either bass or mid.
The original SEAS tweeters look ok and I'll check for open circuit at some stage.
In the meantime, after a fruitless search for suitable replacements and encouragement from this site I decided the look at refurbishing!
The first challenge was to get a 10-inch re-foaming kit and I took a chance on the usual place and bought 4 for £4.39!
Attached are photos I've taken of the refurb from the physical aspect. I've yet to fire up as I'm waiting for foam kits to arrive for the 4.25-inch mids as they seem to be particularly special and are coming from the Netherlands.
The only difficulty was in ensuring the foam stuck down all the way round on the cone since it tried to buckle up in a couple of places. PVA was used, which I had let dry out a little to thicken and I had to re-check and press carefully down until the tack of the glue finally held it in place.
The cones move smoothly with no rubbing.
I'll post more pics when I get the new foam for the mid-range and of course let you know how they sound. I'll check the crossover components as I have an LCR meter and do a visual too.
Attachments
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.
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.
Attachments
The Tweeter are SEAS H107 1-inch domed with 6 Ohms printed on them. They're around that so should be ok.
Next in line for examination and re-furb are the crossovers. They are fixed inside (see photo) and I couldn't tell how they were fixed but no screws were aparent so I assumed some sort of glue.
It was an enormous task to prise the first crossover from the back of the speaker housing. The culprit is 4 large blobs of hot-melt glue and as you can see by the photo it was the MDF surface which actually let go. A hair dryer and blunt knife scraped the majority of the hot melt off and I marked up the connections I for input, B for Bass, M for Mid, T for Tweeter. Photo taken, the rest of the hot melt came away with lots of solvent on a cloth.
The crossovers look in remarkable condition considering the time the speakers have been used at high volume and whilst I'll unsolder one end of each component and measure it looks like I don't need to be replacing Inductors and maybe Resistors.
I'll replace the ELCAP capacitors with Monacor MKT, but if someone has a different suggestion I'm open to ideas...
Next in line for examination and re-furb are the crossovers. They are fixed inside (see photo) and I couldn't tell how they were fixed but no screws were aparent so I assumed some sort of glue.
It was an enormous task to prise the first crossover from the back of the speaker housing. The culprit is 4 large blobs of hot-melt glue and as you can see by the photo it was the MDF surface which actually let go. A hair dryer and blunt knife scraped the majority of the hot melt off and I marked up the connections I for input, B for Bass, M for Mid, T for Tweeter. Photo taken, the rest of the hot melt came away with lots of solvent on a cloth.
The crossovers look in remarkable condition considering the time the speakers have been used at high volume and whilst I'll unsolder one end of each component and measure it looks like I don't need to be replacing Inductors and maybe Resistors.
I'll replace the ELCAP capacitors with Monacor MKT, but if someone has a different suggestion I'm open to ideas...
Attachments
A labour of love. Couple of tips. I don't know offhand if the tweeters are ferrofluid, but you could look that up. Blue Aran at Swaythling near Southampton sell in a 5ml syringe, enough for 5 tweeters.
It goes solid after 20 years, but is easy enough to replace. I think we decided bike repair rubber glue is the right stuff for plastic cones and foam surrounds, PVA for paper cones.
You might try Falcon Acoustics or Wilmslow Audio for NPE capacitors. MKP can be rather large above 10 uF, but 250V types are not too bad.
You need a 40W iron with modern unleaded solder and must tin the tip first time it warms up. Good luck.
It goes solid after 20 years, but is easy enough to replace. I think we decided bike repair rubber glue is the right stuff for plastic cones and foam surrounds, PVA for paper cones.
You might try Falcon Acoustics or Wilmslow Audio for NPE capacitors. MKP can be rather large above 10 uF, but 250V types are not too bad.
You need a 40W iron with modern unleaded solder and must tin the tip first time it warms up. Good luck.
Mission 720 Refurb - the crossovers
Hello Steve - thanks for the comments. The H107 doesn't use ferrofluid so they should be ok. The bike repair glue will be an option if the PVA hasn't worked with the bass drivers plastic cone and foam surround. At least they're easy to remove and since the re-foam pieces from Hong Kong were so cheap at ~£4.40 for four including postage I bought another four.
The original ELCAP capacitors are rated at 50v. I'll checkout NPE type - first trawl through the internet reveals a few concerns about leakage, especially in bass driver circuit. My leaning towards MKP is the 'zero' ageing. I won't say cost is not a concern, however I wan't to do this job once after which they'll definitely outlive me! The MKP types I've seen are all rated at 250v.
I have seen articles where it is recommended to use a certain type of capacitor for bass filter circuit and a different type for the high frequency crossover. Anybody care to comment on that?
Finally on the Mission 720 crossover, the crossover frequencies are given as 700Hz and 2700Hz: These frequencies don't equate to Octave and Decade, i.e. 3 times or 3.4, spread. Each online calculator I've come across so far won't let you put in these frequencies. I was going to put in the frequnecies and see which values popped out the other end.
Curiously also, the SEAS H107 data sheet (attached) shows a recommended frequency range of 3,000 to 25,000Hz and 'suggests' a crossover frequency of 4000Hz, 12dB/oct. So, 2700Hz seems quite low?
Hello Steve - thanks for the comments. The H107 doesn't use ferrofluid so they should be ok. The bike repair glue will be an option if the PVA hasn't worked with the bass drivers plastic cone and foam surround. At least they're easy to remove and since the re-foam pieces from Hong Kong were so cheap at ~£4.40 for four including postage I bought another four.
The original ELCAP capacitors are rated at 50v. I'll checkout NPE type - first trawl through the internet reveals a few concerns about leakage, especially in bass driver circuit. My leaning towards MKP is the 'zero' ageing. I won't say cost is not a concern, however I wan't to do this job once after which they'll definitely outlive me! The MKP types I've seen are all rated at 250v.
I have seen articles where it is recommended to use a certain type of capacitor for bass filter circuit and a different type for the high frequency crossover. Anybody care to comment on that?
Finally on the Mission 720 crossover, the crossover frequencies are given as 700Hz and 2700Hz: These frequencies don't equate to Octave and Decade, i.e. 3 times or 3.4, spread. Each online calculator I've come across so far won't let you put in these frequencies. I was going to put in the frequnecies and see which values popped out the other end.
Curiously also, the SEAS H107 data sheet (attached) shows a recommended frequency range of 3,000 to 25,000Hz and 'suggests' a crossover frequency of 4000Hz, 12dB/oct. So, 2700Hz seems quite low?
Nice job. I refurbed a pair of Heybrook HB3's. I got my mid-range surrounds from the Netherlands. When I researched for glue it was recommended that I used Aleenes Tacky Glue.
Aleene?s Original Tacky Glue
It looks like PVA but I assume it is not.
Something worth doing IMO once you have them up and running is to either buy of make some stands about 200mm high to get them off the floor. I did it with the Heybrooks.
Aleene?s Original Tacky Glue
It looks like PVA but I assume it is not.
Something worth doing IMO once you have them up and running is to either buy of make some stands about 200mm high to get them off the floor. I did it with the Heybrooks.
Hi Puffin - thanks for the link. Yes, it's a PVA type glue being a water-based polymer emulsion. Manufacturers play around with various compositions to alter certain characteristics.
I'll certainly look at some stands as well.
Any takers for the crossover frequencies questions?
I'll certainly look at some stands as well.
Any takers for the crossover frequencies questions?
I'm very hampered by posting from a mobile phone here.
There's very little difference in sound quality between standard NPE and MKP capacitors, but MKP will usually sound louder or brighter due to lower ESR aka resistance which is around 0.5R for NPE types.
There's a lot of different ways to design a three way. I'd suggest you look at Troels Gravesen SEAS 503 refurb. Not sure quite what the mid is, whether it's sealed back etc.
It's also useful to go to Visaton Downloads and get the simulator. Boxsim Projektdatenbank has some similar projekte files you can then import and modify.
The W250S, W100S and SC10N drivers would be good equivalents to sim IMO. But you'll need to study and measure your crossover
There's very little difference in sound quality between standard NPE and MKP capacitors, but MKP will usually sound louder or brighter due to lower ESR aka resistance which is around 0.5R for NPE types.
There's a lot of different ways to design a three way. I'd suggest you look at Troels Gravesen SEAS 503 refurb. Not sure quite what the mid is, whether it's sealed back etc.
It's also useful to go to Visaton Downloads and get the simulator. Boxsim Projektdatenbank has some similar projekte files you can then import and modify.
The W250S, W100S and SC10N drivers would be good equivalents to sim IMO. But you'll need to study and measure your crossover
Thanks Steve - I'll have a look at costs of NPE and MKP. If MKP sound 'brighter and louder' then I'll probably go for them since in the last 36 years my ears have become 'more dull and less sensitive'.
Looking at Falcon Acoustics their new ALCAP capacitors are not finacially burdening, MKP also not out of reach.
I'll keep you posted!
Looking at Falcon Acoustics their new ALCAP capacitors are not finacially burdening, MKP also not out of reach.
I'll keep you posted!
I've not had chance to lift up a leg of each ELCAP yet to measure the capacitance, but as the speakers had been working fine last time they were used and no visible damage I have re-assembled one speaker and tried it.
The results are very pleasing: The bass is punchy and tight, the mids right out there too and the treble crystal clear. This was a test using just one of the re-furbed Mission 720, the other speaker is a Mission 761i being what I have been using over the last few months.
The installation and source is hardly a 'reference' standard, being output from the TV (Samsung Smart TV) via 3.5mm jack to RCA and using a Chinese made A10 EL34 single-ended amp. I will say that the amp. does sound excellent too. The 720 speakers are also sat on the floor so a 200mm stand is planned.
On re-assembly, I have only used half the internal wadding as I've read in several forums specifically that Mission 720 was 'overstuffed'. I can always put some or all of the rest back in, but agree that the two rolls of stuffing were quite tightly rolled up.
With the other 720 speaker still apart I will lift the capacitors legs on the crossover and test shortly and publish the readings. It has been commented they could go on for another 36 years but maybe they could fail in a couple of months if I start using them in anger again.
For the small cost involved maybe I'll just replace them with ALCAPs and be done.
The results are very pleasing: The bass is punchy and tight, the mids right out there too and the treble crystal clear. This was a test using just one of the re-furbed Mission 720, the other speaker is a Mission 761i being what I have been using over the last few months.
The installation and source is hardly a 'reference' standard, being output from the TV (Samsung Smart TV) via 3.5mm jack to RCA and using a Chinese made A10 EL34 single-ended amp. I will say that the amp. does sound excellent too. The 720 speakers are also sat on the floor so a 200mm stand is planned.
On re-assembly, I have only used half the internal wadding as I've read in several forums specifically that Mission 720 was 'overstuffed'. I can always put some or all of the rest back in, but agree that the two rolls of stuffing were quite tightly rolled up.
With the other 720 speaker still apart I will lift the capacitors legs on the crossover and test shortly and publish the readings. It has been commented they could go on for another 36 years but maybe they could fail in a couple of months if I start using them in anger again.
For the small cost involved maybe I'll just replace them with ALCAPs and be done.
You rarely need to unsolder components to measure them. Just ensure that at least one end is floating.
I use a £70 multimeter that includes a mH scale. Along with the more common uF and ohms.
The main work is to sketch the schematic and driver polarities.
Then, if you have done your homework with the SEAS 503 refurb, and do a bit of simming, you will get a grasp of what is going on.
Most 3 ways follow established Steen Duelund target responses, and if they don't, you can improve them.
I use a £70 multimeter that includes a mH scale. Along with the more common uF and ohms.
The main work is to sketch the schematic and driver polarities.
Then, if you have done your homework with the SEAS 503 refurb, and do a bit of simming, you will get a grasp of what is going on.
Most 3 ways follow established Steen Duelund target responses, and if they don't, you can improve them.
Hello Steve - I plan just to lift one leg of each ELCAP, not to completely de-solder and remove from the board. I have both de-soldering pump and de-soldering wick.
The LCR meter I have is a low-cost unit from the usual place but should allow me to measure and sketch out the circuit.
I've looked at SEAS 503 Refurb - very interesting. I take on board what you say about 3-way SD target responses so will be an interesting exercise to analyse the Mission supplied unit.
The LCR meter I have is a low-cost unit from the usual place but should allow me to measure and sketch out the circuit.
I've looked at SEAS 503 Refurb - very interesting. I take on board what you say about 3-way SD target responses so will be an interesting exercise to analyse the Mission supplied unit.
I doubt if there is any reason to lift capacitors at all. I would much rather lift coils and resistors and connection wires if necessary.
Capacitors are fragile with heat. The other components are tough as old boots. And it's usually only the coils that are unmarked. You can read the values of the rest.
Trace that schematic. The Seas 503 should give you an idea what you are looking at.
Capacitors are fragile with heat. The other components are tough as old boots. And it's usually only the coils that are unmarked. You can read the values of the rest.
Trace that schematic. The Seas 503 should give you an idea what you are looking at.
Steve shows a level of self-restraint I cannot. 🙂 There's no way I'd see a cap like that and not know I was going to upgrade it, so I would be clipping it off immediately! 🙂 I'm no adult though, follow Steve's example instead.
Best,
Erik
Best,
Erik
Goodness knows, we've seen enough threads where people announce they are "upgrading" their speaker with higher quality caps. I think a bit more research is in order first.
I have schematics and Sims for the Tandberg TL 5010 and Gale 401C, both of which used this peerless 4" mid. We might be able to make some more useful and cost-effective mods. It looks like a competent crossover, but I still would want to know the layout and some values. Coils, I could probably guess.
I have an old Ditton 44. It has a bit of harshness in the upper mid, but I don't think it is a NPE problem. Just what 5" mids do when not rolled off enough at the top end. The HF 2000 tweeter could probably be replaced too. I also found that one of the basses was wired with the wrong polarity.
You should also beware voicecoil rubbing with old speakers, often fixed merely by rotating the driver 90 degrees.
Which is to say that a proper assessment might reveal a better way to improve a speaker rather than spend big bucks on inconveniently large MKP capacitors.
I have schematics and Sims for the Tandberg TL 5010 and Gale 401C, both of which used this peerless 4" mid. We might be able to make some more useful and cost-effective mods. It looks like a competent crossover, but I still would want to know the layout and some values. Coils, I could probably guess.
I have an old Ditton 44. It has a bit of harshness in the upper mid, but I don't think it is a NPE problem. Just what 5" mids do when not rolled off enough at the top end. The HF 2000 tweeter could probably be replaced too. I also found that one of the basses was wired with the wrong polarity.
You should also beware voicecoil rubbing with old speakers, often fixed merely by rotating the driver 90 degrees.
Which is to say that a proper assessment might reveal a better way to improve a speaker rather than spend big bucks on inconveniently large MKP capacitors.
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Good news... The capacitors all were in tolerance so no need to replace. I must admit that this is only on the crossover board I managed to 'prise' out of the cabinet, but as the other board looks as healthy and is the one I have already re-assembled I am leaving it be.
For community benefit, especially Steve, attached are pictures showing the back (non component side) of the crossover PCB where I have highlighted conductive track in blue. The other picture is a MIRROR IMAGE of the same picture where I have overlaid symbols and values on the components, i.e. as if viewed from the component side. I have measured the resistors and they are in spec. The measured value for the inductors are the ones I've indicated.
I'll trace out the circuit diagram next...
For community benefit, especially Steve, attached are pictures showing the back (non component side) of the crossover PCB where I have highlighted conductive track in blue. The other picture is a MIRROR IMAGE of the same picture where I have overlaid symbols and values on the components, i.e. as if viewed from the component side. I have measured the resistors and they are in spec. The measured value for the inductors are the ones I've indicated.
I'll trace out the circuit diagram next...
Attachments
Update from the weekend: I've re-assembled the second 720, again with half the internal wadding. The results are excellent and I'm very pleased. I have traced out the circuit diagram for the crossover so I'll upload here when I draw it out properly.
Total cost of parts used no more than £30.
I'm more than happy with the existing crossovers however it will be interesting to have some comments on the design.
Next stage will be a new amplifier. PP KT88 type...
I'll take some final pics of the speakers and if anyone would like specific information or photos please let me know.
Total cost of parts used no more than £30.
I'm more than happy with the existing crossovers however it will be interesting to have some comments on the design.
Next stage will be a new amplifier. PP KT88 type...
I'll take some final pics of the speakers and if anyone would like specific information or photos please let me know.
So it may not be worth it to you, but as Steve and I have discovered, those large lands under coils can cause odd interferences. If you were in a prison, and you had nothing else to do but modify these speakers or die of boredom, I'd suggest trying a point to point version of the crossover. 🙂
Best,
Erik
Best,
Erik
Tee-hee Erik... Now another person might tell me the large lands were deliberate because they provide a low impedance signal path and if I changed them that would change how the original designer intended them to sound. Anyway, luckily I'm not in prison and my tired old ears wouldn't probably notice the difference anyway 🙂
I've re-assembled the second 720 and am loving the results of £30 and plenty of patience...
I've re-assembled the second 720 and am loving the results of £30 and plenty of patience...