Mission 753 speaker modding

[Roppu]

Ok Richard. Let me do these measures.
I was contacted by Italian Mission distribution. They said they will try to provide the schematics because Mission was bought by IAG and they're not sure they still have it

[colin.hepburn]

Originally Posted by Roppu

Ok Richard. Let me do these measures.
I was contacted by Italian Mission distribution. They said they will try to provide the schematics because Mission was bought by IAG and they're not sure they still have it

Hi Roppu
Just a thought if you can’t get the schematic from mission you could try Peter Comeau who is a member here he owned W.D at one time and he also designed speakers for mission but not sure if he designed the 753 but he should be able to help you and the bonus is he is the Head of acoustic design for IAG. Here is the link for him http://www.world-designs.co.uk/forum/member.php?u=1
Ps and here is a link to an interview he did https://youtu.be/F1WYNCZqOKY ad here https://youtu.be/yIkEbu8XyKg
Hope this helps you
colin

[Roppu]

Originally Posted by Richard

Hi Roppu.

I’ve attached your latest drawing and the one from the first post in this thread.

If you have one of your speakers still wired you could meter DCR across LF- and LF+

1) if your drawing is correct (mid units in series with no series cap, paralleled with bass units in parallel) you should get a reading of 6.9R // 1.7R = 1.4R

2) if your drawing is correct except you have series bass drivers (mid units in series with no series cap, paralleled with bass units in series) you should get a reading of 6.9R // 6.95R = 3.5R

3) if you get 1.7R you probably have parallel bass drivers and a series cap in the midrange circuit as shown in the other drawing

4) if you get 6.95R you probably have series bass drivers and a series cap in the midrange circuit as shown in the other drawing

Of these I’d expect 4) given the DCR of your drive units.

Thoughts looking at the 2 diagrams; crossovers can be confusing to work out from tracing connections in the speakers. The last 2 speakers I restored (JR149 and KEF Corelli) didn’t have the correct circuits on the web. In addition, both had changes made during their lifespan by the manufacturers.

I would expect yours to have a series cap in the midrange and to have the bass drivers in series not parallel given their DCR.

Re. the first-post drawing it is unusual to see that 2.2R series resistor in the bass circuit and the 2 parallel 1R resistors looks strange as 0.5R would have very little effect on tweeter level.

Many longer running designs had changes made in production so I hope you can get the circuit(s) from Mission.

Ok Richard. I rechecked and my schematic looks reflecting the real situation. Just for you to check, i took some glue away and you can see yourself:
https://1drv.ms/u/s!AgMbnBJhjY8OliNdkh8zWkMpdsaz

I measure 1,8 Ohms but no cap in series in the MR stage!

[Roppu]

Originally Posted by colin.hepburn

Hi Roppu
Just a thought if you can’t get the schematic from mission you could try Peter Comeau who is a member here he owned W.D at one time and he also designed speakers for mission but not sure if he designed the 753 but he should be able to help you and the bonus is he is the Head of acoustic design for IAG. Here is the link for him http://www.world-designs.co.uk/forum/member.php?u=1
Ps and here is a link to an interview he did https://youtu.be/F1WYNCZqOKY ad here https://youtu.be/yIkEbu8XyKg
Hope this helps you
colin

Hi Colin. Thank you. Yes I was hoping he would chime in here but he seems not partecipating here since last august. Anyway I will try to send him a pvt in case Mission would not be able to provide it. As you can read, he answered this thread, first time in this post:

Originally Posted by petercom

Sorry to be late to advising on this one!

I can only advise on things I know will make a difference. For example earthing the baskets (as is done on high-end Tannoy models) may reduce some RF interference in some situations but most people hear 'no difference'.

1. Changing the ferrite cores to air cores is inadvisable. Unless absolutely enormous the air cores will increase the DCR to the bass units and the bass is likely to become flabby as a result. IMO there is very little wrong with good quality ferrites - and these are UK sourced good quality ferrites. You takes your choice - slightly higher dynamic distortion at high listening levels with good ferrite cores, or flabby, uncontrolled bass with air cores - I know which I prefer. (Of course for speakers designed for the higher resistance of air cores you'd be mad to use ferrites!). AFAIK the treble coil was an air core as standard.

2. Rather than messing around with replacement electrolytics you would be better off bypassing them with small value PPs, say between 2% and 5% of the original value, for the bigger caps. Don't bypass individually, wire a cap right across the back-to-back electrolytics.

3. Replace the original polyester caps with PP caps in the treble circuit.

4. Replace the 10uF cap in the midrange circuit with a PP. http://wduk.worldomain.net/acatalog/...propylene.html

5. Replace the ceramic resistors with Vitreous Wirewound types (IMO this reduces the effect of vibration and enhances dynamic capabilities). http://wduk.worldomain.net/acatalog/Resistors.html

6. You can, by all means, try replacing the foam plug behind the dome but you are doomed to experiment a lot with this one as it will alter the frequency response significantly. As a result it is usually better to stick with the foam that was fitted by the manufacturer - removing the dome time and time again to try out different wool/foam etc runs the risk of damaging that highly fragile metal dome!

7. Replacing the foam inside the speakers should bring a substantial improvement (see our recommendations here http://wduk.worldomain.net/Impspkrs.html)

8. Replace the internal wiring. We use this http://wduk.worldomain.net/acatalog/...Cables_21.html for our speakers which, coincidentally, is similar to that which I designed for the high end Mission speakers (Pilastro and Elegante). If the midrange cables pass into the mid enclosure via pins then run the cables direct through sealed holes in the mid chamber instead.

Finally I wouldn't mess about with the dust caps. Dust caps are usually designed to stiffen the cone and removing them also loses up to 1dB of sensitivity (due to the reduction in active cone area at mid frequencies), so leave well alone.

[colin.hepburn]

Oops sorry Roppu I missed peters reply in your thread couple of interesting videos links from him to watch thou

[Roppu]

I will give it a look! Thank you again

[Richard]

Quote:
Originally Posted by Roppu
Ok Richard. I rechecked and my schematic looks reflecting the real situation. Just for you to check, i took some glue away and you can see yourself:
https://1drv.ms/u/s!AgMbnBJhjY8OliNdkh8zWkMpdsaz

I measure 1,8 Ohms but no cap in series in the MR stage!


Hi Roppu,

Yes I see and just 3 inductors unlike the other drawing with 4 inductors.

Your result of 1.8R will be due to the DCR of the 2 inductors which will also be in series with the drivers when you measured. Each inductor will be a fraction of an ohm, so 1) it is.

That doesn’t look right to me though. I do appreciate the difference between DCR and impedance but I’ve not checked any 6 or 8 ohm speakers which measure that low at the terminals.

I would have said consider re-wiring them as per the first drawing but I’m not sure that is correct and you don’t have the other inductors.

So wait and hope for the drawing from IAG, or try and find another pair to check, or ask on another forum if anyone can check their pair for DCR across LF - and LF +

[Roppu]

Originally Posted by Richard

Quote:
Originally Posted by Roppu
Ok Richard. I rechecked and my schematic looks reflecting the real situation. Just for you to check, i took some glue away and you can see yourself:
https://1drv.ms/u/s!AgMbnBJhjY8OliNdkh8zWkMpdsaz

I measure 1,8 Ohms but no cap in series in the MR stage!


Hi Roppu,

Yes I see and just 3 inductors unlike the other drawing with 4 inductors.

Your result of 1.8R will be due to the DCR of the 2 inductors which will also be in series with the drivers when you measured. Each inductor will be a fraction of an ohm, so 1) it is.

That doesn’t look right to me though. I do appreciate the difference between DCR and impedance but I’ve not checked any 6 or 8 ohm speakers which measure that low at the terminals.

I would have said consider re-wiring them as per the first drawing but I’m not sure that is correct and you don’t have the other inductors.

So wait and hope for the drawing from IAG, or try and find another pair to check, or ask on another forum if anyone can check their pair for DCR across LF - and LF +

Hi Richard. I would try to do that too if I was sure. Anyway I'll try to ask somebody else too as you suggest.

Thank you very much again. Have a happy Ester you and the others here

[Roppu]

Originally Posted by Richard

Quote:
[...]
I would have said consider re-wiring them as per the first drawing but I’m not sure that is correct and you don’t have the other inductors.

So wait and hope for the drawing from IAG, or try and find another pair to check, or ask on another forum if anyone can check their pair for DCR across LF - and LF +

Hi Richard.
I tried to find some post on the web where my series (Freedom) and same crossover is discussed. I found this one:
http://myoldvintagehifi.blogspot.com...3-freedom.html
http://3.bp.blogspot.com/_72VgihSgEs.../s1600/070.JPG

It looks just the same and from red and black wires you can see the woofers are connected in parallel. Then my thought is it's not a modding done by previous owners.
Moreover, if you read what "Audiomark" says here (he starts at post n. 17), he says the two polar caps in series were in the midrange section in the original 753's and later they were curiously put in the woofers section in the Freedom's.
If I'm allowed, I'll paste it here too:

Just been working on a pair of very early 753's, so thought I'd revive this thread.
The 753 has always been a favourite of mine, and I've owned two of the original and one pair of Freedoms.
Despite my affection for these things, they are a bit of an enigma, and there's much room for improvement. They were very affordable considering the quality of the cabinet was way above what you'd typically get at the price.
To meet that price point economies were made in terms of components, and compromises made for ease of assembly. And some aspects are just unfathomable.
One interesting fact is that, although they look like they would be a 2.5 way design, they are actually a 3-way, as the upper pair of mid-bass drivers are high-pass filtered.
The most obvious characteristic of the original is the excessive treble, both audible and measured, too hot by about 3 dB. Those Vifa-made tweeters are actually not bad, but are run way too hot. I have found variously no attenuation, 0.5 ohms and 1 ohm in different samples. Substituting a 3/3.3 ohm resistor gives a nice balance, and while you're there, make sure the resistor and cap have been connected in the proper order!
You'll notice that Mission, as they did with all their speakers from that era, use over-sized ring terminals to connect to the binding posts, resulting in poor contact. Replacing with correct size rings is worthwhile.
As is by-passing the series 470 uF electrolytic caps in series with the upper mid-bass units.
If you find the speakers are bass light for your system/room, try shorting out the 2.2 ohm resistor in the lower-bass crossover.
You might be tempted to re-wire your 753's. The problem is, any wiring improvements are going to be compromised by the brass pins that pass through the cabinet divider. these have a lot of mass, and it's difficult to get a good solder joint. Even the factory couldn't.
Best to run the new wire direct to the top cavity, either by punching out the pins and using those holes, or drilling new holes. A bit of a pain.
I've got other ideas for more radical mods. Later.
------
I've just finished modding a pair of 753 Freedoms.
First thing to note is that the Freedom is in fact a 2.5 way speaker, so it's no surprise they sound considerably warmer than the original.
Not so much to do with the much simpler crossover on these. If anything they are a tad dull, so I replaced the original 3.3 ohm tweeter resistors with 2.7's.
Replaced all the ring terminals, cleaned up the binding posts, got rid of the ferrous nuts and washers, by-passed the elcap across the upper mid-bass drivers with a small value Wima, as there's no room for an equivalent value poly cap.
Interestingly, those same back to back 470 uF caps that high pass filter the upper mids in the original, are used to shunt the lower units in the Freedom.
Before the mods the sound was distinctly "meh', rather uninspiring.
Post mods, they've sprung to life, fast and engaging as a 753 should be
-------
Technically a 3 way, as the upper two drivers are high -pass filtered, even though all the drivers are identical.
BSC? Don't think so, as there's resistive attenuation of the lower two drivers. And they were probably intended to be used against the wall. Certainly sound best that way around here.
The likely reason for making it a 3 way is to reduce the excursion of the upper two drivers, in the interest of cleaner mid-range at high levels.
But counter to that, having electrolytic caps in series with the drivers is bad for resolution at any level.
Which is probably the conclusion Mission came to when they re-engineered the design for the Freedom.

[Richard]

Hi Roppu,

I've had a look throught those links but where the red and black wires actually go to is not shown and the crossover is built in 2 sections so connections are difficult to see from web pictures.

However you have traced yours and found your measurement of 1.8 DCR at the LF- LF+ terminals which we accept would be accounted for by parallel bass drivers. I've never found a speaker with DCR or impedance that low though and think yours may have been modified - which may be the reason your amp is struggling and the sound is not good.

What amp are you using? Have you had a reply back yet from IAG?