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mink70
07-13-2017, 06:22 PM
Hi everyone—

I just acquired a set of Altec 846A Valencias and am experiencing a channel imbalance. Opening them up and swapping the back panels revealed that the problem is one of the N-800F crossovers—it's shelving down the tweeter response to where it's barely audible (though the L-pad appears to work). The crossover can is riveted shut so I can't get in. Bill at Great Plains Audio says they only deal with Altec drivers and don't repair or resell crossovers. And there doesn't seem to be another N-800F for sale right now in North Amerca.

Can anyone suggest what to do—I'd love to get these playing.

Thanks so much,
Alex

Earl K
07-13-2017, 07:00 PM
You need to drill out the rivets to get to the insides .

Unfortunately, the components are encased in old tar and wax ( requiring more work to get to the LC parts ) .

All-in-all, not worth the effort ( IMO ).

I would just buildup a set of N800F crossovers using better components .

http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=24113&stc=1&d=1175580320

- If that's all Greek to you, look to hire someone to do it for you .

:)

http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=23788&stc=1&d=1174514671
http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=23789&stc=1&d=1174514704
http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=23790&stc=1&d=1174514728


Both chokes are very close to 3 mh (3.07 and 3.18) with resistance roughly 1 Ohm and Q=16. The smaller capacitor measured 4.44 microfarads, and the larger value measured 12.77 microfarads. I suppose this verifys that the 4.0 and 10.5 microfarads marked on the ELPAC box were correct starting values. The 12.77 mic cap is obviously the one across the woofer in the LF section, while the 4.44 mic cap feeds the signal to the HF section.

The Lpad appears to be a 16 Ohm version (both sections measure 20 Ohms with my Micronta multimeter) and is connected as follows. The junction of the 4.44 mic and the top of the choke are connected to the wipers. The bottom of the choke is connected to ground (common). The bottom of one Lpad section is connected through a 20 Ohm resistor to ground (common), while the top of the other section is connected to the HF output.

I cannot fathom why the HF section uses only a 4 mic capacitor with the same value inductor. I will try to draw a schematic so that I can post it, but don't have that capability at home, and couldn't get on the cad system at the company. It seems to me that the upper crossover frequency with 4 mic's and 3 mh would want to be roughly 1.5 KHz. Can someone explain why Altec would do this?

Jack Gifford :blink:

http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=23824&stc=1&d=1174595804

mink70
07-14-2017, 02:55 PM
Thank you, Earl K. What a PITA!

gdmoore28
07-15-2017, 05:10 PM
Mink, welcome to the board. I second what Earl proposed - have somebody solder up a new set using the schematics on line. There are some really clear examples. Being in NYC, you should have access to thousands of techs and hobbiests who could do this with their eyes shut. I'm an idiot, and still managed to build my own crossovers.

While you are at it, have the tech add the Altec30923 equalizer/attenuator network to the N801-F network. For a few pennies more, you will be dumbstuck by how much it improves the Valencia.

It's not a big deal, so don't fret it. Find a hobbiest who will do the job for you for a song, buy the needed components that the tech will provide you after he sees the N801/30923 schematics, and wait to be impressed.

GeeDeeEmm

cogito
07-15-2017, 06:22 PM
:)

http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=23788&stc=1&d=1174514671
http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=23789&stc=1&d=1174514704
http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=23790&stc=1&d=1174514728





what types of caps are there in the wax?
what is the purpose of encasing caps in wax?

Earl K
07-16-2017, 05:39 AM
what types of caps are there in the wax?
what is the purpose of encasing caps in wax?

The original caps in these networks were made by Elcap ( they may have been electrolytic according to Google pics / but I don't know or have any examples ).

Altec & JBL typically potted their network components back in the 50's & 60's .

I think ( speculate ) that this was because of real engineering lessons learned from WWII & the on-going Age of Rocketry .

:)

gdmoore28
07-16-2017, 11:26 AM
Altec & JBL typically potted their network components back in the 50's & 60's .

I think ( speculate ) that this was because of real engineering lessons learned from WWII & the on-going Age of Rocketry .

:)

Good point! I never thought of that, but with the economy on a war footing during WWII, you just know that practically everything learned during the war trickled down into post-war technology.

Word has it that John Glenn took along a pair of Valencias and the crossovers held up just fine! ;)

GeeDeeEmm