Quad ESL-63 Resurrection

This post is being updated as I do different things to repair and refurb a pair of 42 year old Quad ESL-63 electrostatic speakers. Check back every couple days for updates that will be added to the bottom of this post.

I've been a fan of ESLs since I learned about them and started building them myself in Japan back in the 80s. In the early 90s I had acquired a pair of Quad ESL-63s that I repaired and used for a few years. I was always dissatisfied with their mechanical construction- much too complex and flimsy. I don't think Quad ever had a manufacturing engineer look at the design, because it could have been much simpler, more robust, and probably a lot less expensive to manufacture if they had. I designed and built new frames for the drivers from those speakers using t-slot aluminum, but never got around to rebuilding those speakers in the new frames. I still have all that stuff that I may put to use soon. Quad's follow-up to the ESL-63, The ESL989, was more solidly constructed, as are their current offerings.

I recently found a Craig's List deal on a pair of ESL-63s with their shipping boxes. The seller stated one works fine and the other makes noise. The speakers were listed for almost a month in Chicago, where you'd think there would be at least one audio nut willing to take on the challenge of repairs, or to strip them for parts to sell on ebay. Apparently not. After seeing them listed for a month, I decided to check them out.

Outwardly, they are in perfect physical condition, indistinguishable from new, except both were missing the original plastic feet and a couple screws from the bottom covers. Not bad for speakers made in 1983. You can learn all about Quad ESLs (the original ones, and the 63s) here. Here are the owner's and  service manuals for the ESL-63.

ESL-63 on the left, next to B&W 703 S3 on the right for size comparison.

I handed the man $300 and took them home. Thanks Tom! I assumed that I was going to have to work on them, so there wasn't much point in testing them before I bought them.

When I got home, I tested them. Sure enough, one works fine the other makes noise. Even though one is working, both will need some work to ensure reliable (is that possible with ESL-63s?) operation for the next several years. 

I started the usual way- replacing the electrolytic caps. There are exactly two in each speaker. A 1000 uF 16V cap on the power supply for the arc sensor, and a 220 uF nonpolar electrolytic input coupling cap. 

Accessing the caps is done through the bottom of the speaker. I turned it upside down and leaned it against my work table and removed the screws holding the bottom cover in place. On the inside of the bottom cover there are two strips of foam tape that have rotted and will need to be replaced. This foreshadows MUCH more to come...

Rotted foam tape inside the bottom cover. It only contacts the PCB in the chassis in two small areas, so I'm not sure why they used such long pieces of tape.

 

I ordered new caps from Parts Express- 220 uF nonpolar electrolytics, and a pair of Audyn 4.7 uF PP film caps to bypass them, and while I was waiting for delivery, I printed new TPU feet for the speakers. When the caps arrived, I went in to install them. 

That's where I ran into the first problems. In the first speaker (the one that worked), the 220 uF cap's solder connection to the input terminal had failed and it was making contact only by the strength of the spring force of the leads! I also found a black ground wire that was supposed to be soldered to a lug, just left floating in the air. Oops! 

The PCB that the caps both mount on is held in the enclosure by two + head screws. Those screws are at the bottom of the enclosure, making it almost impossible to remove them to release the PCB, even with a right angle drive wrench and screwdriver bit. Fortunately, there's enough room to get a soldering iron in without removing the board, so I was able to remove the old parts and install the new ones with minimal effort.

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. Hmmmm. Not what I would expect in a very expensive speaker.

Input board screws. Now I'm starting to remember what my ESL-63s were like back in the 90s- a mechanical nightmare! The white wire is the antenna for the arc detector. I had to unsolder it to get to the solder pads for the input and 1000 uF caps.

Unsoldered ground wire. Not really a big deal- it's one of two wires that ground the aluminum bottom cover of the speaker.

After I removed the old input coupling cap and resistor you can see the 1000 uF cap on the input board. 

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.

I had forgotten what a PITA these speakers are to work on. My intention with this blog post was to get to the driver fix(s) quickly and get the thing working again, but as I got deeper into the speaker, I found a LOT of different foam and other tapes that had rotted away. I spent 3 days cleaning the old tape and adhesives off the different parts, replacing it with new stuff that I chose to hopefully last longer than the original stuff.

Keep in mind, that everything I am doing to replace rotted foam tape in the first speaker will also have to be done to the other speaker. The speakers won't work reliably without replacing all that tape- some of it is inside the dust covers. In all, I spent about $100 on all the tapes, solvents, and glue supplies needed to fix both speakers.

Accessing the drivers

The driver enclosure consists of two vertical aluminum rails, plastic top and bottom plates, and two vertical aluminum support rods, front and back, all bolted to the electronics chassis. 

The speakers are covered by cloth "socks" attached to the frame by hook type tape on the top of the driver enclosure. Behind the sock there are metal grids on the front and back side of the speaker that provide some mechanical protection for the drivers and keep curious fingers away from the high voltage inside. 

The step by step procedure to gain access to the drivers is to slide the top wood cover to one side and lift it off, then release the sock from the hook tape and slide it down on the speaker frame. Next, peel off the tape holding the metal grids at the top and bottom of the frame and pry them free of the crumbling foam tape that holds them to the top and bottom plates of the driver enclosure. Then pull the thin wires out from the vertical edges (apparently used to fill the space in the aluminum side rails), and remove the front and rear grids. Finally, snap the frames holding the dust covers off the speaker frame, slide them out sideways, and set them aside.

Sock attachment at the top of the speaker with the wood top cover removed. There's hook tape under the sock holding it in place.

Hook tape on top of the driver enclosure.

Gooey adhesive under the hook tape.

I found the best way to remove this stuff was to start by rolling it off instead of scraping it.

Adhesive booger removed from the top of the driver enclosure. This was the stickiest, gooiest, stuff I've ever seen.

The metal grids have what looks like gaffer's tape covering the vertical edges, and are held in place by more tape along the top and bottom edges, and by double sided foam tape also at top and bottom, and on two vertical supports near the center of the speaker, front and back. All that foam has rotted, and all the tape's adhesive has dried out, so all of it has to be removed and replaced before putting the speakers back together. It's a tedious and messy job.

Front upper left corner of the speaker with the sock pulled down. The metal grid with gaffer's tape on its vertical edge fits into slots in the vertical aluminum frame pieces. The grid is mounted using double sided foam tape at the top and bottom. The green wire fills up the space inside the vertical frame piece so that the grid can't rattle. It is removed by simply pulling it out.

Front upper left corner showing grid coming out of the frame after it has been released from the foam tape. You can see the two vertical supports to the right, just behind the grid.

Front grid removed from the speaker. Gaffer's tape still on the edges. Residue from the rotted foam tape visible along the top and bottom edges.

Brown foam tape residue on the back side of the front grid. This is easily cleaned off with a wire brush. The gaffer's tape at the vertical edges of the grid will also be removed and replaced.

Front bottom left corner of the speaker showing rotted foam tape and residue from the gaffer's tape that held the front metal grid in place.

Front top edge of the speaker showing foam and gaffer's tape residue. One of the two vertical supports (black) is visible at the center.

Poorly positioned gaffer's tape on the edge of the metal grid. That will be replaced, and positioned properly.

Rotted foam tape on one of the four vertical supports. You can see light reflecting off the wrinkled dust cover behind the support.

This is the scraper I use- it has a VERY sharp edge, so be careful!  After scraping, I wipe everything with Goo Gone, Goof Off, or IPA, to remove the adhesive residue. Goo Gone is reserved for parts I can completely remove from the speaker and wash with detergent in a sink after (you have to wash the orange oil off). 

Scraping the rotten foam tape off one of the two vertical supports.

Scraping the rotten double sided foam tape off the top edge of the speaker. The gaffer's tape residue has already been scraped off.

Front bottom of speaker showing the mess created when scraping all the tape residue off the speaker. I keep a vacuum cleaner handy and vacuum this stuff up. You can see the two vertical supports near the center of the speaker.

Wrinkles in dust cover. This is bad, but easily fixed. The wrinkles will cause rattling noises. This will have to be tightened, if I can get it off and back on again without breaking it, or it will have to be replaced. When it's back on the speaker, there should not be any wrinkles. The dust cover is double side taped to a plastic frame that snaps onto aluminum side pieces and plastic pieces at the top and bottom of the speaker frame.

Back side of the speaker with the grid off- requires the same foam tape cleanup as the front side. You can see the dust cover is badly wrinkled here.

 

Dust cover removed, you can see some foam tape hanging down on the driver grid. The black piece is where the dust cover snaps onto the top edge of the speaker. Note- that foam is inside the dust cover- as it rots, it crumbles, and can get inside the drivers and cause all sorts of noise problems.

Bottom corner showing the black piece where the dust cover snaps on, and foam tape. The wire spring like thig is there to ground the aluminum frame pieces and the steel protective grid (already removed). 

Removing the drivers

Each speaker has four drivers, two for "bass" located at the top and bottom of the speaker and two centrally located drivers that are divided into rings that are fed by a delay line to simulate a pulsing spherical radiator. The drivers can be removed without taking the top of the frame off. The procedure is to unsolder the wires from the drivers, then unscrew one of the middle drivers and take it out out of the frame sideways. Then take out the other middle driver the same way, and finally remove the top and bottom bass drivers by shifting them up or down and then remove them sideways through the frame.

You have to unsolder the wires, front and back, in order to remove the drivers. 

This is how you get the drivers out- remove the two in the middle first, then the top and bottom drivers can come out. Each driver is held in by 4 screws that go into aluminum L brackets that run the vertical height of the speaker. Once they are all out, set them aside- there's more foam tape to replace!

The top driver has to be dropped down a bit to get it out of the frame, so you have to remove the driver below it first. The black piece in the green rectangle is the part that the dust cover snaps onto. It comes out with the driver.

This is the top dust cover mount. That rotten foam tape has to be replaced before it falls into the drivers and causes problems. There is a similar piece at the bottom of the speaker that doesn't come out with the drivers.

All drivers removed, the rotten foam tape in the dust cover mount needs to be replaced.

It's a little hard to make out here, but those pieces of foam tape are mounted on an L bracket where the drivers mount. The brackets are on the right and left sides of the speaker. You can also see foam tape that runs the length of the bracket peeking out from behind it. All that stuff is inside the dust covers, crumbling, and has to be replaced.

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

Replace the tapes

The procedure with parts that can be removed from the frame is to first scrape as much of the old tape off as possible, then take them to a sink and spray with Goo Gone, an orange oil based solvent that dissolves the adhesive. Let it sit with the Goo Gone for a while then start wiping with a paper towel, folding frequently. Then spray the whole thing again, let it sit a while, then wipe with paper towels again. And again. And again. Finally, when it looks like all the sticky residue is gone, wash the parts with dish detergent and hot water to remove the orange oil.

For parts that aren't coming out of the frame (the bottom dust cover mount), wipe with paper towels soaked with Goof Off several times. Goof Off is very volatile, so use good ventilation. It will evaporate and not leave any residue of its own, but you may have to wipe several times with it to get rid of all the adhesive residue. 

I measured the old tape and looked for modern substitutes. I selected some EPDM rubber weather seal to replace most of the old foam tape, as it should last a lot longer than that stuff that was in the speakers. The thick foam tape that goes on the vertical driver mounting rails was replaced with some this weatherseal:

Thick foam tape for the driver mounting brackets, C in table above. I bought 1 roll of this stuff. It's 1" wide, so I cut it to the length I need (31") then cut it length-wise so I have two strips that are 1/2" wide. There will be some left over.

Small EPDM rubber foam tape for A, B, D, and E in table above. I bought three rolls of this for the two speakers and will have some left over.

I also bought some 3/4" wide Scotch brand "permanent" double sided tape to use on the hook tape that holds the socks on the speakers, and for the dust covers, and this 1/4" VHB tape to mount the steel grids. Finally, I also bought some 1/2" and 1 " wide gaffer's tape to mount the steel grids.

One of the two driver brackets (per speaker) with new foam tape installed.

Top dust cover mount with new foam tape installed.

I peeled the tape off the vertical edges of grids and found that the adhesive had completely dried out, much of it turning to dust as I peeled it off. The rest scraped off easily. The foam tape residue was a bit more tenacious. I used the scraper to get most of it off, then used a wire brush to get the rest. Finally, I vacuumed and wiped the grids with IPA.

At this point, I have put in at least 12 hours of labor cleaning up the old tape and its residue and shopping for and installing new tape. That's for one speaker, and I haven't even repaired the driver(s) or dust covers yet. This is why 40 year old ESLs can be purchased cheaply, and why it would be cheaper to buy new ones than to have a pair of old ones professionally refurbished!

The Drivers

The drivers have two plastic grids with two stator PCBs glued to them. The grids hold the two stators parallel and about 4.8 mm apart. There is a thin plastic diaphragm with a high resistivity coating glued to one of the grids, between the stators, putting it about 2.4 mm away from either stator. The speaker has a 5.25 kVDC bias supply that connects to the stators and the diaphragm. If the diaphragm breaks and contacts the stators, or the stators come unglued from the grids and contact the diaphragm, or some foreign material has found its way between the stator and diaphragm, the driver will make hissing, whining, and popping noises. My bad speaker makes such noises that you can hear, here.

There are three screws and some metal spring clips holding the two plastic grids together. Carefully remove the screws and then the metal clips to separate the two halves of the drivers. The diaphragm will be glued to one of the grids. Look carefully at both sides of each grid. Look for any loose stators by gently pushing on them with your finger tip, and look for holes or tears in the diaphragms and burn marks on the stators. A torn diaphragm, or one that has holes, has to be replaced. Loose stators can be reglued without replacing the diaphragm(s). 

The best case is that the stator(s) have come unglued, because they can be reglued without replacing the diaphragm. The worst case is that the diaphragm is broken and the stator(s) have become unglued. In that case, the diaphragm has to be replaced, and stators have to be reglued.

Upon inspection, I found that all four drivers had some areas where the stators had come loose from the grids. I will be regluing all of them, in both speakers.

One of the drivers opened up. The gray stuff is the high resistivity coating on the diaphragm- 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.

Regluing stators

There are couple ways to approach this- you can sacrifice the diaphragms and completely remove the stators from the grids, allowing you to clean off the old glue and reglue them to the grids with all new glue, but that will require replacing all the diaphragms- a tricky process, prone to failure.

Alternatively, you can remove the stators from the grids that don't have diaphragms, clean off the old glue, and reglue them with new glue, and then reglue the other stators with the diaphragms in place. A slightly tricky process, but not nearly as bad as replacing the diaphragms. 

I decided on the latter method, using clear polyurethane Gorilla Glue applied with a syringe. 

2/19/25

The last of the gluing and taping supplies I ordered arrived today. 

I took one of the stators out of one of the bass drivers today to assess the condition of it and the grid and to figure out how I want to proceed with regluing it. Taking it off the grid required desoldering two metal connection clips.

Once I had it out I could see that the copper side of the stator, the side that gets glued to the grid, is covered with some sort of clear plastic coating. There are a couple small spots along the edges where the coating seems to have failed and the copper looks corroded. 

I gently scraped the larger bits of glue off the stator, being very careful not to damage the clear coating. I also scraped the glue off the grid. Then I set the stator back into the grid to see how it sits. The three screw posts at the center of the grid protrude loosely through holes in the stator, and it looks sufficient to align the stator within the grid, but I may make some spacers to ensure alignment at the edges anyway.

The stator is pretty flexible and has a bit of a curve to it, so it will have to be weighted down to ensure that it glues onto the grid as flat as possible. It requires extremely light force to flatten the stator in the grid, so I cut three pieces of the 1"x1" foam weatherseal to the width of the stator and will stick it to a board. The weatherseal foam is very soft so the weight of the board I stick it to should be sufficient to guarantee that the foam will compress a bit and the stator will be glued flat in the grid.

Soft foam strips laid out on stator where they will be taped to a board to ensure the stators are glued flat in the grids.

Corrosion along the edge of the stator.

Check back here in a couple days to see how I reglued the stators...