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Rebuilding Beveridge Circuit Board Stators
Rebuilding Beveridge Circuit Board Stators
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Old 2nd March 2019, 08:54 PM   #1
DrJJ is offline DrJJ
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Default Rebuilding Beveridge Circuit Board Stators

Has anyone here rebuilt Beveridge circuit board electrodes/stators? I'd like to know what I would be getting into before I start.

I've always been fascinated with Beveridge ESLs -- in pristine shape, to me they sound compelling. Technically they are a clever series of trade-offs. I think they can be made to sound better with a different set of trade-offs. I'd like to try.

So I bought a pair of Model 5s -- their lowest end model -- that were available locally for a very reasonable price. I'm still assessing what I have and what needs to be fixed. To those not familiar with the Beveridge line, the Model Five is a five foot tall, 18" diameter cylinder that contains a 3'x1' electrostatic element and a 10" woofer. The ESL front wave passes through an acoustic lens (waveguide); the rear wave is damped. The woofer is downward facing, and occupies the lower 2' of the enclosure. The crossover is at about 150 Hz.

This pair dates from 1984, and has had some attention paid to it over the years. There is non-crumbing grille foam, and the woofers work, so they either have been replaced or had the surrounds repaired.

The electrode/stators show quite some loss of aluminum from the diaphragm, mostly down the center line. Interestingly, the capacitance values are not that far off from what was written on the panel edges. They do work, but are very old, and probably should be refurbished before I do much else.

I've scoured the web, and find little information on the Beveridge circuit board electrodes other than that they are notoriously unreliable. I assume they generally follow Beveridge's patent, so use copper on the inside of a PCB (it is backed off from the openings) and insulated with about 250 um of Nylon. The diaphragm claimed in the patent is Mylar (or other PET equivalent) that is coated on both sides with Aluminum. No diaphragm thickness is mentioned, but I assume that it is the standard ca. 4 um thickness. The patent further claims the edges are sealed with a nonconducting silicone elastomer, but if that is used in mine, it long ago has lost its compliance.

Of course the diaphragm should to replaced. I've yet to identify a source that sells double-sided Al-coated PET of a suitable thickness. I assume further that I'll have to examine the integrity of the Nylon that insulates the copper and fix as is necessary. Other than that it should be pretty standard as far as ESL construction goes.

I think. I'm assuming a LOT, and before I dive in I'd like to see if anyone else has done a similar refurb. Is there anything I'm missing/overlooking? Does anyone with experience wish to talk about it?

Last edited by DrJJ; 2nd March 2019 at 09:03 PM. Reason: Correct title spelling.
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Old 3rd March 2019, 02:10 AM   #2
stokessd is offline stokessd  United States
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I'd love to watch.

Unfortunately I don't have any diaphragm material for you.

sheldon
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Old 3rd March 2019, 06:36 AM   #3
DrJJ is offline DrJJ
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Sheldon,

I was hoping that you would ride shotgun (as it were) as I go through this. You may recall (or not!) we corresponded about 15 years ago when I was working on my QUADs. I would appreciate your advice, and I thought you might enjoy the project.

I'll post some more measurements tomorrow -- we went to the Symphony tonight, and I had no time to finish up before we left.

Roger Modjeski lists appropriate Mylar for the Model 2/3 on his web site. I've contacted him, but have not heard back yet. I'd bet it is the same material.
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Old 3rd March 2019, 05:03 PM   #4
stokessd is offline stokessd  United States
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I'll be glad to help out however I can. I have zero experience with those, but maybe some of my quad knowledge will bleed over.

It does seem that metalized diaphragms are a bad idea.

Sheldon
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Old 3rd March 2019, 06:22 PM   #5
DrJJ is offline DrJJ
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Thanks, Sheldon! I think the rebuilding will be very, very similar to the old Quads, with some minor differences such as connecting both sides of the diaphragm and making sure there is no current through the edge adhesive.

Beveridge operates the panels as a quasi-reverse electrostatic in (sort of) constant voltage mode. That has to give higher distortion than a traditional constant-charge operation -- it is one of the trade-offs Bev made that I don't agree with. They sound rather good when they work, so maybe this isn't that important.

FWIW, the stator-stator distance is 2 mm, and there does seem to be Al on both diaphragm sides. I couldn't measure the film thickness, as my micrometer is just too big for the holes. Capacitance (front/rear) on one panel measured 2.83/2.30 nF, whereas the panel had written on it 2.87/3.06.
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Old 4th March 2019, 08:32 PM   #6
AVWERK is offline AVWERK  United States
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If you get a chance, pictures would be nice of this design

I did hear the big 6 foot tall 2-SW back in the day (early 80,s ) and was thoughly blown away by the imaging and the way it illuminated the room like no other and at the same time was way too expensive back then.

The 5 has a different lens design than the 2 series ( 5- 130 degees and 2- was 180 degree arc per speaker)
The 2 used directly driven tube coupling eliminating the typical transformer like the later designs
I can invision a Sanders style panel with the lens design for best reliability overal.
I understand the Beveridge also had excellent impulse response in its “reverse “esl design.
Either Beveridge design is just a marvelous concept although terrible WAF issues, definitely for man cave territorys

Look forward to your progress

Regards
David
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Old 4th March 2019, 08:44 PM   #7
DrJJ is offline DrJJ
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For pictures, I refer you here:

Beveridge Model 5 - Restore | Audiokarma Home Audio Stereo Discussion Forums

It describes the refurbishment of a Model 5, and has better pictures in it than I am able to do at the moment. The AudioKarma Model 5 is an early one, which used the classical epoxy electrodes. There are posts that contain pictures of later ones that have the circuit board stators and connection box. The external looks largely are the same.

For my purposes, there is no information on the newer stators, but the rest of the information is helpful.
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Old 6th March 2019, 08:53 PM   #8
DaveG is offline DaveG  United States
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Hi,
I have some of the Roger Modjeski mylar and it appears to be bad.

I never used it since my epoxy panels still function. I purchased it 8 years ago to have on hand and learn what was inside the panels and only cut off a small sample at the time.

I measure it in the .0009" range or .9 mil. Much thicker than I expected. It is 18.5 inches wide. It is aluminized on 2 sides.

I don't actually know the thickness in the actual transducers having never opened one.
There is mention in the patents of both .25 and .5 mil.

Your thread prompted me to pull out the mylar and remeasure and inspect it and I find it has possibly oxidized and lost much of its aluminum in the center portion and is randomly faded and scuffed to almost no coating with randomly shaped splotchy full density areas.

I would say it is unuseable.

Dave
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Old 6th March 2019, 09:11 PM   #9
DrJJ is offline DrJJ
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DaveG,

Thanks for the reply!

0.25 mil is about 6 um, which sounds about right. 0.9 mil would seem to be way too thick -- is your micrometer accurate in that range? And it is a shame that your Modjeski Mylar is bad. I've received no reply either to email or telephone calls from his company.

Thanks also for confirming that it is Al-coated on both sides.
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Old 6th March 2019, 11:26 PM   #10
DaveG is offline DaveG  United States
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Hi again,
yes, I believe it is actually .0009 inches = .9 mil. = .0228mm = 22.8 micron.

I didn't believe it myself. The thickest I recall reading about are Acoustats at .00065 inches.

10 layers measures at .0093

I used 4 to measure-
Cheap General micrometer.
Brown and Sharpe micrometer.
Micrometer with mechanical 1 3/4 dial.
Mitutoyo calipers with no read out.

All agree.
I cleaned and measured 1 to 10 layers in different combinations. Divided it down.
Still everything matches to .0009 inches. If I was told it was actually .0008 inches I could believe that.

But maybe it is not actually what is in the transducers.

Dave
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