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Old 10th May 2007, 11:09 AM   #1
volkerw is offline volkerw  Germany
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Default Repairing ESL Panels / Corona problem

Hello,
some years ago a friend gave me a pair of trashed Audio Exklusiv P3A stats for free. They are fullrange ESL with integrated direct drive tube amps. The amps are similiar to the acoustat circuit. Both amps were dead due to different failures, but i managed to repair them and they are running stable now.

Each speaker contains 3 identical els driver elements, stacked as a line array.
I inspected one speaker and found all 3 elements to be shot with burned holes in the foils, one foil was missing completely. The stators are build from 1 mm copper clad epoxy perfboard bonded to to egg-crate plastic louvres for stability. Air gap clearence is about 2 mm. The stators are painted with gray paint similar to quad57. Polarisation voltage is around 3KV. I built a stretcher jig and put in new foils with grahite as resistiv coating.

On test, the speakers are basically running, but in the dark i can see corona discharge beetween some holes in the stators and the coated side of the foils. On louder passages, they tend to arc to foils. I tried to fix the corona problem with local application of nail laquer, does not help (and there are too much hole to be covered). I sprayed the inside of one stator with acrylic layqer (Kontakt Plastik70), not much better. Spraying the outside is problematic due to the fixed louvres.

Has anybody an idea how i can fix this problem? Any input welcomed!
Greets,
Volker
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Old 10th May 2007, 11:53 AM   #2
jmateus is offline jmateus  United States
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Try corona dope stuff. It works real well when covering stators.
Available in your country? Don't know....
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JLH 2005, Cyrus and so on...
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Old 10th May 2007, 11:05 PM   #3
Few is offline Few  United States
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Hi Volker,
Can you look at the offending holes through a magnifier and see if there are any sharp edges in the conductor? The electric field strength will be highest around such points so they'll be the first places to exhibit ionization. Carefully eliminating any sharp points before trying to fix the problem with insulating coatings might yield improved success.

Few
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Old 1st June 2007, 10:20 AM   #4
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Hi Volker,

are you shure, that the tension of the membrane is high enough?
To check that you could try to meassure the resonance frequency of the elements.

Just for curiosity, could you place some pictures of the elements in this forum?
Did you manage to backengineer the schematics of the direct drive amp?

Best regards

Dieter
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Old 1st June 2007, 08:29 PM   #5
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I'm not sure, but couldn't a too low resistance coating be the source of these problems?
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Old 13th June 2007, 08:23 AM   #6
volkerw is offline volkerw  Germany
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Hello to all,

many thanks for your response!
Sorry for my late answer, had been to busy at work.
To zelter: yes, i studied the circuit of the HV drive amp. It is VERY similar to the Accustat X amplifier, which is already published on audiocircuit.com. The input amplifier/correction filter appears to be similar too. Sorry, i have no pictures of the elements yet (i have no digital camera). Contact me if you need further info.
To arend-jan: surface resistance is around 100M/cm.
To few: You are right, the holes are quite sharp, i will try to round them with a hand drill. Unfortunately some of the problemetic holes are buried under the egg crate.
To jmateus: Corona dope, yes i know the stuff, but up to now i found no shop who sells it. Must have been more common in the times ehen tv set got repaired and no
disposed.

Regards to all,
Volker
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Old 13th June 2007, 08:38 AM   #7
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rs-components sells Anticorona lacquer,400ml aerosol 48kV/mm (RS stocknummer: 569-313)

But if the Kontact Plastic did not work then I don't expect that this one will. The holes may be sharp but I suppose the speaker once worked correctly when it left the factory. So the most likely candidates are diaphragm tension (but this does not explain the corona when not playing music) or the coating.

Did you apply the coating on the exact same area where the original was?
Can you measure the resistance of the original coating?
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Old 13th June 2007, 11:51 AM   #8
Calvin is offline Calvin  Germany
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Hi,

the problem might be tracking. Maybe -Iīm definitively not sure about this- the used insulation laquer isnīt immune to tracking, than the arcing lead to carbon tracks. These would be īon topī of the insulation and open towards the membrane. Spraying on some relatively thin layer of insulation wonīt stop the speaker from arcing. As far as I know the Pütz features quite high voltage values. So my guess is, that only opening the panels, cleaning them thoroughly and reinsulating them would help.

jauu
Calvin
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Old 20th June 2007, 01:16 PM   #9
volkerw is offline volkerw  Germany
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Hello Calvin,

you are right.
I opened one panel and at least at 3 of the offending holes there is carbon buildup at the epoxy board. I will grind this out. Meanwhile i have build a variable HV supply consisting of a voltage multiplier, isolation transformer and a variac giving me up to 6KV for testing. I have layed one stator on a grounded metal plate and connected the outer conducturs to the HV supply. While winding the voltage up, i can see corona starting and turning to arc where the stator insulation is bad. I have measured the normal polarisation voltage, it is 4.6KV. I wonder if this is not too much for a total air gap width of 3mm including the holes. I allways thought that voltage gradient should not exceed 1KV/mm.
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Old 21st June 2007, 06:56 AM   #10
Calvin is offline Calvin  Germany
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Hi,

Iīm no shure what You mean with "airgap including holes"?
Normally You give the single dimensions....airgap, insulation thickness and material, stator thickness and material, statorīs conducting position, etc.

I assume the Pütz uses a PCB stator with an layer of insulation laquer and the copper side facing towards the membrane?

1kV/mm polarisation is a safe value that would work even with uninsulated stors. Most sources qoute a flashover treshold of 2kV/mm for air under normal conditions, but in praxis low level ionisation would happen and ozone will be generated. From app. 1.5kV/mm the ionization becomes audible as a slight faint hiss. So most manufacturers play safe and set the polarisation to a value around 1kV/mm.
If the insulation is of considerable thickness -and it has to be to be effective and safe- and features a slight conductivity (<10^15Ohms) than the insulation layer and the air function like a voltage divider. If You now measure the voltage at the end of Your cascade, You donīt measure the actual voltage over the airgap! You have to subtract the voltage loss within the insulation layer. So itīs not quite easy to determine the actual voltage gradient in the airgap. The loss depends on the electrical parameters of the insulation material and its thickness. If Your Multimeter reads 4.6kV with an 3mm airgap and hissing/arcing begins, You can assume that a couple of hundred volts are dropped over the insulation layer and that You wonīt have a voltage gradient of more than 1.5kV/mm in the airgap.
Btw. A similar effect takes place with AC-voltaged, where the insulation forms a capacitor, connected in series with the airgap-capacitor. The higher the epsilon-value and the thinner the layer, the lower the losses are. 10% of signal loss are typical! Another reason, why just a few materials work well and many DIY Solutions donīt

jauu
Calvin
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