Here is a list of faults which I have come across while repairing ESL 57's. This list is by no means exhaustive !!! The most common problems are listed first with the more unusual following.
By far the most common failure on ESL 57's is over exuberance with the volume control. This can be caused either by headbangers listening to the latest Black Sabbath album or by owners compensating for a problem with the speaker. What can happen is that over a long period of time (20 years) the power supply can become tired and the panels themselves become less efficient as the coating material wears off. To compensate the volume control is turned up and the treble panel arcs. Drop in output can be caused by other failures. I had one pair of speakers and in one of the speakers a bass panel diaphragm had split causing massive high voltage drop. This had a dramatic effect on the loudness of the speaker and caused it to be only about half as loud as it should have been. When the owner delivered them to me he actually complained that they seemed to be 'clicking' when he turned the volume up. I suspect this was his amplifier clipping and I am even more surprised that he didn't arc the treble panel.
Treble panels arc internally when the voltage between the two stators reaches 2200 volts. They also suffer from the conductive coating deteriorating over time. Bass panels don't generally suffer the same problems as treble panels but they do have problems of their own. These are mostly insulation problems. Due to the way the high voltage power supply is made, any high voltage leakage on the bass panels will drag the treble panel voltage down also. Its not surprising that bass panels suffer from insulation problems as any contamination at 6000 volts will be a potential leakage path. Usually insulation problems can be cured by very carefully vacuuming out any dust from within the bass panel and using corona dope to insulate the panel where leaks are found. Sometimes bass panels rattle - this is caused by loose rivets and the only cure is to replace the rivet. The coating on bass panels doesn't generally fail as it does in treble panels as the bass panel diaphragm is made from a different material.
Faulty Power Supply
Equal to panel failures are the number of speakers I have seen with faulty power supplies. For whatever reason the diodes inside the power supply do get tired over long periods of time. Where the supply can be refurbished this is a simple case of replacement. What will be needed is a means of measuring the high voltage - A MULTIMETER OR DVM CANNOT BE USED TO MEASURE THE HIGH VOLTAGE DIRECTLY. You must use some kind of high voltage probe to measure the high voltages. Another less common power supply problem is the resistors on the input to the power supply. If either of these resistors fails or goes open circuit the output voltage of the supply will be affected. Of the two resistors it is usually the 330KOhm which fails but it could be either. Depending on the age of the supply, they are most likely to be carbon resistors which do have a tendency to fail and go very high resistance.
Faulty Internal Cabling
I have seen some horrible examples of owners fiddling about inside the speaker and really making a mess of the internal cabling. The wire used on the bass panels is especially vulnerable as it does tend to break easily. Also, the connection around the audio inputs on the bass panels can become very poor over time. As well as these problems I have also seen the high voltage bass panel supply pinched between the audio transformer and the wooden frame causing massive high voltage drop and in one case an incorrectly wired up treble panel. In this case the speaker was known to have a new treble panel so I couldn't figure out why it sounded wrong. I noticed that there was no Quad servicing label so I can only assume that the treble panel was bought new from Quad and fitted (wrongly) by the owner. A simple soldering job and the speaker was singing.
Less Common Problems
I am currently working on a power supply which was tested by Quad and they stated the mains transformer was open circuit. It turns out that the speaker was banged hard against a wall with the mains lead plugged in. This caused the mains socket to fracture and in turn caused a short length of wire to become detached from the back of the mains socket. Result no output from the mains transformer and no high voltage. Cure - 30 second solder job.
Dust covers can have tears and in some cases split from top to bottom. I have seen numerous small tears in dust covers but I recently serviced a pair of speakers in which both bass panels had split rear dust covers from top to bottom. This was caused by the wooden frame of the speaker being loose. In the bottom corners of the frame are two triangular braces and both were loose. This caused the whole speaker to move like a parallelogram and caused the split dust covers. The frame was fixed by removing the bass panels and tightening the screws which hold the brackets in place. The bass dust covers were replaced with new dust covers.
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