The Workbench

Re: Penta questions...

GTS — Wed, 29 Feb 2012 10:19:19 GMT

Latest...

Tested every coil for impedance. No open circuits - good news!

Interestingly the coils all come in at 2.9-3.0 Ohms for the Penta 1's, 3.1-3.2 for the Penta 2's. Possibly an age thing.

Ordering foam rings now.

Re: Penta questions...

Leslie — Tue, 28 Feb 2012 17:39:25 GMT

Now it's going to be very interesting "GTS". Will follow your refoaming job with much attention, although you're using lots of words which I don't understand, but that's okay, Google will help

Re: Penta questions...

GTS — Tue, 28 Feb 2012 12:46:09 GMT

Leslie:

Could you colo(u)r up your story a bit by adding some pictures. You know the frase "a picture is worth a thousand words". I'm specially interested in the shimming part of your Penta midrange, cutting of the dustcap, shimming, replace a new one etc.

(Nice pickup on the English spelling!)

Pics? Sure.

Here's what they look like with the original surround removed off my Penta 2's. Note the nice, even bead.

This is one of the better drivers off my Penta 1's after a not-so-good refoam. Note the amount of glue application. You can just see where the original glue line was.

And this is one of the worse drivers off my Penta 1's. Note the poor aplication - some places have glue, some not. The glue, or the interaction between the glue used, the paper and the foam has neatly torn out some of the driver surface (which on some refoam jobs is normal) inboard of the cone perimeter (which is not).

It's taken a lot of work to get the 1's this far. The stresses imparted by not-quite-correct gluing has given rise to some solid misalignment - yes, there are axes in which you can push the cones inwards and there's no mechanical contact, but compared to the Penta 2 set, it's not pretty. And to be frank, given that I take pride in these speakers (as we all should) I'm wondering what I can do to bring them back costmetically also (some pro shops actually paint cones as part of reconditioning...)

Not sure what I'm going to do with them now.

First off I'll to borrow a multimeter tonight to see if why of the coils are open (and thus completely unsalvageable). If we get to the stage where there's too much to invest here, I might just part the speakers out.

Re: Penta questions...

Jeff — Tue, 28 Feb 2012 09:52:32 GMT

GTS:

All valid points Jeff - a great response. I agree that Fs as a lower frequency limit isn't a primary concern, though as a physical property it affects a great deal.

The notion of valid end termination gives further ammunition to the previous question though - loading a cone up with glue at the periphery will have a more significant effect than the same mass added about the center, though we can't completely remove glue at the join with each refoam or a few decades from now we'll have a structural mess and/or little left to attach to.

So what's the smart move? The most I can surmise presently is that there'd be a good tolerance (10-20%) in Fs, a good portion of it in termination and cone mass, primarily if everything's consistent we're onto a better solution than none at all...

To remove all glue traces or to build on top of what's there, cleaned as best possible?

I would imagine a shift of Fs might not be as troublesome as a shift in Qts, depending on how small the enclosure is.

You make a good point re the stiffness of the glue. There would be a difference between a surround terminated with a soft, pliable rubber cement type of glue vs. the PVA hard/stiff glue many recone kits come with, regardless of what that may do to the ability to recone.

Hopefully, if they are made correctly with the right materials, new foam shouldn't suffer the foam rot of the originals. I have a friend who works in the speaker industry, and for a long time driver makers knew that there was a fungus that attacked foam but they were not very forthcoming about it, in many cases when a solution had been discovered certain large speaker companies leaned on the driver makers not to inform competitors of it. From what he described the whole situation sounded kind of sordid. That problem should be solved now, but maybe not depending on who made the replacement surrounds. I can imagine some low end offshore makers might not use the newest formulations, particularly if they are patented. I've never owned a pair of speakers with foam surrounds before the CX50s, so hadn't experienced the rot although I knew it was an issue.

Re: Penta questions...

Step1 — Tue, 28 Feb 2012 03:49:35 GMT

We have recently discuused batteries and stressed that they should not be used!

Re: Penta questions...

Leslie — Tue, 28 Feb 2012 01:18:24 GMT

Could you colo(u)r up your story a bit by adding some pictures. You know the frase "a picture is worth a thousand words". I'm specially interested in the shimming part of your Penta midrange, cutting of the dustcap, shimming, replace a new one etc.

Re: Penta questions...

GTS — Mon, 27 Feb 2012 23:06:42 GMT

Finally cleaned up 16 midrange drivers. Cleaned the original foams off the Penta 2's and finished cleaning a poor refoam off the Penta 1's. Some observations...

Big difference between between cleanup of the original job and the poorly-done refoamed drivers - the latter took three times as long and, quite frankly, are a bit worse for wear. I guess if you're confident doing it, don't - especially if keeping the same speakers into the next refoam.

Choice of glues... critical. The orginal stuff left a thin film of itself. The 'refoam kit' glue is clearly some sort of PVA, and the application went beyond the original glue line (as mentioned here http://www.decware.com/newsite/refoam.htm). Big mistakes both. The glue clearly binds differently to a paper cone and going further into the cone's a big no-no, get the right sized surrounds. The only way to get this mess off is to tear off a surface layer of the cone. Not ideal. The two local 'professionals' suggest a waterproof woodworking PVA - 'a PVA that doesn't dry brittle', but neither business has actually had to refoam a set of speakers that they'd originally refoamed. I'm no closer to knowing what I shouldbe using for my own refoam.

Another observation - the speakers that have had a poor refoam have suffered a bit of misalignment stress in the few months use since the their refoam. Sufficiently so that they'll need more care in alignment - these definitely will be shimmed.

It really is a task that requires care and thought.

A battery seems a bit crude for a 'second rate' approach - wouldn't it be a little more elegant to turn a PC speaker out into a crude signal generator or similar?

If anyone's got detailed thoughts on a refoam adhesive, let me know.

Re: Penta questions...

GTS — Mon, 27 Feb 2012 13:42:34 GMT

All valid points Jeff - a great response. I agree that Fs as a lower frequency limit isn't a primary concern, though as a physical property it affects a great deal.

The notion of valid end termination gives further ammunition to the previous question though - loading a cone up with glue at the periphery will have a more significant effect than the same mass added about the center, though we can't completely remove glue at the join with each refoam or a few decades from now we'll have a structural mess and/or little left to attach to.

So what's the smart move? The most I can surmise presently is that there'd be a good tolerance (10-20%) in Fs, a good portion of it in termination and cone mass, primarily if everything's consistent we're onto a better solution than none at all...

To remove all glue traces or to build on top of what's there, cleaned as best possible?

Re: Penta questions...

Jeff — Mon, 27 Feb 2012 11:06:15 GMT

GTS:

Leslie:

Foam. Have been using rubber before but it's quite difficult to handle!

Would imagine this changes the T/S parameters also, starting with Fs - probably not ideal.

True, but if for a midrange that is operated fairly far above resonance, maybe not as much of an issue? But the upper response of the driver, how smooth, how extended, what rolloff, etc. is definitely a function of the edge termination. Sound propagates radially out from the voice coil, think of a long string or spring as in the old physics classes demostrating standing waves. The manner of termination of said string or spring, into a solid object, into a different value spring, into a damped system, will radically alter the standing wave. Same happens for speakers, when originally designing a driver the engineer will choose the surround and cone thickness, etc. to achieve a certain response (or just wing it and see what happens). The speaker system designer will look at the behavior of the driver at low and high freq extremes when designing a crossover. If the edge termination via the surround changes the upper freq response the crossover will most likely be misaligned.

Of course, if the driver is operated over a range well short of the low and high freq limits any change will be less critical. Not knowing the xover points on the Pentas I can't begin to hazard a guess. But I'd generally try and redo the surrounds with as close to OEM as possible.

Re: Penta questions...

GTS — Mon, 27 Feb 2012 04:43:15 GMT

Still scraping and dissolving glue on the frames of the Penta 1 midrange (the 'professional' refoam)... what a mess. The baskets are glass-filled nylon and end up looking a little worse for wear... until a surface color's applied, then we're getting somewhere - cosmetically original and a nice, flat surface to glue onto.

Quick question on just how much to remove on the cone side:

Getting rid of the foam leaves the old glue residue on the cone. Removing that much tears a thin surface layer of the cone, leaving it on adds where the cone's least rigid and neither option makes doing it 'next time' any easier...

...so who's doing what? Leaving a bit of old glue and gluing on top of that? Scraping that off and thinning out the cone for better attachment?

Re: Penta questions...

GTS — Sun, 26 Feb 2012 17:48:13 GMT

Leslie:

Foam. Have been using rubber before but it's quite difficult to handle!

Would imagine this changes the T/S parameters also, starting with Fs - probably not ideal.

Re: Penta questions...

Beobuddy — Sun, 26 Feb 2012 15:58:16 GMT

Time is a important factor, take your time with refoaming. But that's something you already know

I've done a set of CX's 2 weeks ago. And the result was positive despite that I'm not a real supporter of refoaming.

The tolerance is close to zero when it comes to the cone.

Re: Penta questions...

Leslie — Sun, 26 Feb 2012 15:44:53 GMT

Foam. Have been using rubber before but it's quite difficult to handle!

Have been talking with Martin about this refoaming/shimming thing and my or better our conclusion is, if you don't have the skills or you have two left hands or feet let a pro do the job!

Re: Penta questions...

Beobuddy — Sun, 26 Feb 2012 15:23:33 GMT

@ Leslie,

Did you use rubber instead of foamrings?

It's a know fact that rubber lasts longer, but sounds different.

Re: Penta questions...

Leslie — Sun, 26 Feb 2012 12:53:39 GMT

GTS:

Those having refoamed their own - did you shim? I'm headed that way...

No I did not and never will at leat not for these type of drivers. I will consider using the batterie method for the next refoaming job, but cutting out the dustcaps, no way.

Looks yours has been done by a boatworker with two left feet!

Just finished another 8 and will connect them later this week! Will let you hear about the results....