Altec 291-16L diapragms (21531 and 25885)

[Tom Bellinger]

I'm new to this site. Lots of valuable info that would take lots of time to digest. Really good stuff!!

My questions:
Is the 25885 compatible with the 21531 diaphragm? I have already popped two of the 25885's (after a few days). I did see something about a conversion kit. Anybody help me with this? Also, the driver cover has a red and blue wire. How do you tell which one goes where on the diaphragm? Is there a + or - on the diaphragm?

[Tom Bellinger]

Oops - I have 291-16A drivers not 291-16L

[Jim D]

Firstly, the #2 terminal on the 291 is +, and goes to L2, which is probably not marked on the diaphragm. L1 is connect it to #1 which is -. The 25884, 299 diaphragm is quite tough, and is compatable with the 21531, 291 unit. They both hold the same power and the 21531 does not sound as good and is no longer made. You stated that you popped two diaphragms in the last few days, I wonder if something else is wrong. Please check and see you have no RF in the system. This will either fry the voice coil of turn the diaphragm into flakes. Make sure your amp is not clipping, this will also do similar damage. Is your crossover proper, is your horn loading right, is the driver sealed properly to the horn, have the drivers lost their magnetism, etc? Please post your setup, we need more info. These diaphragms, used properly are quite reliable. To burn them out under normal circumstances would require a lot of power and an unbelievable loudness level. I have used 291s converted into 299s at auto racetracks where they talk over 30 unlimited sprint cars. Jim

[Murray Keidge]

Hello,
Could you explain what you mean by "popped", this will help in finding out what happened. There are a number of ways that a diaphragm can fail. Is the winding burnt out, did the dome suspension fail, did the dome shatter, has the diaphragm "oil canned", has the voice coil former become unglued from the dome etc. It would be good to find out what happened to the 21531 diaphragms, as Jim has suggested, the fault may be elsewhere in your system and putting in more diaphragms may not fix your problem. Speak to you soon,Murray.

[Tom Bellinger]

Thanks for the quick input. Here is what I'm doing. The drivers are used in a piece of scientific weather equipment. It's called a SODAR (SOund Detection And ranging). It's similar to SONAR and RADAR. A pulse of energy at 1500 Hz and 180 or 240 milliseconds long is sent into the atmosphere about every 9 seconds. The driver then switches back to become a big microphone where electronics amplify and discern a doppler shift from the tranmitted pulse. I have 3 drivers (which are located in foam lined enclosures that I call antennaes) one pointed to the north, one east and one vertical. Using geometry, the doppler shifts determine wind directions and wind speeds up to about 2000 feet above ground at 100 foot intervals. The amp is a Crown D-150A. My specs say it puts out 100 electrical watts to the drivers. As for the new 25885 diaphragms "popping" here is what happened in both cases. About 2 days after I installed the last 25885 diaphragm it quit working or after about 20,000 pulses. Both diaphragms have notible damaged on one side and it is has lots of flakes. The Amp did not appear to be clipping after installation. This equipment is located at a remote site so I'm not there very often or long enough see everything that goes on. I just don't understand why I fry the 25885 but not the 21531 diaphragms. Does this info help? Also you suggested that the drivers have lost their magnetism. The system was purchased in early 1990. Is it time? Thanks.

[Tom Bellinger]

The driver is mounted above a 6 foot diameter dish (like a satellite dish). The driver is mounted above the dish at the focal point of the dish. Sound is tranmitted into the dish and the dish reflects the sound up into the atmosphere to give it the proper wave form. The reflect sound off the atmosphere returns along the same path. Now, since I have three of these dishes, each with a 291-16L driver, I am baffled why I have diaphragm problems. I have not changed ALL three diaphragms, only one particular driver to see if the 25885 was compatible. So far no luck. I have been using the drivers in this application since 1990. Any help?

[Todd W. White]

What KIND of band limiting are you using?

<font color="#FFFFFF" size="1">[ February 01, 2003 10:46 AM: Message edited by: Todd W. White ]</font>

[Tom Bellinger]

I'm not sure exactly what kind. The documentation says that it goes through a Xmit Band Pass Filter which is a electronic card upstream of the amp. It seems to me that if this was the problem the other 2 drivers that operate off the system would have trouble. One driver fires waits about three seconds, then another fires and waits 3 seconds, then the last driver fires and waits 3 seconds. Then everything repeats. Each driver fires about 6624 times a day. All I have done is remove one of the old 21531 diaphragms and replaced it with the 25885 leaving the other two with the old 21531's. The new diapragm is the only new component. The system has been operating with the old 21531's for over 10 years. Does this help?

[dennis gilliam]

I'm still a little foggy on just how you are loading these drivers.

You cannot work them into air right out of the throat. A 6-foot dish acting as a Newtonian reflector does not constitute a proper load

They must have something bolted to them in front, even if it is just a piece of pipe 6" long. At best, something physically resonant with the operating frequency.

DG

[Tom Bellinger]

Ok, here is what my documentation says:
the transmit pulse is sent through the band pass filter to make it into a clean sine wave. This sine wave is then sent to a power amp which outputs a pulse of 100 watts to the driver. The driver sends the pulse down to the dish through a conelike horn about 9 or so inches long. I did check with the system manufacturer and they stated they are not doing anything with the +/- phase of the pulse so the idea of which post on the diaphragm they connect to does not appear to be the problem. Here is a new wrinkle. I talked with the manufacturer and they wondered if the new 25885's have a different impedance than the 21531's. Anyway, I thought I had a 25885 here in the office but when I opened the box labeled 25885 there was a 25884 diaphragm in it. Ooops. Somebody goofed. I did ohm out the one 21531 I have here and it is 10.9. Does anybody have a value for the 25885? It is also noteworthy to mention that the new 25885 seem louder in my system.