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Need help regarding crossover removal and capacitors replacement


parlophone1

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I have speakers that are now 40 years old (Infinity Qb). They are sealed enclosure, 3-way with ribbon tweeter.

Sound is acceptable in most recordings. However, I have a strong feeling that mid frequencies are subdued and sound in that area is not as detailed as I wish. On the internet there seems to be unanimous agreement that after such a long service the capacitor on mid-driver (12,5 µF) should be replaced with the new one.

I had a plan to take out the crossovers and to replace capacitors for mid-drivers. While there, I plan to replace clip-on binding posts/terminals with a modern one, to seal speaker boxes with silicone glue and brace them.

 

Here is the crossover scheme:

 

qb_specs.jpg

Now to the point: the only way to access the inside of a speaker box is through the woofer opening at the front panel. Taking the woofer out enables the access to the crossover, since the crossover is placed directly behind the woofer on the speaker rear panel.

I unscrewed the woofer and put the polyfill aside. The internal wires are attached to the speakers with crimps and soldered on the crossover side. It seems that nobody has been inside the speakers until now. Everything is intact and look fresh as the first day the speakers left the factory.

qb_opened1.jpg

Qb_basdriver1.jpg

 

This is a look at the crossover from the inside:

Qb_cross1.jpg

Qb_cross2.jpg

 

1 - Now here is the problem for me, the crossover is glued to the back panel with a sticky elastic glue and additionally densely stapled with narrow staples. It is flush with the chipboard.

Any suggestions on how to remove the crossover from the chipboard? I would like to leave the speakers as intact and original as possible. I suppose that it is possible to do the job with the crossover attached but it would me much more difficult.

2 - Capacitors replacement. I cannot find single 12,5 µF capacitor online. I will do some more search, but that leaves me with the option of putting two in parallel (5,6 and 6,8) which together would make 12,4 µF.

Any suggestions what capacitors I should use keeping in mind price/quality ratio?

3 – While bracing the cabinets, I have to remove the polyfill which seems to be loosely attached to the cabinet walls. I suppose that I would have to use some kind of glue when putting polyfil back again. What glue is that? Spray?

I am doing that for the first time, so bear with me here.

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If things in a speaker are not screwed in but glued, you know they didn't want you to service it or they did the work on the cheap.

 

Caps add up in parallel.  If you are within 5/10% you are probably as good as the company that built the speakers.  

 

Poly fill can be put into cabinet loosely as long as it avoids the drivers.  Spray glue or some other sticky compound such as caulk can help it stay put.

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For sealing the cab, I suggest not using silicon caulk.  I doubt it would make it air tight.  Instead use a polyurethane such as Gorilla Glue or Loctite PL Premium Polyurethane Construction Adhesive which is an expanding glue.

 

I have used Titebond III to seal cabs such as sealed subs with good success.

+++

 

You can put the polyfil in loose, it doesn't need to be glued to the sides.

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Guest wdecho

Silicone 100% is best. Holds mammoth aquariums together without frame. Used and developed my NASA on flights to seal. Just be sure 100% silicone. wvu80's suggestion will probably work just as well but I always use silicone myself. Apply it by pushing and not pulling if this makes since. They way I use to build aquariums. 

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I remember those well. I'm surprised the rubber surrounds on the drivers aren't starting to decompose. You can check those by dipping a Q-tip in some Armour All, and lubricating the surrounds. Press out the excess on a tissue, you don't want the Armour All dripping onto the paper part of the driver. As you go around the surround, gently press and observe. You may see (or not), cracks and splitting. 

 

I would also replace the 6.8uF that's on the tweeter.

 

You can use a 12uF for the midrange. I wouldn't lose any sleep over doing that.

 

Do the work with the crossover attached. Do NOT try to remove it (you've been warned).

 

Use a butane soldering iron.

 

https://www.parts-express.com/dayton-audio-pmpc-12-12uf-250v-precision-audio-capacitor--027-246

 

https://www.parts-express.com/dayton-audio-pmpc-68-68uf-250v-precision-audio-capacitor--027-238

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Thank you all for replies. 

Regarding the capacitor for the tweeter, several poeple have done that and were not happy. They claim that the sound changed to the worse. Do not know what capacitors they used but it seem weard to me.

 

I was also thinking about replacing the internal wires from drivers to crossover. The factory wires look OK but they are so thin. Any advices on that?

 
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Guest wdecho
1 hour ago, parlophone1 said:

DeanG,

 

can I safely buy the capacitors from links that you have posted above?

I notice that Daytons from PartsExpress are 250V,  while my original 6,8 µF capacitor is rated 100V.

 

Could you put the link for the replacement resistor in case I decide to change it?

Thanks...

Higher voltage rating on capacitors mean made better to withstand more voltage. The higher voltage rating increases the price. I can see no need to change any resistor in X-over network. The resistor in question is a wirewound one with a tiny bit of inductance and some like to use a different type without inductance thinking it will make a difference. Splitting hairs to my way of thinking but change if one wants to. Will not hurt anything but doubt will make a difference. Most certainly the resistor in question is not bad. Dayton caps are supposedly made by Bennic, decent caps but I prefer these after using Dayton many times, priced about the same, Jantzen CrossCap Metallized Polypropylene Capacitors. Bottom of page, https://www.parts-express.com/resources-capacitor-selection-guide

 

 

 

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  • 1 month later...

An update:

I have recapped speakers.

It took a while because I have very limited free time. I also sealed the inside of speaker boxes and braced them.

It turned out that one capacitor for mid-range has already been changed because the originals are yellow as I noticed in second speaker when opened.

 

Cross_Untuched4.jpg

 

Opted for Mundorf MCap Evo capacitors (aluminum) because of price and availability, and Mills MRA 12 watts resistors. All components are within or better than factory specifications, except I used 12 microF resistor for mid-range driver instead of 12,5.

 

Cross_Untuched.jpg

 

Soldering was done with amateur soldering station (58 watts). It was tedious job for me since last time I have been soldering was more decades ago. And the job had to be done inside the box as could not take the crossover out. Been careful and tried to execute clean solder joints. The most difficult part was to join three wires going from the speaker drivers together with capacitor wire on a speaker terminal. In such a tight space I once burned my finger and once touched the speaker wire. Happy that were all the injuries.

😃

 

I used new gasket when installing back mid-drivers. Gaskets for woofers were good, so no need to change them. When doing all that I also changed old clip-on speaker terminals for more modern screw type gold-plated terminals (had spares from old Mission speakers).

 

Assembled it and hooked it all up yesterday evening for initial listening. Frankly, I expected more dramatic change. The improvements are subtle.

 

Good thing is that mids are now more articulate than before, but still weak to my ears. The character of the mid driver did not changed. It seems that they were designed that way.

The bas is a bit tighter. Highs showed the least change. If I have to pick out something I would say that they lost some of their mellowness and liquidity and remained detailed but more “technical”. Rumors are this is due to the nature of Mundorfs.

Anyway, I do not believe in burning in the capacitors but I will give the speakers more time for listening.

 

 

Cros_Untuched_Braced2.jpg

 

 

Time is now for some cosmetic changes on the edges of veneer and may be new grills.

 

IMG_20180107_123733.jpg

 

 

 

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No I did not test the tweeter.

How do I do that, sorry I am a newb regarding these issues?

 

But I will mention a few things that seem relevant to me: I did not take out the tweeter. It is a panel-driver with strong magnets which I was afraid to touch cause I might have problems to put them together again. As these were Ok before, I had no reason to mess with them. I was careful working on the crossover so I can only say I did my best to avoid issues like shortages. I have touched one wire with the soldering iron but it did not come through the pastic to the wire, just a cosmetic issue. However, I put the electrical tape over it. The job on other speaker went smoothly and both tweeters seem to sound equal to my ears.

The tweeter is connected to a fuse 1,25 A that is located outside on the crossover. Doesn't it help? Ok, some people advise to replace those with 0,75 or 1 A, but that is not relevant now.

 

I only checked mid-driver and woofer detached from wires with the ohm-meter. They seem to measure ok, if they did not I would think twice regarding recapping.

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