Recapping a Stromberg-Carlson model RS-406 crossover

[marloubow]

This may be a shot in the dark, but I thought it worth a try.

These Stromberg-Carlson, model RS-406 "Acoustical Labyrinth" speakers were found roadside by my brother and given to me.

I've read about all I could find on these and other similar models.
Not much out there, perhaps, because the console version ( which these are) was just too ugly to mess with or as some seem to think, a flaw in the cabinet design.

In their present state, to my surprise, sound pretty decent.
Nice, rich mids and highs. I'm hoping that a recapping of the crossover will improve the lower end. Perhaps.

Anyway, some help with the crossover components would be appreciated.

One component maybe next to impossible, as it is a hand wrapped cap with no values listed.

The metal "box" cap gives me pause as well. Is it polarized?

The woofer is the large magnet "Slimline" 15.5 ohm.
The mid is 13.1 ohm.
The tweet is 3 ohm.
The speaker is rated at 16 ohm.

I'll do my best to supply any info requested.

Any help is appreciated.
Martin

 

[Okie-J]

Wow. Ugly ugly until you see that pearl inside.

Good luck.

 

[PastTechFan]

Can't help with that cap, but it might help if you could trace out the crossover circuit. Looks like the mystery cap, and the one in the lower right are in series. Are they (is it?) also connected to the bathtub cap? Do you have specs for the crossover frequencies?

I'm not sure if the recap will help the bass per se, but should improve the treble. These could maybe use an Lpad for the mids/highs.

Also, that bathtub cap may still be good. Metallized paper (in oil?)

Always loved the red SCs!

 

[marloubow]

PastTechFan
Will do.

 

[RxDx]

I've got a pair of those SCs. That hand-wrapped cap is hiding a coil.

Here's some further conversation: http://www.audiokarma.org/forums/showthread.php?t=653715&highlight=stromberg

I put one of the woofers in a sealed cabinet and did some listening. It's a stout unit, but it won't reach down like the AR3a.

 

[marloubow]

RxDx...
Excellent...reading link now.

 

[marloubow]

Wow...that was informative.
A lot of it over my head, but informative.
I had unwrapped the coil/cap(?) as well.
I see now that it says "foil", not "coil" or "oil" which made me think capacitor.


So what did you end up doing Ken?

Here are some better shots of the layout.

I have a pair of restored AR 3a's that I'll hang onto for a long time.

thanks
Martin

 

[RxDx]

Wow...that was informative.
A lot of it over my head, but informative.
I had unwrapped the coil/cap(?) as well.
I see now that it says "foil", not "coil" or "oil" which made me think capacitor.


So what did you end up doing Ken?

Here are some better shots of the layout.

I have a pair of restored AR 3a's that I'll hang onto for a long time.

thanks
Martin

I ended up doing nothing, for the moment. Well, mostly nothing.
I let the cabinets go, except for the speaker mounts. I haven't done
anything with the crossover. Once I determined it wouldn't match
the AR3a woofer, I basically went on to other things, though I haven't
given up on them entirely.

At some point, I'm going to get my act together enough to measure
them and the rest in this picture. The SC woofer is far left, hard to miss

 

[marloubow]

Well...okay then, I guess its off to "experiment land".
I think I'll move the xover to the exterior of the cabs, replace the known caps and experiment with the rest. Measure what I can and use the "ear test" for the rest.

At least I now have a jumping off point.

martin

 

[marloubow]

By the by...would anybody want to venture an "educated" guess at the value of the unidentified cap?

 

[Okie-J]

I will give you an "uneducated" guess.

Going off just the physical sizes of the caps I think it's going to be 2uf to maybe 4uf.

Wouldn't it have to be something like that? And with the tolerance of those vintage of caps being as high as 50% or more I would go with another 3uf maybe.

 

[Sam Cogley]

That's the big-magnet version of the Slimline woofer. Quite nice. I have a pair tucked away in the basement. Need to measure the T/S parameters sometime.

 

[marloubow]

Seeing how we're on the subject of cap values and the apparent low output
of this woofer. I have a few questions.

Mind you these are questions that I've considered before, but never asked, not wanting to look any more noob than I already am.

1) Say, just for grins, I wanted to increase the output of a woofer that already has a 12uf cap in the circuit. Does increasing (ie; 15uf) or decreasing (ie;8uf)
produce more output...or is it just not that simple?


2) It seems as though the caps used in xovers are of a significant voltage.
Both originally and as with replacements 250 & 400 volts. Do speakers really even come close to seeing that kind of voltage?
I came across this thread and made me wonder...hmmm.
Poll results are at the top of the thread.
http://www.diyaudio.com/forums/mult...much-voltage-power-do-your-speakers-need.html


And 3) Any one ever replace the older stiffer accordion surrounds with the more flexible rolled surrounds?
One of my 12" red woofers has a mouse hole chewed in it.
It's 'bout the size of a quarter, half in the cone and half in the surround.
Just musing over my options.

Martin

 

[RxDx]

The known 3uf cap measured 7.93uf on my meter, and the
one under the coil measured 7.71uf. So, for purposes of
rebuilding, it would make sense to start with a couple of 3s.

Toward the goal of increasing low end, the driver could
use some cabinet help. Like I said, I pulled it from the
cardboard factory boxes. I think something like a
transmission line or "long tailed phase inverter", which
reinforces the bass, could make a big difference.

The other part, though, is actual driver response. I'd like
to see T/S parameters.

 

[marloubow]

Thanks for the input.
I've had some good success with voight pipes and some tapered pipes designed by David Weems as well.

martin

 

[marloubow]

To continue...
First of all, to you guys who have a real handle on speakers...from xover design to cab modeling to
driver parameters...I salute you! Getting down to the particulars of this stuff makes my head hurt.

Anyway, I recapped the crossovers with the exception of the "bath tub cap", but have the caps if need be.

They overall sound is "okay", really nice in the midrange, but lacks at the top and bottom end.

So I've been pondering about what I might do about these SC 406's.
Since they are not necessarily "sought after" speakers, I am open to significant modifications.

The material they used for the console inserts is only about twice the density of acoustic ceiling tiles.
I can literally stab a screwdriver thru it.

The only reason I can come up with as to "why" is the additional weight that plywood would add to an already gear weighted console. Again, I'm just guessing here.

The insert cabinets weigh a mere 16.6lbs empty and 29.6lbs loaded. How much more do you think plywood would have added to that? Keep in mind that these are a Labyrinth design with added ply inside.
I know that they made a non console version of this model that is only 3/8 taller and 3/8 wider than the console inserts. I assume that it was made with quality plywood and probably offered more stable environment for the woofer.

I'm going to give the designers the benefit of the doubt and hope they actually designed a cab that mated well with these drivers and that the cheap insert cabs were a solution to a "weighty" problem.

So as a first step, I'm going to build a new set of cabs out of mahogany plywood and add the extra 3/8 inch H&W.

Here are the dimensions of the inserts and photo of the insert cab.
Martin

 

[RxDx]

I think the cabinets were "whatever the traffic will bear". By the time they
were made, General Dynamics was letting S-C die. It shut down completely
a couple of years later.

I'm pretty sure you can improve on these if you want. Here's a thread on some boxes
of similar philosophy I built in high school, seriously updated with several great links on design.

On my pair, I dumped the original cabinets, except for the front mounting boards.
But I'm lazy, so I stuck the S-C drivers in some old house-brand cabinets
from the 70s. I agree, nice but not exceptional sound. And they are seriously overbuilt.

 

[RxDx]

The main design challenge is finding the parameters of the woofer. I posted what
numbers there are in the thread I linked. I have slowly started assembling the pieces
to take my own measurements, but I'm not there yet.

 

[marloubow]

Thanks Ken
I can certainly postpone this project till you complete further testing on your end. One thing I don't lack is projects.
Please feel free to PM me with any findings you come up with.
Thanks
Martin

 

[RxDx]

Finally did some testing... As my ears suggested, the S-C woofer can't keep up with my
reference speaker, the AR3a. I know these curves don't really match the official versions, but
it's good for comparison at any rate.