The Quad ESL - Treble Panel

This is the part of the Quad Electrostatic that everyone wants to refurbish. Why? It's very likely that they've broken it, or someone has, and run the thing to death by arcing it. In fairness to those (including me) who've arced a treble panel, it has to be said that as the panels get older the efficiency declines for a variety of reasons, including loss of tension in the diaphragms, and loss of coating. The efficiency loss is about 15% over the first 5 years, and after that the decline is slower with a plateau of about 70% of factory original efficiency after 10 years. We have this phenomenon sneak up on us, and then one day - poof - lightning strikes!

Arced panels can go on working reasonably well, in fact, provided that the arc point was not near one of the EHT pickup rivets that enter the panel. In that case, the diaphragm will probably not charge very well, if at all, resulting in significant loss of efficiency, or no sound at all!

The original panel diaphragms were made from 6 mm tensilised Mylar^® which is not made any more by DuPont. Other tensilised 6 mm polyesters are available - Skyrol^® - for one, and if you are prepared to buy 8 tonnes of it, SKC will run it off for you right away sir. Personally, I don't think I could afford the postage on that. There is made-to-original-spec 6-micron tensilised P.E.T. film (and superior!) available at ER Audio.

The materials mentioned above are probably the best ever made for the diaphragms of electrostatic speakers. Quad Electroacoustics use a 3.5 mm
Melinex ^® in the new Quad 988 and 989, but I think that working with a film this fine for the DIY refurbisher would be more than a little tricky.

Many refurbishers use 12 mm Mylar^® and use a higher diaphragm tension. The results are very good, although tensioning the diaphragm may be more critical to keep the resonance in the supersonic region. Getting the resonance in the supersonic region is a piece of cake with 6 mm material which has a resonance, when properly (and lightly) tensioned at 28kHz - 32kHz. If your diaphragm tension is too low the panel will buzz or make flapping noises like a stack paper being shuffled.

One caveat with these notes is that I am not providing as many pictures as for bass panel reconstruction, since the panel construction is pretty much identical, except for a few details that I will call out in the notes as we go along. For the sake of some completeness, though, here is a picture of a disassembled treble panel.

You can see much more detail in the (larger but similar) bass panel pictures, if the structure is not clear enough here.

Materials List:

o Fine Sand Paper (600 grit and finer)
o Silver Loaded Paint
o Gray Enamel Paint
o High Voltage Wire
o Solder & Soldering Iron(s)
o Dust Cover Shrink Film (12 mm or less in thickness)
o PVC Tape (2")
o Aluminium Tape/Foil
o Mylar^® or other suitable 6 mm or 12 mm polyester film
o Isopropyl Alcohol and/or Acetone
o Corona or Circuit Board Lacquer
o Diaphragm Coating Material (CALATON CB, ELVAMIDE, DIY soluble nylon, graphite)
o Screw Drivers (Flat blade & Philips)
o 60 x 12mm x 3mm machine bolts
o 60 x 3mm Hex nuts for bolts
o 120 x 3mm washers for each side of panel
o Hobby Knife
o Masking Tape
o See Parts Section for Suppliers

N.B. I have described a coating method below using graphite, but it is NOT the best coating to use, by far. Soluble nylon is the original coating, and I believe, the best for this speaker. You can buy soluble nylon, and, I have to say, a superior coating, at , ER Audio. , if you wish, along with instructions on how to use it.

Stators & Dust Covers

Physically disconnect and remove the panel from the speaker. This procedure is described in the Disassembly Section, and this section should be read, and well understood, before attempting to remove any panels from the speaker. With the panel now removed, you can (almost) see the structure.
It is a five layer composite. The two obvious outer layers are the dust covers. The next two (still obvious) layers are the perforated stators. The centre-most layer is the diaphragm - not obvious, you cannot see it at this point. The actual treble panel looks like this when disassembled.

Fresh out of the speaker it will have the dust covers on front and rear and be held together with clear tape around the perimeter.
Remove the dust covers by carefully slitting the tape around the periphery of the panel and unfastening the four 2mm diameter bolts and the felt washers that penetrate the dust covers and the panel. N.B. Note the position of these items carefully as they must be refitted. Keep a vacuum cleaner handy, you'll generate a lot of dust here with old panels! Be careful not to cut into the stators, they're only plastic; or, indeed, the wooden frames as these are very light wood. Put the dust covers aside for replacement separately. You can now clearly appreciate the physical state of the outside surfaces of panels. There will probably be spots of missing gray paint. The gray paint is just that - gray paint - on the outside. Clean it up and touch it up as you like. Spots of "corrosion" may be evident around the rivets, although this is very much less likely than with bass panels. You can clearly see that the treble panel is held together with about 60 rivets - what a bastard! Why rivets? They are: 1) quick & easy to use, 2) don't present much sharp surface for corona to form, 3) are cheap and 4) allow a close fitting dust cover. They are also a complete bastard to remove!
Remove the rivets holding the two stators together. This can be done by crimping the rivets in two directions at 90^o to each other and pushing the rivet out. I use a high speed drill, like an Arlec Super Tool^tm. A fair bit of heat is generated, and the stator will often soften slightly. I view this as somewhat of an advantage, if you don't overdo it(!). The rivet will push easily out through the original hole after a couple of seconds drilling with the appropriate abrasive head on the tool. I use a 3.5mm carborundum ball. The hole will be slightly enlarged, but this is not a problem, since we re-fasten with 3mm bolts and nuts, with washers. This operation is a pain in the bum, takes a long time, generates a lot of dust and dirt, and you need to clean the whole of each stator when finished, whichever method you use. Try not to knock off any more of the paint on the outside. If any red plastic is showing, then you will only need to re-coat with gray paint, for cosmetic purposes.
Once the rivets are removed, remove the two small bolts that attach the EHT line (thin red wire) to panel and then you can separate the two stators. The stators are not glued together, but they may separate reluctantly because they've been squeezed together in there for a lot of years. Use a fine blade screwdriver if necessary to gently prise them apart. The diaphragm is only attached to one of the stators. The diaphragm , if it is original, will have white, streaky marks all over it. This is a CALATON^® coating used by Quad at the factory. The diaphragm can be removed, since you have decided to replace it.

Old Diaphragm with White Coating
Carefully scrape off, or sand off, any remaining diaphragm material and glue at the edges of the stators. Use a fine paper, about 600 grit wet & dry is OK. Try not to make any deep scratches - anywhere!
Thoroughly vacuum clean the stators so that all dust, paint, et cetera, is removed. You should now have a clean treble stator, with perhaps an obvious spot on the inside in one of the "wells" where the panel has arced and the gray paint and the conductive coating on the inside have been burned off.
Examine the stator structure thoroughly. A normal stator looks like a smaller copy of the bass panel. The aluminium tape around the edges seems to be missing and the carbon tracks on the two centre “ribs” seem to be missing. The ribs in towards the centre and the ribs at the edges of the stator have strips of what looks like masking tape running down the length of the rib. They cover gray paint, and under the paint - carbon tracks. The wax paper is there to damp the membrane at the much higher frequencies that this panel works at. The paper is not needed in the lower frequency bass panels. So, there is the mystery of how the 'treble panel gets its charge - solved. It gets its charge from the two EHT bolts which contact the carbon 'frame' the same as in the bass panel. It is just that in the treble panel these have been covered by gray paint and the paper strips.
Clean any arced areas with fine (600 grit) sand paper. You may have to take badly arced areas right back to the red plastic base material. Do not remove more than is necessary and then clean carefully with mild soap and water or isopropyl alcohol. Isopropyl alcohol is not aggressive on this type of plastic. DO NOT USE ACETONE.
Paint the area that you have just cleaned down with conductive paint. Silver loaded paint is best (and most expensive), but other types, such as those used to repair windscreen heater tracks also work. Be careful that none of this 'drools' into the perforations. Even really bad areas of arcing can be coated easily and quickly with dabs from a small artists brush.
When the conductive paint is dry - usually about 5 minutes, or less - you can cover the conductive layer with gray paint. A spray of corona lacquer or Circuit Board lacquer won't hurt once the gray paint is dry, and will slightly decrease the risk of future arcing.
Once all arced areas are repaired as noted above, you only need to ensure that the panel is cosmetically acceptable, and that the strips of 'masking tape' are OK. These strips damp standing waves in the membrane and are not there for decoration, so if you damaged them, replace them.

Diaphragm Replacement

OK, you've got this far, and you're feeling pretty pleased with yourself. Let's see what we can do about that, then!! By this stage you should have two clean, repaired stators, with no arced spots or conductive layer discontinuities.

Cut a piece of 12 mm Mylar^® (or 6mm P.E.T.^®) which is about 10 cm larger than the treble panel stator in each dimension.
Clean a smooth laminex or glass topped table with ordinary household detergent. Dry well, and then clean with either isopropyl alcohol or acetone. N.B. Both of these materials are flammable. Acetone in particular!! Do not allow anyone to smoke or bring a naked flame into the same HOUSE with acetone. Both materials are relatively innocuous, otherwise. Your objective here is to remove anything of a lumpy, or gritty nature from the work area.
Lay out the diaphragm film smoothly on the table top, and fix the corners with masking tape, stretching the film slightly as you do this. Then attach masking tape to the centres of each side, stretching the Mylar ^® again, in each direction. Continue taping each side by "splitting the difference" between previous tape points, until you have the Mylar^® reasonably taught. Don't try to get it very tight at this point, but do tape it firmly all around.
Using the inner stator dimensions as a guide, mark, or mask off, a rectangle to be coated.
If using graphite, rub the powdered graphite into the Mylar^® as hard as you can without tearing it (of course). Try to really grind it in, and get a uniform coating. Test the surface for uniform conductivity with a surface resistivity meter or DVM that can read to at least 100 MW. This will produce a diaphragm which is far too low in resistance to be really useful in a Quad Electrostatic. Don't panic!! N.B. Soluble NYLON is the best coating to use, and it just wipes on in alcohol solution. I would also highly recommend the coating from ER Audio!
Try to rub off all the graphite, using a paper towel soaked in isopropyl alcohol. A graphite glaze will be produced, and the colour of the diaphragm will appear a very light gray. Do try to rub off all the graphite. IF you ground it into the film well enough, it won't come off completely.
Check the resistance with your meter. If it is not at least 100 MW then try rubbing the graphite off a little more. You also need to avoid making large discontinuities in the membrane coating. This is why this method is such a pain, even if the process is, practically speaking, very simple. When the membrane measures as you would like it, clean the general surroundings, but leave the Mylar^® taped to the table top.
If you want to avoid this rubbing and grunting and carrying on, just wipe on some DIY soluble nylon, or CALATON CB or ELVAMIDE any of which will duplicate the original Quad diaphragm coating, and wait for it to dry off. Diaphragm coated - no effort.
Mix up some two part epoxy resin glue - the PLAIN kind - no metal fillers!!
Choose a stator to which you will glue the membrane.
Apply a thin (1/8^th inch) bead around the perimeter where the old brown (factory) glue was. Do not put epoxy on the tapes at the edges. The bead should be run between the tape and the outer edge of the stator plastic.
Place the stator, glue side down(!) over the Mylar^® film being very, very careful to align the inner rectangle of the stator with the coated area. If you mark up the position accurately with a felt pen, this is not a real problem - just plan ahead.
Press down with both thumbs all around the perimeter of the stator where the bead of glue was run. The idea is to squeeze the glue into a very thin film. N.B. Placing weights on the stator at this time, alone, will not do a good enough job.
Place a layer of books, a sheet of steel, or something solid, over the stator to spread the pressure, and weight the whole thing with bricks. Wait until the glue is well cured. I leave the stator overnight, usually, no matter which epoxy I use.
When the glue is well cured, remove all the weights and other paraphernalia, and lift up the stator with diaphragm attached. Trim the edges very carefully with a very sharp knife, (e.g. Stanley Knife).
Heat Shrink the diaphragm with a heat gun set at about 400 Watts, working about 20 to 30 cm from the diaphragm. There are so many variables involved in this, that it is impossible to give precise instructions. If you have not done this before, then you must practice with a spare piece of Mylar^®. Tape a 20cm x 20cm piece of Mylar^® film to a table top and practice heat shrinking until you can shrink the film tightly (no little creases) without melting a hole in the film! If you melt a hole in the diaphragm, then you can start again.
Coat the exposed side of the diaphragm with DIY soluble Nylon if you are using the original coating. This is what Quad did at the factory, and what you should do if you want the speaker restored to its original condition. See the FAQs for an explanation of why this is so.
Melt holes in the diaphragm for each stator hole (~60 holes) with a fine-pointed tip on a small wattage soldering iron.
Double check that the heat shrink job is OK. If not, carefully heat shrink again. Leave overnight and repeat the heat shrinking if not sure. The diaphragm should be perfectly smooth, for treble panels.
Using 9mm x 3mm (M3) bolts, hex nuts, and two washers (one each side), bolt the stators together. Tighten each nut and bolt firmly, but not too tight.
Take the refurbished Dust Covers and identify the rear cover. This will need new, large holes drilled in it to accommodate the hex nuts on the rear stator. Otherwise the panel, overall, will be thicker than the original and will not fit into the frame without other unpleasant maneuvering.
Re-fit the dust covers such that the signal leads (brown and blue) and the EHT lead (red) exit through the side. There's a couple of small checks in the timber frame, so this is not hard to identify and do. Tape all around with PVC tape to seal and insulate, but don't overdo it. Thick layers of tape will make it hard to fit the panel into place in the speaker frame.
I recommend fitting the dust cover film such that is glued reasonably tight and free of wrinkles, as far as possible by hand. Then, when both covers are re-fitted to the panel I heat shrink both at that time.
Melt four (4) holes in the dust cover film where the four small bolts and felt washers were originally located. Re-fitting of these felt washers is essential as they prevent the dust cover from rattling and buzzing at high frequencies.
The panel is now ready to be refitted to the frame and the appropriate solder connections to the EHT and Audio transformer can be made now or later. When re-wiring to the audio transformer, make sure that FRONT and BACK connections for the brown and blue wires are strictly observed, or you will have one part of the panel running out of phase with another part of the panel or the bass panels, or both.