Martin Logan Theater center-channel panel rebuild

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Jazzman53

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A while back I posted an offer to walk someone thru a panel rebuild, for the purpose of creating a panel rebuild guide.

Someone took me up on this offer and I've just completed rebuilding a ML Theater panel. The process is unique to the Theater panel but can be adapted to other panels.

I was unable to post the rebuild here (exceeded the 1k word maximum), but I've posted it on my blog page.

Click this link to view the write-up:
http://jazzman-esl-page.blogspot.com/2011/11/compensating-diplole-phase-cancellation.html
 
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A while back I posted an offer to walk someone thru a panel rebuild, for the purpose of creating a panel rebuild guide.

Someone took me up on this offer and I've just completed rebuilding a ML Theater panel. The process is unique to the Theater panel but can be adapted to other panels.

I was unable to post the rebuild here (exceeded the 1k word maximum), but I've posted it on my blog page.

Click this link to view the write-up: https://www.blogger.com/blog/post/edit/1858103255701027780/6793096460258332283
Thanks, Jazz. However, the link whether clicked on or copied/pasted does NOT go to a document but to "..../reading"?!
 
Thanks, Jazz. However, the link whether clicked on or copied/pasted does NOT go to a document but to "..../reading"?!

Is the link not working for anyone else?

It's working fine for me... maybe it was saving an edit. Please try it again.
 
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Not to "look a gift horse in the mouth" but the pdf when opened has YELLOW font color on WHITE background...very tough to read. Have not figure out a way to change font color.
 
Not to "look a gift horse in the mouth" but the pdf when opened has YELLOW font color on WHITE background...very tough to read. Have not figure out a way to change font color.

The link leads to my blog page, which shows as yellow font on a black background when I open it. I suppose it could have been in EDIT mode when you opened it, or our computers translate it differently.

Is anyone else seeing a WHITE background on the writeup page?
 
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Is anyone else seeing a WHITE background on the writeup page?
I see it as Yellow text on black background, but I'm running Chrome in forced dark mode (laptop with OLED screen), so even this site (MLO) is white text on black.

Even the Edge browser shows a black background, and so does Mac Safari. So I think it's good.
 
I see it as Yellow text on black background, but I'm running Chrome in forced dark mode (laptop with OLED screen), so even this site (MLO) is white text on black.

Even the Edge browser shows a black background, and so does Mac Safari. So I think it's good.
Yes, it's black on the latest Edge browser for me too. Looks great.
 
To clarify... I see it as YELLOW on BLACK at the link but saving to pdf for offline printing/reading makes it YELLOW on WHITE. Printing in GRAYSCALE makes it barely readable light GRAY on WHITE. BLACK on WHITE would be great.
 
Update on the rebuild:

I noted in my write-up that the diaphragm coating did not extend over the spars, as it was visually evident that the area over the spars, and about 3/16" on either side of the spars, had been masked-off before the coating was applied.

I figured ML had a reason for this, so I did the same.

At first I pondered that perhaps the adhesive tape doesn't adhere well to the coating so ML omitting it over the spars to facilitate a better bond between the spars and diaphragm, but I've since had an epiphany about another possible reason.

See the attached sketch.

Unlike a flat-panel ESL which can be fully symmetric in its construction, a curved panel can't be symmetric because it can't constrain the diaphragm equidistant from both stators.

Specifically; the diaphragm's tension parallel to the curve pulls it toward the rear stator. This is visually evident by the diaphragm taking a "saddle" shape between the spars.

The reason for the spacer asymmetry is easy to see but that doesn't explain why the diaphragm coating was omitted over and near the spars. But I think I know why... charge migration.

Ideally we want the ESL operating in "constant charge mode". That is; all those electrons dancing on the diaphragm remaining stationary, but those electrons are attracted to areas on the diaphragm that are closest to a stator. Hence; those areas collect greater charge, and become more strongly driven than the areas with less charge.

Areas with greater charge and closer proximity to a stator are more prone to arcing. Typically, diaphragm areas between the spars would be less anchored by a spar and freer to move closer to a stator. This is certainly the case with a flat panel ESL but an asymmetric curved panel with saddle-shaped distortions in the diaphragm between spars presents a different condition.

I'm thinking the diaphragm's conductive coating was omitted over and near the spars because of their closer proximity to the front stator, which would otherwise strongly attract charge to those areas-- resulting in not only unequal charge across the diaphragm but a higher probability or arcing to the front stator.

This condition would be compounded by a high conductance/low resistance coating, and the coating on that Theater panel diaphragm was about 3X more conductive than the Licron Crystal that I put on the new diaphragm.

Hey, my 2 cents.

stator sketch.jpeg
 
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Update on the rebuild:

I noted in my write-up that the diaphragm coating did not extend over the spars, as it was visually evident that the area over the spars, and about 3/16" on either side of the spars, had been masked-off before the coating was applied.

I figured ML had a reason for this, so I did the same.

At first I pondered that perhaps the adhesive tape doesn't adhere well to the coating so ML omitting it over the spars to facilitate a better bond between the spars and diaphragm, but I've since had an epiphany about another possible reason.

See the attached sketch.

Unlike a flat-panel ESL which can be fully symmetric in its construction, a curved panel can't be symmetric because it can't constrain the diaphragm equidistant from both stators.

Specifically; the diaphragm's tension parallel to the curve pulls it toward the rear stator. This is visually evident by the diaphragm taking a "saddle" shape between the spars.

The reason for the spacer asymmetry is easy to see but that doesn't explain why the diaphragm coating was omitted over and near the spars. But I think I know why... charge migration.

Ideally we want the ESL operating in "constant charge mode". That is; all those electrons dancing on the diaphragm remaining stationary, but those electrons are attracted to areas on the diaphragm that are closest to a stator. Hence; those areas collect greater charge, and become more strongly driven than the areas with less charge.

Areas with greater charge and closer proximity to a stator are more prone to arcing. Typically, diaphragm areas between the spars would be less anchored by a spar and freer to move closer to a stator. This is certainly the case with a flat panel ESL but an asymmetric curved panel with saddle-shaped distortions in the diaphragm between spars presents a different condition.

I'm thinking the diaphragm's conductive coating was omitted over and near the spars because of their closer proximity to the front stator, which would otherwise strongly attract charge to those areas-- resulting in not only unequal charge across the diaphragm but a higher probability or arcing to the front stator.

This condition would be compounded by a high conductance/low resistance coating, and the coating on that Theater panel diaphragm was about 3X more conductive than the Licron Crystal that I put on the new diaphragm.

Hey, my 2 cents.

View attachment 25005
Hum. Will having a coating that's only 1/3 as conductive as the original affect things in a negative way?
Your hypothesis about why it's not coated by the spars makes a lot of sense. I'd be interested in seeing what happens if you coat the whole thing.

Your diagram shows the spar being thicker in the rear than the front. Any idea why its not equidistant on both sides, front and back?
 
Hum. Will having a coating that's only 1/3 as conductive as the original affect things in a negative way?
Your hypothesis about why it's not coated by the spars makes a lot of sense. I'd be interested in seeing what happens if you coat the whole thing.

Your diagram shows the spar being thicker in the rear than the front. Any idea why its not equidistant on both sides, front and back?

My point was that the original coating is too conductive for the panel to operate in the more ideal constant-charge mode. Moreover, my measurements showed a large variation in surface conductance across the diaphragm (ranging from zero conductance to 3X more than needed).

I don't know what the coating was but graphite has that characteristic so I figured it could be some kind of graphite slurry (graphite mixed into a clear paint carrier). It works fine... but there's a downside to having more conductance than needed, which I explained in my previous post.

Imagine the diaphragm being a leaky jug, its surface area being the jug's volume, and the conductive coating being a hose filling it with water. The hose must flow enough water to keep the leaky jug filled, but once the jug is full, a bigger hose (more conductance) can't put more water into an already-full jug.

Peter Walker (inventor of the Quad 57 speaker, and mathematician who first modeled ESLs in his infamous "Walker equation") determined that the optimal resistance for a diaphragm coating is about 1MOhm. "Optimal" defined as conductive enough to carry charge (fill the jug), yet resistive enough to restrain the charge's mobility across the surface. This allows the panel to operate in constant-charge mode (i.e. all areas on the diaphragm holding roughly equal charge and giving equal output).

Licron Crystal has somewhat less than optimal resistance (about 700kOhms), and I measured some areas on the Theater diaphragm which had 3X more conductance than the Licron sample I compared it to. So, a ballpark figure would put the Theater panel coating at about 200kOhms, average (way below optimal).

But the clincher is that the Theater panel played louder with the less conductive Licron Crystal coating, even though I added 4 mils (the added tape) to the diaphragm-to-stator gap (more gap = less drive force and lowered output). So; we see that more conductance does not produce greater output than merely adequate conductance.

The spars are thicker on the rear stator for two reasons:
- The curved panel forces the diaphragm to take a "saddle" shape between spars, which puts the diaphragm closer to the rear stator and farther from the front stator, in the center area.
- The curve is a hoop restraining the forward movement of the diaphragm. Moving the diaphragm closer to the front stator (thicker rear spars and/or thinner front spars) increases the forward drive force to offset the hoop constraint.

The thicker rear spars compensate both issues.
 
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My point was that the original coating is too conductive for the panel to operate in the more ideal constant-charge mode. Moreover, my measurements showed a large variation in surface conductance across the diaphragm (ranging from zero conductance to 3X more than needed).

I don't know what the coating was but graphite has that characteristic so I figured it could be some kind of graphite slurry (graphite mixed into a clear paint carrier). It works fine... but there's a downside to having more conductance than needed, which I explained in my previous post.

Imagine the diaphragm being a leaky jug, its surface area being the jug's volume, and the conductive coating being a hose filling it with water. The hose must flow enough water to keep the leaky jug filled, but once the jug is full, a bigger hose (more conductance) can't put more water into an already-full jug.

Peter Walker (the inventor of the Quad 57 speaker, and the mathematician who first modeled ESLs in his infamous "Walker equation") determined that the optimal resistance for a diaphragm coating is about 1MOhm.

Licron Crystal has somewhat less than optimal resistance (about 700kOhms), and I measured some areas on the Theater diaphragm which had 3X more conductance than the Licron sample I compared it to. So, a ballpark figure would put the Theater panel coating at about 200kOhms, average (way below optimal).

But the clincher is that the Theater panel played louder with the less conductive Licron Crystal coating, even though I added 4 mils (the added tape) to the diaphragm-to-stator gap (more gap = less drive force and lowered output). So; we see that more conductance does not produce greater output than merely adequate conductance.

The spars are thicker on the rear stator for two reasons:
- The curved panel forces the diaphragm to take a "saddle" shape between spars, which puts the diaphragm closer to the rear stator and farther from the front stator, in the center area.
- The curved diaphragm's forward movement is restrained by the hoop force (a hoop is easy to compress but difficult to expand). Therefore, the diaphragm needs to be closer to the front stator to increase the force and offset the hoop constraint.

The thicker rear spars compensate both issues.
Wow, great explanation. Thanks for taking the time. I used to teach high school Chemistry, Gen. Science, and Biology. I think you would have made a good teacher!
 
I wanted to express a huge thank you to Charlie for rebuilding my Theater panel. When I initially spoke to him by phone, I was immediately put at ease with his friendliness and his generous nature. I was blown away at his knowledge of electrostatic speakers. One look at his webpage and I knew I found a real perfectionist. He used to build his own speakers and his web page shows his impressive work! I sent my panel to him and when he received it, he did some basic tests to confirm that I didn't have another issue such as a power supply issue or something else. My panel sounded flat to me, loss of highs, and a very evident dead area of the panel as well. I think my Denon processor tried its best to compensate through its room correction, but it wasn't enough. This is a 20-year-old panel, and the loss of performance was to be expected. Charlie stated that it may be sometime the following week when he would start the work, and I was fine with that. I picked up a cheap ML Motion 8 center channel speaker to use in the meantime. Well, less than 24 hours later, Charlie had already dived into the work. He has an infectious enthusiasm for the hobby, and selflessly enjoys sharing his knowledge. He kept me updated every step of the way.

Now, I am not a technical person, but after having just received my panel back this past week, I am in heaven. I can now hear every breath, every strum in a song, top to bottom, and the sound is evenly dispersed throughout the panel, unlike before. It plays loud and clear, and I can understand dialogue like never before. I also hear a cohesiveness between the panel, the mid-bass speakers, and the triple array tweeters. I can imagine that many people aren't even aware of what they are missing until they hear a refurbished panel.

You may think I'm giving Charlie a plug for business, but he doesn't want any! He made it clear that was not interested in doing this again. I appreciate what he did for me, but more importantly the knowledge he's shared on this forum. Hopefully it helps others to do this rebuild, especially since replacement panels are no longer offered by Martin Logan. Good luck with yours!
 
I appreciate what he did for me, but more importantly the knowledge he's shared on this forum. Hopefully it helps others to do this rebuild, especially since replacement panels are no longer offered by Martin Logan. Good luck with yours!
Thanks so much for sharing the story from your perspective and glad it worked out well for you.

The lessons learned and shared are welcome, and everyone should also check out Charlie's page, as it's chock full of great information: https://jazzman-esl-page.blogspot.com/
 
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