I recently prepared a very long post on my efforts to restore a pair of Quad ESL-63 electrostatic loudspeakers made in 1983. That post was made almost daily as I did the work, and includes a lot of things I tried that didn't work. This post leaves out the stuff that didn't work and summarizes the stuff that did. I have omitted a lot of detail here, so check the other post if anything isn't clear.
Main parts of the rebuild:
1) remove tapes and their adhesive residues that were used in the assembly and replace them
2) Reglue stators
3) Replace diaphragms
4) Replace dust covers
5) Repair Electronics
It's best to do this work in a room that's free of dust, dirt, and pet hair. Keep a vacuum cleaner handy and stop to vacuum things off at each step in the process. It doesn't take much dirt to cause the drivers to hiss and whine.
I used lint free microfiber cloths whenever I wiped anything with either a solvent or water. Paper towels might leave lint behind that could turn into a source of noise.
I wore 5x and 2.5x loupes when doing detailed work- being able to see what you're doing is really helpful. Use plenty of light in the room, too!
The Tapes
For some reason these speakers had a bunch of different foam and other tapes used in their construction. After 40 years, some of the adhesive became brittle and the tape wasn't sticking well, and foam tapes were rotting away, usually leaving sticky residue behind.
Removing the old tape typically involved peeling it off, then scraping as much of the adhesive residue off as possible. Finally, solvents were used to remove the last of the residue. I used Goo Gone, an orange oil based solvent, and Goof Off, a very volatile solvent.
The purpose of some of the tapes used was questionable, but I replaced almost all of them anyway.
Note: some folks may get the idea that they can use double stick tape to mount the diaphragms on the driver grids. No, you can't. It doesn't work. The problem is that the tension on the film will pull the adhesive from the tape inward, toward the center of the speaker, more importantly, the tension on the diaphragm will be reduced when it does. That will lower resonance and may cause the diaphragm to stick to one of the stators whenever power is switched on.
Here are all the locations of the tapes that need to be replaced. A, C, and D are all inside the dust covers, so it's absolutely critical to clean the old stuff out and replace it with long lasting materials. Double sided tapes are used to mount the steel grids on the speakers and to mount the dust cover film. All others have adhesive on one side only.
Adhesive tapes for 2 speakers
location
total length
width
thickness
adhesive
type
A
driver mounting brackets
48"
3/8"
1/4"
single
foam
B
vertical frame supports
248"
3/8"
1/4"
single
foam
C
driver mounting brackets
120"
1/2-3/4"
3/4"
single
foam
D
top and bottom dust cover mounts
184"
3/8"
1/4"
single
foam
E
inside bottom panel
8"
3/8"
1/4"
single
foam
F
top and bottom of frame, holds grids
200"
5/16"
1/8"
double side
foam
G
top and bottom of frame, over grids
234"
1/2"
single
gaffer
H
vertical edges of grids
256"
1"
single
gaffer
J
dust cover frames
448"
3/4"
1/8"
double side
foam/other
Note: F tape seems redundant- the G gaffer tape wraps around the driver enclosure, preventing the grids from moving. Maybe I'll use some of the same tape used at B and D to prevent the grids from rattling...
I used EPDM rubber weather seal foam tape that should hopefully last longer than the original polyurethane foam tapes that the factory used. For the dust covers I used 3M VHB double stick tape. I didn't use double stick foam tape at the top and bottom edges of the metal grids, and I didn't apply gaffers tape around the top and bottom of the driver enclosure. I want it to be easy to go back in and fix things if it becomes necessary. If they're still working right in 6 months, maybe I'll put the tape around the top and bottom edges of the driver enclosures.
The Stators
Each speaker has four drivers. Each driver is made of two plastic grids that clamp together, one at the front side and one at the back side of the speaker. The front side grid has the diaphragm glued to it and the back side grid has conductive metal strips that contact the diaphragm. Both grids have stators glued to them. The back side stator has some fine nylon (?) cloth glued to it to dampen the resonance of the diaphragm.
The stators are thin, perforated PCB material that has copper on one side, covered with some sort of clear coating. The diaphragm sits about 2.5 mm away from the stators. When the stator glue bonds fail, the stators tend to curl inward toward the diaphragm, resulting in contact that makes noise even when the speaker isn't playing music, due to the 5.25 kV bias.
Once the stators start to let go, they need to be removed completely and reglued. If the stator glue bonds fail on the front side grid, the diaphragm has to be removed to reglue the stator, and then the diaphragm has to be replaced.
Usually, some of the stator glue bonds will have failed, so you break the stator free of the grid by pushing on the stator where the bonds are already broken and keep pushing until all the bonds are broken. You'll need to unsolder the electrical connection clips to remove the stator completely, and then clean flux off the solder pads using IPA.
Here's how you find broken stator glue bonds:
Original solder on one of the center drivers. Do you think they used enough solder? The black stuff on the edge is where one of the metal clips that hold the two sides of the driver together was positioned. All those clips had a little rust on them.
Aluminum clamped to the stator for desoldering. You have to remove the solder so the stator will sit down flat in the grid. The aluminum acts as a heatsink to protect the cloth that's glued to the bottom of the stator (back side stators, only).
Stator with flux from desoldering wick. That needs to be cleaned up.
This is what it should look like after you clean the flux off by wiping with a towel soaked in IPA.
Scraping the stator glue off one of the grids:
I also used the scraper to gently remove any excess glue from the stator. Don't get too aggressive- you don't want to cut into the plastic coating that covers the copper on the stator.
Once the grid and stator have been scraped and are ready to glue, vacuum them off to get rid of the glue dust. You don't want any foreign objects to get into the drivers or they will make noise.
This is the glue I used for the stators. One bottle is plenty to reglue all the stators for two speakers.
I applied the glue using a 5 ml syringe with an 18 gauge gluing needle. The syringe can be reused, but you'll need a new needle for each stator you glue, so buy a bunch of them. They're cheap.
Applying glue to the grid. I use clear polyurethane Gorilla Glue and wipe the grid with a cloth dampened with distilled water just prior to applying the glue. Then I wipe the stator with water before setting it down on the grid/glue. The glue needs the water to set properly, so don't skip this step. Glue only goes on the vertical ribs of the grid. It took me about 10 minutes to apply glue to all the ribs on one grid.
This is the stator clamping tool I made. The 1" thick foam strips are mounted on a piece of plywood about the same size as the driver. When gluing a stator, a couple kg of weight is placed on the plywood to ensure the entire stator is pressed against the grid.
This is how the clamping tool sits on the stator and grid. When you are actually gluing a stator, put a layer of saran wrap down before you put the clamping tool on the grid/stator. You don't want excess glue to stick to the foam.
I dampened and applied glue to the vertical ribs on the grid, placed the stator, covered with saran wrap, then placed the clamping tool and some weight on it to hold the stator flat against the grid.
The Diaphragms
The diaphragms are very thin (3 um) polyester film (Mylar is one of many brand names) stretched tight on a rigid frame, and they have resonances like a drum. I tested the factory diaphragms before I pulled them off their grids and found a consistent 86 Hz primary resonance. The test was performed by tilting the driver up vertically, then "thumping" the diaphragm close to its center, using a steel ball hanging from a thread. I placed a UMIK-1 measurement mic within 1 cm of the diaphragm and ran Room Equalization Wizard (REW) software on a laptop. I used the Real Time Analyzer (RTA) in REW to plot the spectrum of the sound captured by the mic. I used the same test method to tune the replacement diaphragms.
One of the drivers opened up. The piece on top is the back side driver grid, the bottom is the front side grid with the diaphragm attached. The gray stuff is the high resistivity coating on the diaphragm. No, it's not dirty, it's supposed to look like that. The metal tape at the edges of the top part makes contact with the coating on the diaphragm and allows charge to transfer to the diaphragm. You can see a thin cloth covering the stator. That's there to dampen diaphragm resonance. Don't remove it, and try not to damage it. Be careful when unsoldering the electrical connections to the stators.
Diaphragm coming loose in a driver. The problem is that the glue the factory used doesn't bond to the polyester film. This diaphragm has to be replaced, even if the driver plays OK.
This video shows how little effort it takes to peel the factory diaphragm off the grid. The glue they used didn't bond to the film.
I used a pneumatic stretcher to put tension on film and tuned the resonance to match the factory diaphragm resonance, before gluing it to a driver grid. The stretcher is a table with a hole in the center that matches the size of the driver grid, and has a wood lip. I put double stick tape on the lip on the underside of the table, neoprene foam on the top side of the table, and a bicycle tire tube is stretched over the lip of the table. The table is built so that it can tilt up for resonance testing/tuning while a diaphragm is being stretched.
Tilting stretcher table to allow easy and safe resonance testing of the diaphragm before gluing it to the driver grid.
The original diaphragms were a multilayer mystery, but comments by the engineer who designed the speakers indicate that they used a 3.5 um thick polyester film, and then applied multiple coatings to make the diaphragm slightly conductive, a necessary condition to apply charge to the diaphragm and have low distortion at large excursions (usually low frequency signals). I was unable to locate 3.5 um film, so I used 3 um film I was able to buy via ebay.
Replacing the diaphragms is a multistep process.
1) Remove the old diaphragm and glue that held them on their grids. I used a scraper and a solvent called Goof Off to remove the glue.
2) Lay the film on the top of the stretcher table, start at one edge and pull the wrinkles out of the film and stick it to the tape on the lip of the table. Then go to the opposite side of the table and do the same, then do the same at the other two edges.
Back in the 80s an engineer from 3M recommended 4693H contact cement for gluing the diaphragms and he was absolutely right about it. The stuff bonds to both the diaphragms (they way the original glued didn't) and to the grids. I used a silicone squeegee that I cut down to about 15 mm wide to spread the glue on the diaphragms and the grids.
How well does 4693H bond to the diaphragm? Here's me trying to peel off some film:
Rolling the film out on the stretcher table- film is stuck to the stretcher on the right end of the table to keep it from flying around. Note- my stretcher requires a significant margin of film around the driver to allow the film to attach to the stretcher. Be sure you order enough of the film!. I went through an entire 20m roll of film to replace the diaphragms on 8 drivers (two speakers).
This is what the diaphragm looks like on the stretcher before it gets stretched. Don't worry about the wrinkles, they will disappear as soon as the tube is inflated. The black stuff is neoprene foam that lets the slightly concave grid perimeter contact the glue everywhere.
3) Once the film is stuck to the table, I tilt it up vertically to inspect it and make sure the film is stuck down on the tape everywhere and make adjustments as needed. Once it looks good, I connect a tire pump and put a few strokes of air into the tube. That takes out all the wrinkles as it puts the film under tension.
This is what the film looks like on the underside of the stretcher before the tube is inflated. This is when you want to make any adjustments to the film position, before inflating the tube and tensioning the film.
The diaphragm thumper- just a ball bearing glued to a piece of thread. Simple but VERY effective!
The tight diaphragm being tested for resonance. The mic and thumper are positioned as close to the center of the diaphragm as possible. I made a couple 3D printed clamps and a short "boom" for the thumper. The RTA in REW displays the spectrum of the sound that the mic picks up.
The tight diaphragm glued to the grid (and stretched on the table) behaves like a drum and has a primary resonance and multiple other resonances based on the dimensions of the driver. The diaphragms that came from the factory had a consistent primary resonance of 86 Hz.
This is what the resonance typically looks like in the REW RTA. The primary resonance is the highest peak at the lowest frequency due to the longest dimension (the width) of the driver. The lower peaks at higher frequencies are due to the shorter height of the driver and maybe interactions/interference between the waves moving on the film. I set resonances of the new diaphragms slightly higher than the factory numbers, assuming that over 40 years the factory diaphragms may have "relaxed" a bit, and to allow for the extra mass that will be added by the conductive coating.
4) Tune the resonance of the film on the stretcher by pumping more air into the tube (or letting some air out, though I never had to do that). Once you have the resonance where you want it, disconnect the the air pump to prevent slow leaks through the hose or pump. Apply 4693H contact cement to both the grid and the film on the stretcher. I spread the glue with a silicone squeegee that I cut down to about 15 mm wide. Try to keep the glue on the film in the area of the neoprene foam. Let the two pieces sit for 20 minutes to allow the glue to set.
5) Once the glue is set, tilt up the stretcher and check the resonance one more time. Adjust the air pressure as needed, then carefully set the grid down on the tight film on the stretcher. The film and grid will bond instantly, so be very careful! I manually pressed the edges of the grid down on the film then put some weight on top of it and let it sit for a couple hours.
6) When you're ready to free the grid from the stretcher, let some air out of the tube and slice the film close to the tube, leaving a wide margin of film around the grid. Lift the grid up and turn it over to inspect it- there should be absolutely no wrinkles anywhere. If there are, you'll have to do it all again.
Grid stuck to the diaphragm on the stretcher.
After releasing air pressure, cut the grid free by cutting the film all around the grid near the tube, leaving excess film attached to the grid.
The wrinkle free diaphragm on the grid is ready to have the excess film trimmed off. Pull gently on the film and slice it away from the edge of the grid with the razor knife blade pressed against the edge of the grid, using it as a guide.
7) Now carefully use a fresh, sharp razor knife to trim the excess film from around the driver grid.
8) The next step is to make holes in the film around the the three center posts where screws pass through the driver. I use a soldering iron with a small tip and temperature set to 265C. Hold the soldering iron with one hand and use your other hand to steady it. Put the tip down on the film right next to the post and use the plastic post as a guide and just move the soldering tip around the post, keeping it in contact until you've made a complete circle around the post. Remove the soldering iron by lifting it straight up. Next do the same at the other two posts. Finally, use some tweezers to remove any hairs or discs of film that remain stuck to the posts.
Small tip on the soldering iron.
This is what the holes in the diaphragm should look like after removing film disc and any hairs that are produced. Use your other hand to steady the hand holding the soldering iron, and use the post as the guide for moving the soldering iron.
9) Now you can check the resonance of the diaphragm, you just installed. Tilt the driver up vertically and adjust the mic and thumper positions as close to the center of the diaphragm as possible. Thump the diaphragm and read the resonance in REW.
Checking resonance of a diaphragm after gluing it to the grid and making holes for the center screws.
In my setup, the driver resonances come out about 10Hz higher than the resonances I measure on the stretcher. I think this is because the open area in the stretcher is a little larger than the open area of the driver grid. When I stretch film I set the resonance on the stretcher about 10 Hz below the target resonance.
10) Apply the high resistivity coating to the diaphragm. The coating doesn't cover the entire diaphragm- some specific areas have to be masked off. I 3D printed a set of masks to make this job very easy.
This is the area (gray) of the diaphragm that needs to be coated. The left and right edges are left uncoated to reduce leakage current due to proximity with the metal frame that the drivers are mounted on. The center circles also need to be blocked out because of the metal screws that go through the holes.
Driver with masks and blue masking tape on long sides, just after spraying with Licron Crystal.
The masks are made based on the uncoated areas of the original factory diaphragms. The coating process is easy- put masking tape on the long sides of the driver grid, place the masks on the ends and on the center holes, shake up the can of Licron Crystal and spray. I spray in two passes, right to left, then left to right, then turn the whole grid 180 degrees and do two more passes. Then I take the masks off and set the grid on a shelf for a few hours to dry.
End mask to block conductive coating on new diaphragms. Dimensions in mm. If you make holes in the specified locations you can use screws to align the mask with the driver, otherwise, just put some masking tape on the edges to hold it in place while you spray with Licron.
One of the hole masks showing the M3 screw that fits into the holes in the grid supporting the diaphragm and ensures proper positioning when applying the Licron.
11) After the the Licron coating has dried, I test it with a resistivity meter to see that the coating is very weakly conductive.
Checking the resistivity of the coating, in this case 10^9 Ohms/square- great! The coating process is so reliable, I would consider the resistivity meter optional.
12) Once the diaphragms are all coated, reassemble the drivers by installing the metal clips at the edges and the 3 central screws and nuts. Then you can mount them in the speaker again and reconnect all the wires you unsoldered.
The Dust Covers
Some people claim the speakers sound better without the dust covers. Some go so far as to remove the socks, metal grids, and the dust covers, exposing the drivers to everything floating around in the air including dust, pollen, pet hair, and bugs, and running the risk of electric shock to anyone foolish enough to touch the drivers while the speakers are operating. I don't recommend operating without all those things in place.
There are two dust cover in each speaker, one at the front and one at the back of the driver enclosure. The dust cover frame snaps onto the aluminum side pieces and a couple plastic pieces at the top and bottom of the driver enclosure. But the frame is barely rigid enough to support itself when it isn't snapped onto the speaker. The dust cover is made from the same film that's used for the diaphragms, and like the diaphragms, it should be wrinkle free. The only way to achieve that is to put the film under some tension, then attach the frame pieces to it, then mount it in the speaker and use a heat gun to shrink the film and take out the wrinkles that will inevitably appear. This is by far the worst part of the design of these speakers.
Start by marking out the outline of the dust cover frame on the work surface using masking tape. Make sure you get it square by measuring diagonals! The inside edges of the tape are where the outside edges of the frame pieces will be placed.
Applying the VHB tape to the frame pieces. The tape is wide enough to cover two pieces at a time, so I stick it to the two pieces laying side by side, then cut them apart with a razor knife.
The frame pieces cut apart.
Roll out the film and tape opposite corners to the table, pulling and applying tension as you go.
Next tape opposite sides, pulling and applying tension as you go.
Now add more tape on opposite sides, pulling and applying tension as you go.
Place the metal bits into the ends of one short side of the frame, and peel the backing off the VHB tape. Use the edge of the frame piece and the two metal bits to align the piece with the tape on the table top, and stick it down- be careful not to let it touch in any wrong spot as the tape will not let go of the film!
First frame piece stuck down on the film. Now take the metal bits out of the ends and put them on one of the long frame pieces...
Note the orientation of the metal piece- they can fit two ways, but only one is correct! Peel a couple cm of the backing tape off.
Hook the metal piece into the short frame piece that's already stuck down, and start sticking the long frame piece down, peeling the backing away as you go. I flexed the frame piece a little to set the center of it about 1mm inside the blue tape outline- this will help tension the film when the frame gets installed in the speaker. Now stick down the other long frame piece following the same steps.
Second long frame piece stuck down. Now prep the other short frame piece...
Put the metal piece on the end of the short frame piece, peel some of the backing off the tape, and get the frame piece into position- don't set it down yet!
Holding the frame piece up, peel off the rest of the tape backing. Keep the center elevated so it doesn't stick to the film and then bring the end down and hook the metal piece into the long frame piece, and finally let the short frame piece down to stick to the film. The hard part is over!
Now cut the frame free of the film that's taped to the table using a fresh razor knife held at a very low angle.
Frame cut free of the film on the table. There are going to be wrinkles- don't worry about it. Take it to the speaker, slide it into position, and snap the frame down onto the mounts in the speaker. There will be wrinkles- don't worry about it! Start working on the wrinkles with a heat gun- use a low temperature setting and keep the thing moving- you don't want to burn a hole in the film!
Dust cover on the speaker. Notice how smooth the reflection looks in the surface of the dust cover!. But there are a few small wrinkles at two corners...
Small wrinkles in one corner that could not be removed with the heat gun.
Small wrinkles in another corner that couldn't be removed with the heat gun. The other two corners were wrinkle free.
That's it! Heat shrunk film relaxes over time, and eventually, wrinkles may reappear in the film. Also, the adhesive on the tape remains soft. Tension on the film will pull the adhesive toward the center of the dust cover, also releasing tension - this is why you don't use tape to hold the driver diaphragms! The ideal attachment would be to use the 4693H adhesive I used on the driver diaphragms- it will not move under tension.
The Electronics
The schematic of the speakers changed a bit during the early years of manufacture, so your speakers may be a little different from mine. In my case, the electronics was all working fine so the only thing I had to do was replace the nonpolar electrolytic caps at the inputs of the speakers (based on their age, not performance). I ordered new 220 uF NP electrolytics and some 4.7 uF PP film caps to bypass them. I also replaced a 1000 uF 16V cap used in a power supply for the protection circuit in each speaker.
An ESL-63 schematic from 1989. Earlier versions might be a little different. The red box is the input circuit, the green box is a HV clamp, the pink box has the delay line, the orange box has the LPF for the bass panels, the blue box is the HV bias circuit, and the yellow box is the arc prevention circuit.
A lot of the circuit is there to protect the speakers from misuse. Looking at the input circuit in the red box, there's a MOV (a type of self resettable fuse) that protects the audio transformers from too much current in the primary windings. You'll see the 220 uF cap in parallel with a 1.5 Ohm resistor connected to the low voltage side of the audio transformers. The transformers step up the voltage from the amplifier to a few kV (hence the warning stickers on the transformers). You'll also notice a triac that, when switched on, shorts out the primary side of the input transformers (and puts the 220 uF cap and 1.5 Ohm resistor across the amplifier output! That's probably the only condition when the MOV would open up, protecting both the amplifier - we hope - and the speaker from the large currents that would flow were the MOV not there).
That triac is driven by the arc preventer circuit (yellow box) which detects the ionization of the air that occurs just before an arc forms. So if the speaker is being badly overdriven, and the air starts to break down, the arc preventer will turn on the triac as described in the above paragraph. This only works if the speaker is powered on!
The blue box is the voltage multiplier that provides the 5.25 kVDC bias to the speakers. It's a standard Cockcroft-Walton voltage multiplier that charges the caps in parallel and discharges them in series. There's a small neon lamp and capacitor connected to the output as well as a 10 Mega Ohm resistor. The resistor limits current available to charge the diaphragms and helps keep them operating in constant charge mode for low distortion. The neon lamp only lights up and conducts when the voltage across it is about 90V. That happens at power up when there's little charge left on the diaphragms, and if the charge leaks away from the diaphragms indicating that there is some dirt or foreign object (like a dead bug or pet hair) trapped in the speaker between the diaphragm and the stator or frame of the speaker. There will always be a little charge leaking away from the diaphragms, so the neon lamp will flash. The frequency of the neon lamp flashing is a good indicator of the condition of the speaker. That lamp should only flash briefly once a minute or so and frequent flashing indicates that it would be a good idea to inspect the speaker and maybe do some cleaning. The neon lamp is inside the electronics enclosure where you can't see it unless you take off the bottom cover.
The green box is a high voltage clamp/limiter circuit. It sits across the HV output side of the audio transformers. If the voltage rises above about 7 kV, the zener diodes in the circuit will turn on and prevent the voltage from rising further. That protects the speaker from damage caused by arcing if the speakers are powered off (which prevents the arc detector from working) and a loud audio signal is applied (maybe some doofus turns on the stereo and doesn't know the speakers are powered off, doesn't hear any sound, and turns up the volume to try to get the speakers to play). There's an LED in series with the zener diodes that will light up when the clamp is active, but it's inside the electronics enclosure where you can't normally see it.
The pink box is the delay line that is responsible for driving the rings (segments) of the stators at slightly different times to simulate the pulsing of a spherical driver.
The stuff in the orange box is a LPF that drives the outermost ring and bass drivers (the top and bottom drivers in the speaker). The 360k Ohm resistors work with the capacitance of the panels to roll off the high frequencies sent to those drivers.
Bottom of one of the speakers with the cover removed. The nonpolar cap is the blue part located under the cement resistor, circled in green. There's plenty of room for the replacement caps I ordered. The only thing supporting the cap is the wire that wasn't properly soldered to the input connector and that thin yellow wire that's going to the PCB. Note- the empty space to the right of the green circle would allow for much larger film caps to be installed in place of the non polar electrolytic caps I used. The transformers at the top left and right corners are the audio transformers. The green PCBs at the bottom are the delay line. The board in the lower right corner is the HV clamp. The top center section has the power transformer, HV bias circuit, and arc preventer circuit.
After I removed the old input coupling cap and resistor you can see the 1000 uF cap in the arc preventer circuit power supply.
New input coupling caps installed. The new 1000 uF cap is under there, too. I was able to unsolder the old parts and solder in the new parts from the bottom side of the PCB without having to take the board out of the chassis.
