Kazoo buzz in 802 8d

[Scottf]

I recently purchased some a7 500's with 802 8d drivers. One sounds great but the other makes piano particularly, and other instruments also sound like they're playing through a kazoo. Both drivers are hardly broken in and appear to have what I've read is the older style diaphragm. The diaphragms both appear perfect without creases or indentations. I didn't have the nerve to remove or fiddle with them. Can anyone say what is causing this obnoxious buzzing? What, if anything, can I do to adjust or tweak this noise away? Does it sound like I need a professional fix? Who might be recommended? I'd really appreciate any suggestions you Altec veterans might offer.

[joyspring]

Hi Scott,

My guess regarding your noisy 802 would be that the voice coil is out-of-round, the diaphragm assembly is off-center (unlikely since it's securely indexed by fixed locating pins), the voice coil former has separated from the diaphragm, the voice coil has separated from the voice coil former, the voice coil is burnt/shorted, the lead(s) from the diaphragm rim terminals to the diaphragm are broken/intermittent, or that there is foreign material in the voice coil gap.

Though I will not take responsibility for any damage that may be incurred, I would recommend the following checks; unfortunately all would require that the diaphragm is removed from the driver...

Carefully remove each diaphragm. Check the DC resistance of each. Look at the voice coil and former - check that either is not burnt, burnished or loose, and look particularly the junction between the former and diaphragm. Adhesives do harden and age over time.

Also check the leads from the terminals on the diaphragm rim to the diaphragm itself - these are very delicate, doped ribbon.

Use a piece of masking tape folded over such that the adhesive side faces out. Carefully insert the tape into the voice coil gap to its full depth and clean, using fresh tape until no debris appears on the tape.

You may reuse the diaphragms if you were careful in their removal and handling, however new diaphragms are relatively inexpensive. I'd recommend a new set and having the Alnico magnets recharged at the same time.

Members: please correct or make addenda as necessary ;-)

Good luck,
BobR

[RonSSS]

Just take the diaphragms out and inspect them. Despite their looks, they aren't all that fragile
IF YOU ARE CAREFULL. Just work slowly, and hold the screwdriver at the handle and near the tip. The magnet is pretty strong and will pull the screwdriver away from you if you let it.
Mark the diaphragm and driver with a felt pen so yo uget the thing back in the same position.
The possible problems BobR mentioned are right on.
If you need to replace one, i'd opt to just go ahead and replace both.
Ron

[Steve Burger]

If repairs needed I will take this opportunity to plug Bill Hanuschack at Great Plains Audio. GPA is the only place I would send an Altec product to get fixed! His contact info is available on this website.

[jnorv]

I seem to be having a similar problem with one of my 902-8B's. No kazoo but high distortion. Took it apart and the voice coil seems to be in good shape ( eyeballed the roundness, no burns, no sign of rubbing, 7.1 DC ohms). How do I take the plastic cover off of the diaphram assembly? Can I put these in to my 802 d's to give a cross check. Tried the masking tape and recovered no gunk.

Jim N

[TonyBlues]

What about a problem with the high pass filter in the crossover? Try swapping the drivers between the two speakers. It seems like a shorted capacitor might produce similar symptoms.

[Mike Edwards]

If high gauss causes high stress when working around diaphragms with screwdrivers might I recommend the purchase of non-ferrous drivers, either brass or beryllium copper. I'm going to get some for myself and reduce the stress factor.

-Mike Edwards