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moldyoldy
05-31-2006, 10:58 PM
Awhile back, I bought a pair of Altec 1231 wedge monitors with messed-up XOs. They were originally installed in a church and had line xfmrs and HF attenuators added. The guy I bought them from unhooked the xfmrs and attenuators, and for some strange reason, also removed all wiring and the XO caps and put it all in a coffee can. Shouldn't be a big deal, right?

The 1231 specs say 2000Hz at 12dB/octave with 8 ohm drivers. My problem is that there are two coils and THREE caps. Assuming they used a 2nd order lowpass and a third order highpass gives the appropriate component count, but I can't make the values work out for that configuration. There are two matching 1mH coils, and 3, 8, and 21uf caps.

Can't find a schem, but I did find a picture of one on eBay that verifies I have the right components (too bad the pictures not good enough to see how it's wired!);

Help!

spkrman57
06-01-2006, 05:43 AM
The 1231 specs say 2000Hz at 12dB/octave with 8 ohm drivers. My problem is that there are two coils and THREE caps. Assuming they used a 2nd order lowpass and a third order highpass gives the appropriate component count, but I can't make the values work out for that configuration. There are two matching 1mH coils, and 3, 8, and 21uf caps.
Help!

Going from your posted values, a 3rd order crossover would probably use the 8 ufd then shunt 1 mh coil then 21 ufd cap for the top end.

Using the 1 mh coil on the bottom with shunt of 3ufd does not make any sense, but going with the values posted would be the most reasonable result.

Are you sure there are no other parts??? Or are 2 of the caps in parallel to equal a new value????

Just my 2 cents on this!

Ron

Art J.
06-01-2006, 09:35 AM
The upper coil in the picture looks different.
if it is half the value, their is a combination
that will work out. Can you test it?

moldyoldy
06-01-2006, 10:10 AM
Hi Ron,

My parts are identical in appearance to those in the eBay photo, and seller said they were the ones he removed. I considered the series/parallel cap possibility, but neither result computed either. Haven't yet measured driver impedance, but at around 2kHz, they should be real close to ratings. For a third order, the coil is nowhere close either.

Hey there Art!

My coils appear identical and both are numbered 18180, but I don't have an inductance meter to verify the values. Both DCR at 0.2.
I've got some extra 8uf caps, may just use them in a second order Butterworth at 1800Hz if I can't figure it out otherwise. With an 808-8B on a 32A plastic horn, should be good down to around 1kHz, but I'm assuming Altec used the 2kHz for better power handling in the MI app. My only info on the 1231s is from the '78 MI catalog, though I've seen so many errors in mid 70s Altec lit I wouldn't take it to the bank as gospel.

spkrman57
06-01-2006, 12:41 PM
Best to build from scratch and use 2nd order on the lo-pass and 3rd order on the hi-pass and approx 1.2khz might be good place to start at!

Ron

moldyoldy
06-01-2006, 02:54 PM
The drivers are stock, 417-8HII, and 808-8B. I retired the old Symbiotic 'frams and installed new Pascalites. Will be used as floor monitors as intended.

spkrman57
06-01-2006, 05:57 PM
To the best of my knowledge the 417 drivers were made for guitar use. If you are going for audio use they might need a sub.

Ron




The drivers are stock, 417-8HII, and 808-8B. I retired the old Symbiotic 'frams and installed new Pascalites. Will be used as floor monitors as intended.

Art J.
06-01-2006, 06:56 PM
My friend in Bella Vista, an interesting puzzle indeed.
The coils must be the same if they are all 18180. The part numbers
have nothing to do with component value unfortunately. I have some 18145's
that equal 3.6 MH.

I tried different combinations in the calculator until hair pulling time.
The closest I came to anything practical using the 3 and 21 on top was
a 3rd order Bessel at 1600 presuming the rate of rise on the tweeter
was already at 12.5 ohms at that frequency. And then rounding off to
manufactured values.I wouldn't put that combo in service without verifying
the results with a scope and oscillator. Results often show quite differently.
Specially with me calculating.:D

The safest way is as you mentioned using another 8 on top second order.
If you can notice a dip in response between 1500 and 2000 you may want
to try that overlap to see if it fills. Its tough without test equipment.

I remember a diagram in Sound Practices long time ago that used a Bessel
to overlap or gap a problem in the crossover region. Think it was in the A-7.

moldyoldy
06-01-2006, 11:59 PM
Many thanks to you both for your efforts. I ran the numbers through the calculators in every way I could think of too, but thought I ought to see if I was missing something obvious. At this point I've concluded the coils must be a different value than I was told, but having no way to measure the value I'll cut my losses and start over from scratch. Only had to give $300 for the pair, so buying some new XO parts won't hurt a bit. I have an RTA, so I can experiment till I get it right. Maybe I'll stumble on a combination that'll actually make my singing and playing sound good....but that's asking a lot from any monitor, even an Altec!

(Yeah Art, it's me, only the names have been changed to protect the innocent....);)

Live Long and Prosper!:cheers:

spkrman57
06-02-2006, 06:50 AM
How about trying out some reverse engineering and see where the input is and what takes place in the travels to the drivers.

Not too many parts and should be pretty easy to do!

Ron

sparky2
08-17-2018, 04:03 PM
OK, I know this is a very old thread, but I was given an old Altec 1231c slanted stage monitor cabinet in 2017. It had a 90 degree bend Altec 32c horn with a blown Electro-Voice driver. The 12 inch bass driver was also an Electro-Voice with a tear in the cone, and one in the surround. To my pleasant surprise, when I opened it, this driver was an EV 12L, the best 12" guitar speaker ever made, in many people's opinion. They are used exclusively by Joe Bonamassa. I also had the previous model 12M, in my Music Man guitar amp, so I excitedly removed the 12L from the monitor, and repaired the cone with a piece of coffee filter and PVC Glue. I repaired the surround with coffee filter and rubber cement. I put it in a closed cabinet, and now use it along with the open-back guitar amp, so am getting good bass out of the 12L, and good midrange out of the 12M. Awesome combination.

Now to the crossover: When I saw the large dimensions of the cabinet, I asked myself' "Why would Altec build it so huge for a 12" driver, as it could hold a 15 inch?" My thought was that they changed the design at the last minute before manufacturing, and put a 12 inch in instead of a 15 inch to reduce costs. I also looked at the old literature which says the crossover point is 2000 HZ, but my calculations with the actual crossover components indicate a 4500 Hz third order crossover on the tweeter (with a 1mh instead of a 0.21mh shunt inductor), and a second order Butterworth on the woofer. The two inductors should have been two different sizes, not the same 1mh value. The original design, using the components I found in the box, creatively rearranged, indicate a 2nd order Butterworth filter at 1800 Hz. for both sections. More mystery. I also found this thread, and another one somewhere else where people were asking the same questions. I found that Altec used a 417 twelve-inch guitar driver , and I once owned two 418 fifteen-inch drivers (both outstanding). Hmmm. When I calculated the cabinet resonance with the existing sized ports, it came out to match the resonance of the 418, not the 417. I rearranged the existing capacitors and added one 10 mF capacitor in parallel with the existing 2mf. to get a 2nd order Butterworth crossover frequency of about 1800 Hz. on the woofer (using component values from a 3rd order design, without the L3 inductor, which was not found in the box), and a 3rd order 1800 Hz. crossover on the tweeter, with the different inductor, as mentioned above. So these crossover points are not exact, and may have a bit strange frequency response around the crossover points. These are just low-demand vocal monitors, so I went cheap and put in a low-cost Internet ordered 125W bass guitar speaker with a top end of 3000 Hz, and a Pyle HF horn driver. They still sound great in a Denon-based 5.1 Surround system as a center channel. My theory is: Altec wanted to reduce costs, so substituted the 12 inch for the 15 inch. They also had probably bought all the crossover components, and wanted to utilize those parts, so rearranged them to get the approximate 4500Hz crossover, with a mismatch on the parallel tweeter inductor value. This may have been done also to reduce the bandwidth and load on the tweeters to avoid burning out diaphragm coils. They also had probably already cut the ports in the cabinets, so they shipped them as-is anyway. Just my theory, and I got a great guitar speaker and a lot of audio-nut pleasure out of the reverse-engineering. I wrote this mainly to document my work, and hope that someone can use the info.