Before attempting any repair or restoration work on the Bass Panels, you need to determine if there is actually anything wrong with them. Damage to dust covers will usually be obvious, or the dust covers will be starting to become very “brittle” to the touch. If so, replace the dust covers, at least...see Dust Covers.
Otherwise, life can be tricky with bass panels. The bass panels never show arcing damage, as treble panels do. They suffer from excessive leakage to the frame, and via the rivets. They also lose their diaphragm coating over a period of about 10 to 15 years. If the diaphragm coating is partly or mainly(!) missing then the bass output will be significantly down on the output of a factory fresh panel. This is one reason why, I suspect, that people who listen to aged Quads notice that "lack of bass" that everyone writes about when reviewing the Quads. A lot of this palaver is sometimes about a panel which is well out of specification. A simple series of tests to determine if the bass panel is leaky is described in the EHT Fixit section of this site. Alas, the only really definitive way to determine if the bass panel coating has seen better days is to pull the panel apart. If you do this, then you may as well replace the diaphragm while you're at it.
Materials and Tools:
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
o PVC Tape (2")
o Aluminium Tape/Foil
o Mylar® or other suitable 12 mm polyester film
o Isopropyl Alcohol and/or Acetone
o Corona Lacquer or Circuit Board Lacquer
o Diaphragm Coating Material (CALATON CB, ELVAMIDE, DIY soluble nylon, graphite; et cetera)
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
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. However, ER Audio offer a superb replacement which is in many ways superior in performance if you do not really want a 100% original restore. For my money, I would be using ER Audio coatings.
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 bass panel looks like this , except that it will have clear tape holding the dust covers on, not gray PVC tape.
- Remove the dust covers by carefully slitting the tape around the periphery of the panel. 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. N.B. To completely remove the dust covers, you will need to also de-solder the three thick (white, red, black) wires from the small phenolic connection board at the bottom of the panel, see Fig. 1 below:
Put the dust covers aside for replacement separately. You can now clearly appreciate the physical state of the panels. There will probably be spots of missing gray paint. Spots of missing stator coating (conductive coating is on the outside of bass stators). Spots of “corrosion” around the rivets.
You can also clearly see that the bass 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 90o to each other and pushing the rivet out. I use a high speed drill, like an Arlec Super Tooltm. 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 white plastic is showing, then you will need to re-coat with conductive paint. More on that later.
- Once the rivets are removed, you need to de-solder the thick white wire that carries signal to the front stator from its solder tab. Use a heat sink on the tab, and a very hot soldering iron. Just heat the tab enough to pull the wire off the front, or you'll melt the stator for sure. Push the wire back through both stators to the rear, so that you can separate the two stators. The panel is now in two parts. One stator, and one stator with the diaphragm attached. The original diaphragm , if 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.
- Carefully scrape, or sand off the 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 white bass stator, that looks a bit like this...
- Examine the stator structure thoroughly. A normal stator - carbon tracks and tapes - looks like this all around.
The aluminium tape around the edges can be corroded. A typical cause of corrosion is absorbed water from the dust cover frame, in humid climates. The dust cover frames also, very occasionally, have had "sappy" wood in some places on the frame that absorbed extra moisture, helping to corrode the perimeter tape. If corrosion is present, it will be obvious as a white calcification, or white powder. This will tape need repair, if corrosion is present, as the tape is essential to charging the diaphragm.
- Clean any corroded tape areas with sand paper. You may find that the corrosion has eaten clean through the tape and you will need to scrape the area clean with a sharp knife (e.g. Hobby Knife, Stanley Knife). Be careful not to scrape the stator plastic too much. Wipe clean with alcohol, or mild soap and water.
- Bridge the gap with aluminium foil that is affixed to the plastic with conductive paint (see Parts Section ). The conductive paint will provide a small amount of adhesion to hold the foil in place. Use two layers of foil in all. When the stators are re-attached to each other the foils will be firmly held to the diaphragm, and the conductive paint will put put to rest any worry of discontinuity that the use of glues might bring up. A repaired stator tape looks something like this...
- Examine the outer surfaces of the bass panels. If there is any white plastic showing through, then both the conductive paint layer and the gray paint layer have been lost (somehow). A typical panel in need of this sort of repair is shown here:
- Repair this type of problem, or where white plastic shows through, by gently cleaning loose material away with very fine sand paper (600 grit or finer) and then re-painting (firstly) with conductive paint (See Parts Section) and then covering with a gray finish. The gray paint is there to stop the panel reflecting too much light back through the grille, and it really doesn't matter if the gray doesn't match. You could re-paint the whole stator for a nice, cosmetic effect, if you like, but be careful that you NEVER allow paint of either sort to drool into those perforations!!! N.B. There is one exception to this rule. Around the rivets, simply sand back, and re-paint with gray paint only. Extending the conductive paint to the rivets will only increase (or create!) panel leakage.
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 tape breaks, corrosion, or conductive layer discontinuities.
- Cut a piece of 12 mm Mylar which is about 10 cm larger than the bass 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 (not recommended), rub the powdered graphite into the Mylar® as hard as you can. 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.
- 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 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 glue the membrane to.
- Apply a thin (1/8th inch) bead around the perimeter where the old brown glue was. Do not put epoxy on the metal tape. The bead should be run between the tape and the outer edge of the stator.
- 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.
- 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 both sides 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, although this is not as critical as it is for treble panels.
- Using 12mm x 3mm (M3) bolts, hex nuts, and two washers (one each side), bolt the stators together. N.B. Do not forget to run the front stator high voltage wire back through the holes in each stator and re-solder it to the tab on the front stator. Tighten each nut and bolt firmly, but not too tight.
- Run a line of PVC tape around the perimeter of the panel, covering the outermost bolts. This is added insulation to prevent leakage to the frame. Everything conducts (it seems some times) at 6 000 Volts.
- Take the refurbished Dust Covers and identify the rear cover. This will need 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 small phenolic connection board to the bottom of the rear dust cover - it bolts to the bottom of the frame with two small (12mm x 2mm) bolts. You may have to re-drill one mounting hole to do this if the new framing arrangements overlap the original holes.
- Melt holes in the dust cover film to match the holes in the small board, and run the HT and signal wires through the appropriate holes. LEFT - REAR Stator (white). MIDDLE - EHT Connection (red). RIGHT - FRONT Stator (black). N.B. The older speakers have all white wires, so watch what you're doing!
- Tape the dust covers in a “sandwich” with the rebuilt panel between them using 2" PVC tape. Make sure there are no gaps, or there will be panel leakage somewhere.
- 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.