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View Full Version : Crossover for 9812-A loudspeaker system



Ken Norris
03-21-2006, 01:48 PM
Howdy,

Last message didn't post (don't know why).

After a year we finally got our church's no.1 loudspeakersafely on th ground. It was missing HF reproduction. Isolation testing shows the HF driver works, but the HF output from the crossover does not. Piece of a large resister was cracked and fell off when I pulled the rear panel out. Darkening of the circuit board around the base of the resister and blistering on the reverse side, plus loss of signal and continuity on the HF output wires, indicates the crossover (well, the HF side of the crossover) is fried.

The thing is riveted to the rear connection panel, which also has the speaker connections and the hipass attenuation switch.

So far I have been unable to locate the part online. If some one could please point me in the right direction, I'd be appreciative.

Thank you,
Ken N.

Earl K
03-21-2006, 03:40 PM
CLICK HERE ! (http://www.lansingheritage.org/html/altec/specs/pro-speakers/9812-8a.htm) to view the product cut sheet for the 9812-A speaker system. It's a start . You'll see that the crossover point is listed as 500 hz . That compression driver is actually a rebadged driver that was made by Electro Voice .

- The bad news is that Altec is out of business ( in case you didn't know ) complicating a proper repair .

- The good news is that these days , the parts for passive crossovers are easily obtained online from a place like Parts Express. A new coil, cap(s) and resistor(s) for a 500 hz point will not be that expensive. Of course you need to come up with the schematic for the network ( or find someone with it ) so that you can determine the correct value of the replacement parts .

- I would also ask your question(s) over at Todds' Unofficial Altec Forum . (http://www.hostboard.com/cgi-bin/ultimatebb.cgi?ubb=forum&f=3729) Someone there may actually have the schematic for this network .

- Failing to get the real values for parts you may need to fake it. A 500 hz , 2 pole Butterworth hipass with about 10 db of padding , with a bit of CD horn compensation (EQ ) is pretty straightforward.

EDIT :
Ken, it would be worth yourwhile to snap some pics of this crossover from a few angles. The whole idea is to give other people a few look sees , closeup of the components in the crossover The advantage of many pairs of eyes is that someone may be able to identify the values of the capacitors . The coil ( inductor ) may still be alright. Disconnect is from the circuit and then measure it for continuity and take note of its DCR ( DC impedance ) .

:)

Ken Norris
03-23-2006, 04:35 PM
Hi Earl,

Thanks for the info. I'm not really sure about methods for posting on this forum. It's asking for a title (title of what? -- it'a already a thread, right? I don't want to start a new thread).

Anyway, yes, I'm aware of what happened to Altec, and knew the driver is actually an EV part (though it has an Altec part number).

For sure the crossover blew a 15 watt 5 ohm resister (the board is slightly blisterd on the back side, and black under the resister, which, like other parts, are glued to the board with silicone glue). I got to the mainland yesterday and got a 10 watt 4.7 ohm to replace it (closest they had), but it's still not working. I think the foil on the board is OK (pretty big sections).

I think the 16 MFD capacitor right next to it may be fried as well. I ran a clip lead across the capacitor, bypassing it, and got the driver working, but I don't have a clue what that does to the crossover. I'm not that great on testing this circuit, but the coils look fine.

Bill at Great Plains Audio is looking for parts in his shop and will email me tonight. I'm thinking it may change the sound a bit, but wondering if a Voice-Of-The-Theater crossover could be adapted (it won't fit in the opening, that's for sure).

Thanks again,
Ken N.