Mission 753 speaker modding

[Roppu]

Originally Posted by Richard

Hi, here's the impedance curve, nominal 6 ohm,

https://www.stereonet.com/forums/upl...0091c12aa8.png

I'd expect approx 4 DCR drivers if they are in series or much higher around, 12 - 16 DCR if in parallel.

Thank you!

[Brian]

Looks like you could move the crossovers outside the cabinets without any hole drilling in the terminal plates. Others may find it straightforward but it looks a complicated crossover to me, I would want to move them outside the cabinets though, that would be my first step given you want to tweak them even if they were working fine.

[Roppu]

Originally Posted by Richard

Difficult to say how accurate either of the circuit drawings are; yours may be modded and the one in the first one may have an error. You could try checking the DCR of the drivers to see if it gives a clue.

Hi Richard, hi guys.
Just for those who might happen to get here and have my same series, here I will insert the schematics as a viewable image:
https://1drv.ms/u/s!AgMbnBJhjY8Olh-tDAoU_oHsPjhm

I finally found some time to desolder one lead of caps and speakers to take some measurement. Here are results:
https://1drv.ms/u/s!AgMbnBJhjY8Olh5bFEDbTUeKoaDq

Originally Posted by Richard

There are quite a few 753 sites which may be worth a read, just google “mission 753 schematic”
etc etc, looks like problems and mods were common so try to get the original correct circuit, then test your drive units and xover components, then rebuild them as standard.

Yes, this one is very interesting (see last "Svenr" post):
https://www.stereonet.com/forums/top...pgrade-thread/
even though I can't understand everything. And unfortunately it's based on the schematics of another series. However "Svenr" says having two polar caps connected back to back can lead to problems... My 470 uF polar ones belong to the LF sections but they do't seem to be faulty. Unfortunately I don't have a meter for checking their ESR... Anyway I will change them to give this a try. Should I then replace them with a single 235uF non polar?
You can see the C4 "square" cap too here:
https://1drv.ms/u/s!AgMbnBJhjY8OliDP4Y-M0AN9i_n7
What kind of cap is this?

Originally Posted by Richard

Hi, here's the impedance curve, nominal 6 ohm,
https://www.stereonet.com/forums/upl...0091c12aa8.png
I'd expect approx 4 DCR drivers if they are in series or much higher around, 12 - 16 DCR if in parallel.

I measured them one by one. What about the measures I found?

I will send Mission an email now asking for the schematics of my one and hoping they will provide it.

The mids and tweeter are in a inner, smaller cabinet. Wiring from crossover to them is not direct but it's made through metal pins fixed to the base of this small cabinet:
https://1drv.ms/u/s!AgMbnBJhjY8OliFSvmVltP9D0LwV
I decided to eliminate them and use continuos wires.
Do you have ome recommendation for what kind of wire to use? I used solid copper LAN cables to make my power cables and I was thinking to do the same inside.

I know somebody thinks it's snake oil but I have not enough experience to decide myself: do you think there's a brand of caps I might find more proper to improve LF replacing C1 and C2?

[Roppu]

Originally Posted by Brian

Looks like you could move the crossovers outside the cabinets without any hole drilling in the terminal plates. Others may find it straightforward but it looks a complicated crossover to me, I would want to move them outside the cabinets though, that would be my first step given you want to tweak them even if they were working fine.

Thanks Brian.
My problem with tweaking is that actually I'm not that good. I should say I'm just a beginner. I would do an external crossover as you say but honestly reaching the goal of improving the LF for me would be more than enough if that was doable by changing caps and wire

[Roppu]

Originally Posted by Brian

The 3rd link from Richard shows a schematic that looks far more likely to work properly, imo. ( Note: I viewed the page at the 3rd link once and it won’t allow me to view it again without joining....very odd)
[...]
I’d start by building the crossover following the one in the link from Richard.

I had to register to see it!
Really do you think I should move to that schematics? But we don't know if the speakers have the same features... What if they are different and with different impedance? That's why I'm writing to Mission to ask for my schematics

[bob orbell]

Originally Posted by Roppu

Thanks Brian.
My problem with tweaking is that actually I'm not that good. I should say I'm just a beginner. I would do an external crossover as you say but honestly reaching the goal of improving the LF for me would be more than enough if that was doable by changing caps and wire

To be honest with you, if you think you are not that good at tweaking then you really should not be tampering with speaker crossovers, they can be a real problem, as I have said in another post, listen to and buy speakers you like, if it is more bass you are after then bigger or transmission line, stop trying to change something that has been designed by people who know what they are doing, if getting more bass from the speakers you have was possible, it would have been done, at Mission . Bob

[Roppu]

Originally Posted by bob orbell

To be honest with you, if you think you are not that good at tweaking then you really should not be tampering with speaker crossovers, they can be a real problem, as I have said in another post, listen to and buy speakers you like, if it is more bass you are after then bigger or transmission line, stop trying to change something that has been designed by people who know what they are doing, if getting more bass from the speakers you have was possible, it would have been done, at Mission . Bob

Seems like you don't even understand. I'm just changing caps and wiring. That's not called tampering. Save your time for more useful recommendations

[bob orbell]

You are not going to get more bass by simply changing components, but if it pleases you, then go ahead. Bob

[Greg.]

Originally Posted by bob orbell

You are not going to get more bass by simply changing components, but if it pleases you, then go ahead. Bob

IMHO and experience that statement is wrong. ‘Better’ (emphasis applies to my ears) components most certainly can improve bass response.

[Richard]

Quote:
Originally Posted by Roppu
Hi Richard,


I measured them one by one. What about the measures I found?

I will send Mission an email now asking for the schematics of my one and hoping they will provide it.


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.