Need help resuscitating old speakers

Gary Mettus

Gary Mettus

Member
I picked up this pair of Magnavox S-8766 speakers yesterday at Goodwill for $15. The 15" alnico woofers are stamped with the CTS code and are dated 1972 so I know I need to at least replace the caps.

After inheriting my Dad's 1968 Westinghouse console with the exact same woofers and similar tweeters, blowing the internal amp, hooking up another $15 Goodwill purchase of a 1990 Optimus receiver with 100wpc, I've become hooked on this very simple speaker design of 15" woofers and horn tweeters. Both systems use sealed cabinets and really need a lot of power (way more than they were originally paired with) to come alive but when they do they sound really good and can fill a huge volume great room.

The Westinghouse simply uses a 10mf cap as an approximately 1,500Hz high pass filter. The Magnavox has a bit more pieces including an adjustable treble dial. I'd like to go simple with just a cap
but I don't know the range of the tweeter nor do I understand what all the existing pieces are doing or the resulting effective filter they produce.

Suggestions, advice, please.

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Wow... those are archaic.. yet beautiful!

And look at the weight.. almost 50 pounds :eek:

Most of the parts can be had at Parts Express.. and the like online. The woofer appears to be intact which is amazing given the materials at that time were not as durable.

Great find!!
 
I'm a firm believer in the automotive world's axiom; there's no replacement for displacement! Ipso facto; diameter, watts, and weight :rflmao:

The other set of CTS woofers I have are just as pristine and I know there are a lot of others out there still working really hard in guitar amps.
 
Good enough is the enemy of great.

That looks like a first-order crossover: inductor for the woofer, capacitor for the tweeter. A rebuild is therefore simple and inexpensive and should greatly improve the sound quality.

I suggest:
(a) Replacing the inductor with a low-gauge (i.e. thicker wire) air-core inductor. Cored inductors commonly use either a laminated steel core or ferrite core to increase the effective inductance while reducing the amount of copper wire, thereby saving manufacturing cost. The problem is the magnetic core tends to saturate and has very non-linear properties, so the inductor's properties vary by frequency and power. Because ferrite is a magnetic material it has hysteresis, i.e. it retains magnetic state as the frequency fluctuates. Think of this as averaging signals, much like the dielectric absorption in a capacitor. Plus the core adds Barkhausen noise. Many chances to win the distortion lottery. Any of the standard 16 gauge air-core inductors from Parts Express will be suitable.

(b) Replacing the worn-out non-polar electrolytic (NPE) capacitor with a polypropylene film capacitor, such as the Dayton house brand of Part's Express. Electrolytics add distortion, and film improves the sound. The PP film Dayton house brand of Parts Express are great capacitors; the only slightly better dielectric is PTFE (tradename Teflon) at much higher cost. I further suggest bypassing the capacitor by placing in parallel small capacitors of value 0.1 uf and 0.01 uF. (Capacitors in parallel sum.) This removes virtually all of the ESR and ESL, already negligible with PP, and improves the performance at the higher frequencies reproduced by the tweeter. Multiple smaller capacitors work better than a single larger value because of how the charge distributes across the plates, and how the dielectric (insulator between the plates) functions.

(c) Add damping material, such as modelling clay, to the tweeter horn to reduce its ringing.

(d) Clean and lubricate the potentiometer.
​
Just because the manufacturer had price-point constraints does not mean you must continue to use poor quality components in the crossover, which is most definitely in the signal path. The improvement will be clearly audible.
 
Not much to add, but if the pleated rubberized cloth surrounds are showing signs of deterioration, a fresh coating of rubber cement brushed on the pleats after a careful cleaning can stop any cracking.
 
Watthour said:
Not much to add, but if the pleated rubberized cloth surrounds are showing signs of deterioration, a fresh coating of rubber cement brushed on the pleats after a careful cleaning can stop any cracking.
Click to expand...

NEVER use rubber cement on a surround as that will ruin the speaker's characteristics.

The only acceptable surround treatment is butylene rubber dissolved in toluene, same as for any coated surround.

The problem with cements, caulks, and similar treatments is that the applied material gradually hardens, stiffening the surround. Rubber cement, for example, is neoprene, a hard, stiff rubber, which will stiffen the surround, raising Fs.

Many threads on AK have debunked alternate materials, and RoyC at ClassicSpeakerPages did the experiments and monitored over time. The truth is out there.
 
More information debunking alternate surround treatments which I wrote up nearly five years ago:
Again, RoyC deserves the entirety of the credit as he did the heavy lifting an the meticulous experimentation. I merely summarized it and linked to it, and explained the formulations for some promoted products which are no different. White Glue Cult is like a zombie: it never gives up trying to destroy speakers until one double-taps the head and burns the entire body.
 
Gary Mettus said:
How about this for a quick clean solution? I know at $60 for two it is way more than the parts but... https://www.parts-express.com/dayton-audio-xo2w-2k-2-way-speaker-crossover-2000-hz--260-140
Click to expand...

That's really not a good solution for this application. Nor for any application, but that's a separate issue.

(1) The inductor has a laminated core, i.e. inexensive and distorting. Read what I wrote.

(2) This is a second-order filter, not the first-order you currently have. You will need to carefully construct the midrange-tweeter bandpass at the proper frequencies to remove the lower-frequencies which, at best, cause distortion and, at worst, may damage the tweeter. I suggest reading about second-order crossovers specifically for horns. The Electro-Voice T-35 would be a commonly described unit. I suggest researching the horn midrange-tweeter you have to find its exact characteristics.

Better to purchase good quality components, as I above described, and assemble the crossover. A second-order crossover for the tweeter is easily constructed as it only requires the addition of an inductor to remove lower frequencies.
 
Retrovert said:
NEVER use rubber cement on a surround as that will ruin the speaker's characteristics.

The only acceptable surround treatment is butylene rubber dissolved in toluene, same as for any coated surround.

The problem with cements, caulks, and similar treatments is that the applied material gradually hardens, stiffening the surround. Rubber cement, for example, is neoprene, a hard, stiff rubber, which will stiffen the surround, raising Fs.

Many threads on AK have debunked alternate materials, and RoyC at ClassicSpeakerPages did the experiments and monitored over time. The truth is out there.
Click to expand...
So...duct tape is right out?
 
Here is how you may calculate the corner frequency for any capacitor:
Given:
f is the crossover frequency (Hz)
R is the drivers' nominal impedance (Ω)
C is the capacitance (Farads)

Then:
// NB: if C is given in µF, multiply value by 10^-6 or 0.000001, that's one-millionth, or five zeros and a 1
f = 1 / (2π × R × C)​

Plugging in 8 Ω and 10 µF yields f = 1,989 Hz ≈ 2,000 Hz, a point which a horn tweeter should be able to reproduce without harm.

But you must verify that the nominal impedance of the driver is 8 Ω.

Using a second-order filter (capacitor plus inductor) offers 12 dB / Octave rolloff which protects the midrange-tweeter from the more dangerous low frequencies, but a first-order crossover at that point would be fine for most tweeters.
 
RobRoy said:
So...duct tape is right out?
Click to expand...

That depends upon whether or not the duct tape comes pre-loaded with phlogiston.

I myself prefer the aluminum sealant tape for HVAC ducts because it reflects all of the stray electrons beamed by the CIA installation parked at the curb. Plus, ooooh, shiny! Any extra may be used to tape together the aluminum foil helmet, as it is import to avoid galvanic corrosion in the helmet.

Not sayin', just sayin'.
 
Retrovert said:
That depends upon whether or not the duct tape comes pre-loaded with phlogiston.

I myself prefer the aluminum sealant tape for HVAC ducts because it reflects all of the stray electrons beamed by the CIA installation parked at the curb. Plus, ooooh, shiny! Any extra may be used to tape together the aluminum foil helmet, as it is import to avoid galvanic corrosion in the helmet.

Not sayin', just sayin'.
Click to expand...
I use that in all my hats. Depending on the look I'm going for, sometimes I apply to the inside of the hat and other times on the outside. Both are equally effective at reducing the voices to an inaudible whisper.
 
Those old 15 inch woofers and horns still have some life in 'em. Just like B&W speakers, feeding those Magnavox units with a powerful amp is how you get the most out out of them.
I'd hate to have to replace surrounds like those.

Good luck with the restoration.
 
I LOVE aluminium tape, I can tell you stories of when I flipped a house :rflmao:

Sadly, one of the tweeters has no reading now. I could swear it worked when I first brought them home and tried them out. I think I'm going expand the project and replace both tweeters as I essentially have no money in it yet. Replacement horns look very reasonably priced. Apparently the horn will be ABS plastic so would this also benefit from some dampening material?
 
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