Refurbishing ESL 57's


DISCLAIMER

The Quad ESL 57 operates with internal lethal voltages. DO NOT attempt any servicing work unless you are totally confident about what you are doing.

The sections contained within this web page are :

Power Supply Faults

Problems with the power supply rank about equal in number with diaphragm damage. Its not wholly surprising that this is the case as a drop in the output of the power supply causes reduced output from the speaker. Users then compensate for this by increasing the volume and eventually arc the treble panel. The first thing to note about the power supplies is that they will either be of the epoxy type or of the beeswax type. The latter are serviceable. The former are only good for the bin if they are found to be low on output. It must be stressed that measurement of the power supply should be done with the panels disconnected from the supply. This is to find the output voltage under a no load condition. If it is subsequently found that the panels when connected are loading the supply down, the problem will be elsewhere. It must also be stressed that a probe specifically designed for high voltages be used when measuring the power supply.


The two types of power supply. Wax type on the left, epoxy type on the right.
Later replacement power supplies were just a PCB dipped into wax.

The power supply itself is only made up of 4 different electronic components. These are :

Of these 4 components, the ones which fail most frequently are the diodes and the two input resistors. The diodes are easily replaced with modern alternatives. The two input resistors can also fail. Quad used cheap carbon resistors which have a tendency to go high in ohmic value. If your refurbishing the power supply its just as well to replace these as a matter of course. The other components immediately next to the diodes on the PCB are the capacitors. These do not fail. The only failure of a capacitor which I have seen was one of the lead off wires having snapped off.


Naked power supply pcb.

Removal of the power supply should be done with extreme care. There's nothing worse than seeing a blob of solder go straight through the dust cover on its way to the diaphragm - result you will need a new diaphragm. The power supply block is held on with two nylon screws. If your careful you can remove these and de-solder the wires without removing the whole mains assembly. If your not happy remove the whole assembly for safety. Make a note of what wires come from where. There should be a fat red wire carrying the 6000 volts for the bass panels, a thinner red wire carrying the 1500 volts for the treble panel, a red and a black wire from the mains transformer and finally a black wire which goes back to the audio transformer. It should be obvious but you must leave the speaker unplugged over night to make sure there is no residual voltage lying around inside the speaker. My soldering iron is earthed so the first wires I disconnect are the panel power supply wires. If there's any voltage left in the power supply the earthed iron will kill it. Also it is advisable to disconnect the speaker from your amplifier. The audio transformer has a stepup ratio of 110:1 with no current limiting. This means that if you touch the wrong wire and theres music playing the next person you will see is Gabriel or Hadies depending on what sort of life you lead. In fact the audio transformer is much more dangerous than the power supply. I have had a few shocks from the power supply and I am still here to talk about it. Once the main power supply block has been removed you will be ready to melt the wax out. This is easy to do - just put it in a small tin and put it in an oven. I usually use a temperature of about 150 degrees C. You don't want the oven too hot otherwise the wax will burn. Too cold and it wont melt. Its probably best to start off a little cooler than 150 and gradually turn the heat up. Keep an eye on things as you go. When all the wax has melted and everything has cooled down you can start removing the diodes and the input resistors. Take them all out but make a note of which way round the diodes go. Replace all the diodes and resistors with new components. When I do this I find that the leadout wires for the new diodes are slightly larger diameter than the original ones so I have to drill out the holes in the PCB a little. Put the PCB with all the new components back into the little box and screw the nylon screws back into the side. Start pouring the melted wax back into the box but only fill it about half an inch at a time allowing it too cool and solidify each time. This allows the wax to set and contract. If you pour it all back in one go you will get gaps caused by shrinkage all round the edge of the wax and possibly voids in the wax where the diodes are. Refitting the power supply is simply a reversal of dismantling. You should check the output of the supply before you reconnect the panels just to make sure you have done everything OK.


New power supply (as made by me) ready for fitting.

The only other fault I have seen with the power supply is the mains transformer itself. These rarely fail. If you find that you have a dead mains transformer the only thing to do is get a local company to wind a new one for you. Show them the old transformer and just ask for a replacement. It shouldn't be too expensive.

Dust Cover Problems

Many problems associated with the ESL57 are to do with a drop in the high voltage on the diaphragms. Most of the time this is caused by a faulty power supply but can be caused by a failure of the dust covers which are attached to the panels. The dust cover is nothing more than a very flimsy wooden frame which has a piece of film stuck onto it.


Split Bass dust cover with a previous repair.

Tears and splits in the dust covers are bad news. The high voltage inside these panels acts as a dust magnet. You will probably have noticed when you took the back off the speaker how dusty they were. Any ingress of dust will affect the high voltage charge on the diaphragm. In my experience a torn dust cover will either have a small tear or a split from top to bottom. A small tear can be fixed with some sticky tape. Try to use some decent sticky tape and not the cheap and nasty stuff which goes yellow after 6 months. I would advise that you carefully clean around the area of the split to get a good contact for the sticky tape. If you do it right the repair will be good for 20 years. I have seen speakers with this type of temporary repair that have been like it for 15 years. Once the sticky tape is on its staying there. A bad split will need a new dust cover. This will need the panel to be removed from the speaker. See the section on diaphragm faults on how to remove panels.

The dust covers themselves are made of the same film as the diaphragm in the panel to which they are fitted. Bass panel dust covers are 12 micron PVC film (Saran wrap/cling film) and treble panel dust covers are made from 6 micron Polyester (Mylar/Melinex/Hostaphan) film. The front covers on bass panels are sprayed gun metal grey to reduce reflected light coming out from behind the front grill. I use 12 micron Polyester film regardless of which dust covers I am repairing. It doesn't make any difference. The most tricky part is getting the tension on the cover just right. If the tension is to low it will rattle and resonate and sound horrible. I use a hot air gun (as used for stripping paint) to shrink the film. This takes some practice as too little heat and the film wont shrink enough, too much heat and you have to start again. Tensioning the cover should be done with the panel re-assembled and fitted back into the speaker. The cover should also be a uniform distance away from the panel inside all over its surface. If the wooden frame has become twisted in any way or the diaphragm is closer to the panel in any places you will probably get an arc jumping across the gap from the head of a rivet. This sounds a lot worse than it actually is - there really is no excuse for it. I use epoxy 2 part glue for holding the dust cover onto the frame but any good adhesive will do. Once the panel is put back in the speaker and the covers are tensioned properly the front covers will need to be sprayed with metallic grey paint to reduce light reflecting back out the front of the grill.

Fixing the dust covers back on is a little tricky. What your looking to do is fix both front and back covers on with no gaps between the wooden frame and the panel itself. You also have to try to avoid putting your fingers through the new covers. The covers are held together and sealed with heavy duty sticky tape. I use 2 inch wide heavy duty tape. See the tools section for information on this.

One important thing to look for when the dust covers have been removed from the panel is the insulation around the edge of the panel. Quad used nickel plated brass rivets to hold all their panels together. The brass rivets around the edge of the panels needed to be insulated to stop leakage to the frame of the dust cover. This insulation was two layers of the same sticky tape that they used to hold the dust covers onto the panels. Over time some rivets can corrode and cause the insulation to fail. If this happens you will probably see areas of corrosion on the wooden dust cover frame. This will put extra strain on the power supply and needs to be fixed. This is simply a case of replacing the damaged insulation and removing as much corrosion as possible. In extreme cases the rivet will disintegrate and should be replaced with either another rivet or an M3 nut/bolt.


Corroded rivet causing high voltage loss.

Diaphragm Faults

Inside the speaker there are two different types of panels. There are the bass panels (2 of these) and the treble panel. As there are differences between both these panels I shall cover them separately.

By far the most damage that can be caused is over exuberance with the volume control and the part which always suffers is the treble panel. The design of the treble panel is such that the stators which carry the audio signal are only 1.5mm apart. This means that if the voltage on the stators exceeds 2200 volts an arc will jump the gap and put a hole through your diaphragm. Quad designed the ESL57 protection circuit to clip at 2200 volts so this is a safe working voltage. However I have still seen damaged treble panels from speakers which have protection circuits fitted. Protection circuits reduce the likelihood of arcing but they don't eliminate it. This is why I make protection circuits to clip at 2000 volts. Usually when an arc jumps the gap it not only causes damage to the diaphragm but usually burns a hole through the dust covers also. From all the treble panels I have repaired the most likely points for an arc to jump are at the points where the solder tags are riveted onto the panel and halfway up the panel where the bend in the panel is at its greatest point.


Badly arced treble panel.

Repairing any panel is boring, usually involves getting very dirty and is very time consuming. Removal of the panel involves first removing the front and back speaker covers. The rear cover is held on with small wood screws around its edge. Be careful with the rear cover as its edges are usually razor sharp. You should find an earth point at the bottom of the cover. This should be held on with a small nut and bolt. The front grill is held on with staples down the sides and screws along the bottom. The staples are removed by un-screwing the wooden side rails and levering the staples out with a screw driver. As with the back panel there should be an earth point along the bottom of the grill which will need disconnecting. When the front and back covers are removed you will probably need a vacuum cleaner and a small brush to remove years of dirt and dust. All the panels are held in with a small aluminium bracket at the top and bottom of the treble panel. These are held in with 2 screws. When these are removed the panels are free to come out. I usually find that the panels have stuck together (the sticky tape goes all yucky) so a small amount of persuasion can be necessary. Do not try to remove the panel until you have electrically disconnected the panel. The treble panel has 5 wires going to it - 2 brown, 2 blue and a single red wire. The red wire is the 1500 volt diaphragm voltage and the other 4 are the audio signal. It is very important to mark which brown or blue wire comes from where. It is important because if you get either the brown or blue wires reversed you will reverse the phase of the panel. Access to the brown and blue audio wires is underneath the audio transformer. This is removed by unscrewing the 4 bolts from underneath the speaker. Be careful not to strain the wires as you remove the transformer.

Once the panel is out of the speaker you are ready to start dismantling the panel. The first part of this process is to remove the four nuts and bolts which hold the dust covers close to the panel. Exercise some care as these are removed as the dust covers can tear easily. Once all four nuts, bolts and felt pads have been removed you are ready to cut the dust covers off. This is done with a sharp knife. Its simply a matter of going round the edge of the panel and cutting through the tape which holds the dust covers onto the panel.

Remove the 4 screws and felt pads


...and then split the panel with a knife.

Next comes the boring bit. The stators of the panel are held together with 68 nickel plated brass rivets. The rivets around the periphery of the panel will be covered with insulating tape. All these rivets will have to be removed. Before you do any of these its a good idea to notice where the red high voltage wire is connected to the panel. The red wire itself comes in and is soldered onto two eyelets. These are held onto the panel with two small nuts+bolts. Both these bolts can be removed. I usually find that these bolts have become corroded over time and a small drop of light oil works wonders. If the bolt shears off (and they do, frequently) I replace them with stainless steel M2.5 nuts and bolts. They are expensive but worth the extra as they don't corrode. This reveals the actual high voltage connections into the panel. Notice how similar they look to the panel rivets. DO NOT REMOVE THESE RIVETS !!! These four rivets (2 front, 2 back) do not hold the panel together and MUST NOT be removed. Mark them in some way to remind yourself. Removal of the panel rivets can now begin. I do the centre two rows first. This is because it means leaving the removal of the tape around the outside as late as possible. When you remove this tape you will see why - it will probably leave most of its sticky stuff stuck to the panel. I have yet to find a quick and easy way to remove rivets. DO NOT try drilling out the rivets - they will spin in the hole because the riveting is usually quite poor. Grinding the head off the rivet is also a bad choice as it will melt the plastic stator. I find a small pair of pliers or cheap wire cutters works best. You will notice that on one side of the panel the heads of the rivets look a proper dome shape, while on the other they look a little untidy. This is the side you want to work on. Using my pliers, I first squeeze the rivet one way then rotate the pliers through 90 degrees and squeeze the other way. This should squeeze the riveted end of the rivet so you can push it back through the hole. Whatever method you use, after doing 68 of them you will wish you had never started. You may get a few rivets which are tight in their holes. DO NOT use excessive force to push the rivet back through the hole. You will only end up cracking the plastic. If you find an awkward rivet that refuses to budge do not get angry with it, put the panel down, go and make a cup of tea, drink the tea and when your frustration has gone return to the panel. Patience is what you will need plenty of in order to do a good job.


Exposed panel - note the black arc point halfway up the centre cavity



Note the position of the HT connections - the rivets underneath the connections are NOT to be removed !!!



First squeeze the end which has been riveted over one way....



...then rotate the cutters through 90 degrees and squeeze the other way



...and finally split the panel in half


When all the rivets have been removed the panel should come apart with ease. When the panels splits apart one of the stators will have the diaphragm stuck to it. The other stator will be clean. You will now be looking at the inside of the panel and probably at some nasty arc points. You will probably also notice the coating on the diaphragm. The coating on an old diaphragm may be transparent or milky white or somewhere in between and you should see how little care was used when the coating was originally applied to the diaphragm. It sometimes looks to me like someone got an old rag and wiped it on like they didn't care about what they were doing. In any case it doesn't matter as this diaphragm's life has come to an end. You want to remove this diaphragm without leaving any of it still attached to the stator. I find a small screw driver works best by finding a place where you can slide it in between diaphragm and stator and working your way round. Try to remove the diaphragm in as complete a piece as you can. Once its off the first thing I do is wash the plastic stators. I use washing up liquid and a sponge to remove any debris left by the stator or the arcing.


Carefully remove the old diaphragm



Nasty arc point !!!



Wash the panel with a soft sponge


At this point you will have the panel totally disassembled. Its a good idea at this point to consider the modification I make to the dust covers. The problem is that when you replace the rivets with nuts/bolts it will make the overall thickness of the panel about 5mm thicker. This may not sound much but it makes it very awkward to refit the panel into the speaker. So what I do is drill some clearance holes in the wooden part of the dust covers. To do this I mark where the head of the bolt or the nut will foul the wooden frame of the dust cover. It is therefore easiest to do this when the panel is apart which it should be at this stage of the process !!

coming soon pic of marking holes

Once the stators have been cleaned ready for the new diaphragm you can start preparing the diaphragm itself. There are two areas that I am not going to cover here. Firstly which coating to use and secondly how to tension the diaphragm. Much has been written about various coatings - in fact it could probably be the basis for writing a book. Personally I use a coating which is almost identical to that of Quads original. As far as the tensioning is concerned you can use whatever method you feel easiest with. I use a rectangular frame to tension the diaphragm. Quad used weights hanging off the edge of the film. Others use heat to shrink the film. Whatever. It all has the same end result.

UPDATE See the section What no diaphragm coating material ??? to see what coating I currently use.

coming soon pic of tensioned diaphragm

You should now have the diaphragm ready for gluing into the panel. This is now your last chance to make sure the panel which you glue the diaphragm too is in good order. Quad always glued the diaphragm to the rear panel. Things to check for are :

  1. is the grey paint on the inside of the panel un-damaged ? if it is re-insulate it with corona dope. DO NOT UNDERESTIMATE HOW IMPORTANT THIS CHECK IS - even a small spot of chipped grey paint could cause an arc to jump the gap
  2. is the audio connection in good order ? I sometimes see burning round the rivet which means the connection to the panel could be damaged
  3. are there any high spots on the inside surface of the panel (usually caused when arcing occurs). If there are smooth them down and re-insulate the panel

Once you are satisfied the panel is OK you can apply the glue. I use Araldite but any good epoxy will do. I put a bead of glue around the edge of the panel about 5mm wide. You don't need much. I always mix too much. Lay the panel on some newspaper (in case you get excess glue squeezing out) and lay the pre-tensioned diaphragm on the panel. With the diaphragm laying in place, go round the edge of the panel squeezing the glue out to the outside edge. I then put some sheets of clean paper on top of the diaphragm, then a towel and lastly some magazines to weight it down. I use a towel because it takes care of any possible surface unevenness on the panel. Put some old magazines on the towel to weight it down. Depending on which type of glue you use you will now have to wait until it sets. My Araldite takes 24 hours.

The last step before re-assembly is burning the holes through the diaphragm through which the bolts will go. These are the bolts which hold the whole panel together. I use a hot soldering iron to create the holes. This has the advantage of sealing the edge of the hole and thus reducing the likelihood of the diaphragm tearing. You may find that some glue has gone into the holes - do your best to clean the excess glue out without compromising the diaphragm.

coming soon pic of hole burning

Once all the bolt holes have been made you are ready to re-assemble the panel. Start by putting just a couple of bolts through the front stator and securing the nuts on the back. Don't tighten these just yet as some freedom to move will be needed to line up all the other holes. Go round all the holes putting in bolts and leaving the nuts loose. When all the nuts/bolts are in place start tightening the nuts from the center outwards. You don't need the bolts super tight - just tight enough to pinch the front and rear stators together. Finally re-connect the high voltage using the two bolts. At this point I am ready to test the panel. Its still suspended in the stretching frame but thats OK as it means its insulated from everything. Once I have tested it I disconnect the panel and cut it out of the frame. To stop the nuts shaking loose I put a little drop of thread lock onto the nuts. I only do this on the inner two rows of nuts as the nuts round the edge of the panel are held by the insulating tape. I then put the tape round the edge of the panel (as Quad did), vacuum out any dust which may have found its way into the panel and re-assemble the dust covers. If you have done the modification to the dust covers the panel should be the same thickness as it was originally.

coming soon photo's of cutting out panel, connecting high voltage wire, tape round the edge, thread lock, dust cover mods

You can now give yourself a pat on the back. If you have done everything carefully you will have a new panel which will give years of service. Before its put back into the speaker I give it a final test. This is easy for me as I have spare power supplies and audio transformers. There shouldn't really be anything wrong with it but its just for peace of mind. I recently fixed a treble panel which tested fine out of the speaker but when using the speakers own audio transformer caused a horrible crackly noise to be produced. It turned out that one of the resistors on the audio crossover had a hairline crack and needed replacing.

Audio Transformer/Crossover Faults

First thing to say about the audio transformer is that it is potentially the most dangerous part inside the speaker. YOU MUST DISCONNECT THE AUDIO FROM YOUR AMPLIFIER before you start working on the audio transformer. The housing itself contains two separate parts - the step-up transformer itself and the audio crossover to feed each individual panel.

The audio transformer is used to step the voltage up which comes from your amplifier. The transformer has a single primary and two secondary windings. The primary winding is what you connect your amplifier too. The secondary windings eventually connect to the bass and treble panels (after the signal goes through the crossover). This means that all the components in the crossover have to be rated at high voltages. The secondary windings are nominally 55:1 and 110:1 for the treble and bass panels respectively. This means that at the maximum input voltage of 36 volts you are getting about 4000 volts on the stators of the bass panels !! The transformer is totally encased in wax. This gives the transformer very long life. In fact I have only ever heard of 1 failed audio transformer. If it has failed you may be able to get it re-wound - otherwise you can get a new one from Quad in Germany.



Naked transformer, thanks to Sheldon Stokes for this.

The crossover circuit is a simple yet effective set of resistors and capacitors. Speakers with serial numbers after 16800 benefit from a simple modification of a resistor and a couple of capacitors (see Quad Service Manual for the differences) to improve power handling. If your speaker doesn't have these it is a very simple modification to make. The most common fault on the crossover is a fractured resistor. Quad used ceramic carbon resistors which can fail due to a hairline crack. In my experience the capacitors don't fail. The easiest way to check the crossover components is to put a DVM across each resistor. In some cases the resistor has a capacitor in parallel with it. This can be a nuisance - if you actually want to check the resistor you will have to un-solder the capacitor. Of course if the resistor is open circuit you don't need to un-solder it. Just put the DVM across them and you should see high resistance.

coming soon pic of early and later type of crossover circuit


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