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robertbartsch
07-11-2008, 02:18 PM
Is there any way to determine whether a working driver needs to be, or should be rebuilt? Is the measure of impedence an indicator?

Do certain Alnico or other magnets lose strength with passage of time; do voice coils deteriorate with time?

Do rebuilders typically re-charge magnets or is this an additional service that should be discussed prior to sending a driver to a rebuilder?

grumpy
07-11-2008, 02:52 PM
Is there any way to determine whether a working driver needs to be, or should be rebuilt? Is the measure of impedence an indicator?

Test for T/S changes, preferably under power (special test equipment).
DC resistance test? If open, shorted, unstable, or drastically different than expected,
then yes... it's an indicator... but only a gross one. AC impedance plot might tell you
more, but again, it's not normal to have the required equipment. I've had a good
experience with WooferTester2 http://www.woofertester.com/ . Also, looking for
physical changes or damage (surround or spider shape changes, foam surround
condition, tears, splits, loose or corroded connectors, ... :blah:)


Do certain Alnico or other magnets lose strength with passage of time; do voice coils deteriorate with time?

They can, and they can ... both require some level of abuse (physical and/or
electrical). Some drivers are apparently more prone to this than others... a
search of this forum might turn up some interesting conversations on the subject.


Do rebuilders typically re-charge magnets or is this an additional service that should be discussed prior to sending a driver to a rebuilder?

If Alnico, it's worth asking for it to be checked, in advance. I'm not sure there is a
"typical" case.

Robh3606
07-11-2008, 02:59 PM
Great question!

I have some very old drivers at home some from the 50's and they seem just fine. No obvious damage or degredation you can see. Now measurements??? Not sure what to look for very low FS?? Very broad imp peak??

It would help if we knew what drivers. Alnico structures can be demagnitized with drive signal however underhung coils are immune from this as an example.

Number one question is are recone kits available??

If no well then you are limited.

Rob:)

grumpy
07-11-2008, 03:30 PM
Great question! :yes:


I have some very old drivers at home some from the 50's and they seem just fine. No obvious damage or degredation you can see. Now measurements??? Not sure what to look for very low FS?? Very broad imp peak??yeah... without a baseline, specs, or other knowledge/experience with a particular
driver or type, things get a bit more interesting...

At that point, I'd be thinking about why I had that driver before
having any 'work' done. :)

Gary L
07-11-2008, 06:53 PM
Great question Robert and I have had it bouncing around inside my head for a long time now.

I buy and use mostly old Vintage Altec drivers and specifically 802-8G HFs and 416-8B woofers.

I can tell you for sure that a meter reading of the VC is no indication of proper function. All it really tells us is there is a complete circuit through it.

Magnet strength is one very few of us have any ability to check!

I have purchased numerous 802s and 416s that are claimed to be working, looking and sounding perfect but in the cabinets they just did not sound right. They appeared fine and had meter readings that were within the acceptable norms.

A quick trip to GPA and Bill will tell you straight up if they need recharging or more extensive work.

I got to a point that every driver I purchased just automatically went to GPA for a 30 year physical.

We have to consider that many of these Vintage drivers have seen some ruff duty either from being over run with high power and heat or by being used on amps that had a hard time producing good clean power.

It does not take much in the line of electrical spikes to damage the delicate VCs of any speaker.

Spiders and accordion and foam surrounds do tend to loose some elasticity over the many years and all we need to do is look at our own faces to see that things begin to droop after 30 years.

There are numerous things that can deteriorate over time and sagging cones, weak magnets, overheated VCs or diaphragms are just the beginning but will render a nice driver quite useless or even worse when it shows it's ugly head at high output.

I buy the drivers from unknown life situations and just send them in for a complete check up before I even put them into newly built or refurbed cabinets. Almost like when you buy a used car and the seller tells you he just changed the oil--------10,000 miles ago!:( I think I actually drained good clean oil once but the rest was mostly TAR!

Gary

robertbartsch
07-11-2008, 07:36 PM
Yeh, OK. thanks.

I recently assembled two JBL monitors - 4312 from components and from what I can remember from 35 years ago, these do not sound the same.

I guess, from what I remember, there should be more dynamic responce from the system - e.g., lower SPLs from the low passages and higher SPLs from the higher passages.

Granted they do not have HF compression drivers but
over all, the sound is OK, but i wonder weather the magnets or voice coils have lost their orignal power.

When I compare these monitors to some current JBL 15s and HF compression drivers that I have recenty purchased they are kinda-patheitic. Granted, they do not have HF compression drivers, but I suppose they should be more comparitve, IMO.

Gary L
07-11-2008, 08:33 PM
The first place I would look is in the XOs! 35 year old caps are always suspect and just one bad cap can change the entire output.

I can't even say for sure that any of the drivers I ever sent to GPA needed recharging of the alnicos. They just do it as a matter of corse when your driver gets there.

Most of the woofers I buy have accordion surrounds but I think they too suffer when left in one position for 20-30 years. Foam is a given and has to be replaced.

I can't honestly think of anything that after 30 years does not require at least some maintanance!

I do have a Sansui G 8000 receiver that was made in 1978 and has never skipped a beat--------YET and has never been touched inside!

Gary

Robh3606
07-12-2008, 09:44 AM
What specific drivers do you have??

Rob:)

robertbartsch
07-12-2008, 10:17 AM
...don;t know the model # of the drivers since I 'm not at home but they are suposed to be the correct drivers for a 4312....

...the factory x-overs were tuned up and work flawlessly in the factory cabinets....

robertbartsch
07-12-2008, 10:19 AM
...so all the capacitors in the x-overs should be swapped out? Is this something that an electronic repair shop can do?

Gary L
07-12-2008, 11:52 AM
I don't know what that means, "factory XOs were tuned up and work flawlesly in the cabinets".

Others here are allot more familiar with JBL XOs. I usually find Dayton caps at PE and do the simple switch myself in Altec XOs.

If your XOs really were checked and all is within tolerances then I would leave them be and look elsewhere for the cause.

Gary

robertbartsch
07-12-2008, 12:10 PM
OK - thanks

...previous owner claimed the X-overs controls were sticky so he had them tuned up by a pro... not sure if that is accurate but the controls are now working without any flaws (e.g, no cut -out when adjusting etc.)....

Gary L
07-12-2008, 12:57 PM
Yes Robert, chances are the tech just sprayed them with a cleaner/lubricant to get them touching at all positions.

I know there is a way to test caps to see if they are in spec. I don't know how or what meter you would need.

On my altec XOs switching caps is very simple, two wires and remove one and replace it with a new one.

Some here will advise you to go for the fancy and expensive designer caps while others are perfectly satisfied with ones like the Daytons from PE.
That will be your decision and you can either get the values needed by looking at what is in the XOs or possibly there is a schematic of your XO somewhere that will list the cap values.

The speaker is a pretty common one so I am sure someone here has the numbers and can guide you better then me.

I had a set of model 19s and one speaker sounded lousy. A cap was visibly leaking or cooroding on one end. I think I spent around $25, each XO had 4 caps and after I changed them all was great.

Here is a 19 XO so you can see how simple it is.

http://img.photobucket.com/albums/v722/Gamalot/XOs/Altec_Model19_Xovers_03.jpg

Gary