AR4x Crossover Rebuild Check

[JKent]

Two thoughts:

1. As Roy said your pots are almost certainly the cause of the tweeter problem. They can be cleaned (sometimes) but you’re better off replacing. I don’t know what you have on hand but L-pads work well. 
2. In my experience the Aida cloth has too tight and regular a weave to look right and maybe even to sound right. Michaels sells CharlesCraft Irish Linen cross stitch material in “Tea” color, 28 count, that works well. One roll is enough. 
https://www.michaels.com/product/charles-craft-28-count-irish-linen-needlework-fabric-10357706?michaelsStore=1098&inv=1

Edited December 17, 2023 by JKent
Added link

[jviss]

6 hours ago, JKent said:

Two thoughts:

1. As Roy said your pots are almost certainly the cause of the tweeter problem. They can be cleaned (sometimes) but you’re better off replacing. I don’t know what you have on hand but L-pads work well. 
2. In my experience the Aida cloth has too tight and regular a weave to look right and maybe even to sound right. Michaels sells CharlesCraft Irish Linen cross stitch material in “Tea” color, 28 count, that works well. One roll is enough. 
https://www.michaels.com/product/charles-craft-28-count-irish-linen-needlework-fabric-10357706?michaelsStore=1098&inv=1

Thanks.  I installed the cloth and it looks great and sounds great, too.  I might try the one you recommend, though, as I have more speakers to do.

The pots I have are new Ohmite RHSR15R 15Ω 25W rheostats, which is what I believe are what is originally in my speakers.  

I just ordered new caps for the crossover, Dayton 20µF 250V polypropylene caps.  I feel stupid, here I am a retired electrical engineer and I don't know why I selected this value and type except that's what I gleaned reading these posts.  

I'm open to recommendations on refurbishing speakers, and on sealant for the speaker to cabinet interface.  

Thanks.

[JKent]

Glad the cloth worked out. 
Many of us have used Ohmiites. The thing is you have to make an enclosure or use a little box to keep fiberglass out of their open backs. The originals were made by Aetna-Pollak (sp?) and had closed backs. 
Those caps are right   Poly instead of lytic will last generations. 
How bout some pics?

PS. Parts Express sells nice foam gasket tape. In a pinch you could use adhesive foam window weather stripping or get a brick of duct seal in the electrical department. It’s like modeling clay and you can roll 1/4” diam worms to seal the driver to the baffle

[jviss]

[jviss]

I've just ordered a set of expensive stands, Kanto SX26W Universal Fillable Speaker Floor Stands for Bookshelf Speakers | 26".  I'm sure you know these sound much better when you get them off the floor, and out of a corner!  Good stands can make a huge difference.  

[jviss]

The walnut cabs look pretty good.  I haven't touched them.  The plan is bronze wool and Watco Danish oil.  

What do you use to reseal the speakers once you've removed them for service?

[jviss]

30 minutes ago, JKent said:

PS. Parts Express sells nice foam gasket tape. In a pinch you could use adhesive foam window weather stripping or get a brick of duct seal in the electrical department. It’s like modeling clay and you can roll 1/4” diam worms to seal the driver to the baffle

Ah, gotcha!  You know, I have some really good butyl tape, I think I'll use that.  

[jviss]

1 hour ago, JKent said:

\Many of us have used Ohmiites. The thing is you have to make an enclosure or use a little box to keep fiberglass out of their open backs. The originals were made by Aetna-Pollak (sp?) and had closed backs. 

Interesting.  I'll have to see what's inside when I open them up.  My recollection is not 100%, but I recall when I purchased these two new Ohmites that I was convinced that was what was in there originally.  I wonder if they have boxes to keep the insulation out?

I also wonder why they go bad!  Ha, ha.  Corrosion, I guess.  I also recall that these Ohmite units can be refurbished, so I'll save the old ones.

Then I'll have to do some research on adjusting them.  

[RoyC]

jviss,

-The original controls/"pots" were notorious for forming aggressive corrosion, and are almost always the reason for the silence of high frequency drivers. Ohmite controls were never used as original equipment.

-This is the link to the Parts Express gasket products: https://www.parts-express.com/speaker-components/cabinet-hardware-speaker-grill-cloth/Speaker-Gasketing-Tape-Caulk The 3/8 inch version of the gasket tape works very well, but is back-ordered at the moment. If using putty, the black rope "caulk" sold by PE is much better than "duct seal".

-I agree with Kent regarding your choice of caps. They will outlast the rest of the components of your speakers...and us.

-I also agree with Kent regarding the aida cloth. It is much too tight.

-make sure to place the cabinet stuffing in separate bags so you put the same amount back in each cabinet.

Roy

[jviss]

51 minutes ago, RoyC said:

-I also agree with Kent regarding the aida cloth. It is much too tight.

 

Oh, interesting.  Have you experience with this particular cloth?  The weave seems quite open to me.  How does one measure this?   

[jviss]

1 hour ago, RoyC said:

-I also agree with Kent regarding the aida cloth. It is much too tight.

BTW, I just ordered the cloth @JKent recommended!  I'll try that, too.  

[jviss]

1 hour ago, RoyC said:

Ohmite controls were never used as original equipment.

What was used as original equipment?

[JKent]

See my earlier post: Aetna-Pollak potentiometers 

[jviss]

1 hour ago, JKent said:

Aetna-Pollak potentiometer

Ah, thanks.  I don't suppose they're still available? 

[RoyC]

5 hours ago, jviss said:

Ah, thanks.  I don't suppose they're still available? 

Not for about 40+ years.

[jviss]

It would be interesting to see a crossover plot at various pot settings for this network.  

Also, I wonder if folks don't just punt on restoring these networks and just install a crossover board form, for example, Parts Express, which can be had for around $12.  No adjustment, but then, so what?

(I paid $8 for just the caps!)

[JKent]

I’ve seen it done. But it totally changes the speaker system so it’s no longer a classic AR. Some people throw out original woofers if they have foam rot and replace them with something off the shelf. Crossover design is a complex art/science, not “one size fits all”.  You’re doing it the right way. 

[jviss]

3 minutes ago, JKent said:

I’ve seen it done. But it totally changes the speaker system so it’s no longer a classic AR. Some people throw out original woofers if they have foam rot and replace them with something off the shelf. Crossover design is a complex art/science, not “one size fits all”.  You’re doing it the right way. 

Oh, that's a shame to throw away the original woofers.  One can get foam kits, as I'm sure you know.  

Yes, when I get mine apart I'm going to test the old cap, and characterize the crossover - I think my audio generator has a sweep function, I don't recall!  If so I can put it all on the scope and see how it behaves, adjust the pot and see how it moves the cutoff.  Then replace the cap, clean/replace the pots, and test it again.  

[RoyC]

The original crossover is easily as important as the individual drivers. It doesn't matter if an expensive off-the-shelf replacement driver is used or a generic crossover. There is much more to achieving a successful speaker "system", much less the replication of a particular design. The original design work is often overlooked.

Roy

[RoyC]

I just found some photos of some later AR design and response work...

[jviss]

Wow, awesome pics!  Thanks!

I'm sorry if I've hijacked this thread with unrelated things; but, I bought the recommended cloth, and I really need a tutorial on installing it.  I just can't get the corner folds done correctly or consistently. 

 

[jviss]

36 minutes ago, RoyC said:

The original crossover is easily as important as the individual drivers. It doesn't matter if an expensive off-the-shelf replacement driver is used or a generic crossover. There is much more to achieving a successful speaker "system", much less the replication of a particular design. The original design work is often overlooked.

Roy

O.K., I get that.  But other than the schematic, component material types, and component values, what could there be to the crossover network?  

I recognize that my Ohmite rheostats might not be original equipment, but they're probably indiscernible from a rheostat or pot they are replacing.  Perhaps not so much if replacing an L-pad, I imagine.   

[RoyC]

8 hours ago, jviss said:

O.K., I get that.  But other than the schematic, component material types, and component values, what could there be to the crossover network?  

I recognize that my Ohmite rheostats might not be original equipment, but they're probably indiscernible from a rheostat or pot they are replacing.  Perhaps not so much if replacing an L-pad, I imagine.   

Your 15 ohm Ohmites are an exact electrical match to the originals, and will work just fine. You just have to cover the open backs.

I was only responding to your statements below while agreeing with Kent's comment regarding the replacement of non-original components.

"It would be interesting to see a crossover plot at various pot settings for this network." 

This has actually been discussed a number of times over the years in the forum. In summary, the variable series and parallel resistance provided by the original 15 ohm pots influences the overall response of the speaker, as the behavior of the capacitors is affected by this resistance. The pot's  "white dot" setting on the back of the speaker is what AR considered to be the "normal" tweeter-attenuation setting for most rooms. If the pot is completely eliminated, a 15 ohm resistor placed in parallel with the tweeter will fix the tweeter's output at the former pot's maximum setting by eliminating the pot's corresponding series resistance. In other words, the pot cannot simply be removed if the goal is to achieve the original characteristics of the speaker.

Also, I wonder if folks don't just punt on restoring these networks and just install a crossover board form, for example, Parts Express, which can be had for around $12.  No adjustment, but then, so what? (I paid $8 for just the caps!)"

You purchased nice film capacitors for $8. Does Parts Express sell a crossover board with the necessary components (including the inductor) in accordance with the schematic? If so, it would likely be more difficult to install. Of course, this point is moot given the level controls (or equivalent components) are necessary for the appropriate response of the speaker system. As Kent mentioned, people have replaced/omitted a variety of original items, resulting in no longer owning the AR speaker under discussion.

Roy

 

[jviss]

Thanks, @RoyC, for the detailed response!  

I will replace the caps, repair, refurbish, or replace the pots, and look into doping the woofer surrounds.  The cabinets look very good already!  Just have to get some glue circles off the bottoms where some self-stick felt pads were applied.  Then touch them with bronze wool and Watco.  

I have three pair of these to do!  I hope they are all about the same vintage.  I'll have to check the serial numbers.

[JKent]

16 hours ago, jviss said:

Wow, awesome pics!  Thanks!

I'm sorry if I've hijacked this thread with unrelated things; but, I bought the recommended cloth, and I really need a tutorial on installing it.  I just can't get the corner folds done correctly or consistently. 

 

This tutorial is a start but not exactly relevant for linen https://www.humanspeakers.com/howto/grill-cloth.htm

For the AR grilles don’t bother with messy glue. Use 1/4” stainless staples.  Last step after the cloth is attached and pretty tight: spritz it with water from a spray bottle and dry with a hair dryer. It will get nice and taut.