Do not repair what is not broken ! But read this: https://theaudiobeatnik.com/electrostatic-solutions-original-quad-esl-loudspeakers-introduction/
The speakers degrade in three ways:
- The conductive coating degrades reducing efficiency typically 2-3dB. This takes decades to have any impact.
- The carbon comp resistors in the filter network go high in value. The network carefully balances the output of the bass panels relative to the tweeter panel. When the resistors go high it reduces the output of the bass panels upsetting the balance between the panels. Loss of bass leaned out midrange, rather hot on the top is the sonic result.
- The worst way they degrade is the film used in the bass panels. Quad only used Mylar film in the tweeter panel. I imagine Mylar was quite expensive in the 50s-60s.
The film that was used in the bass panels was polyolefin. Polyolefin has a lot of plasticizers in the film Mylar does not. Over time the plasticizers leech out of the film causing it to become brittle and tight. This raises the panel resonance. I have seen it raise as high as an octave. This kills bass response the reason some say they have no bass capability. They don’t in this condition!
Even if one managed to find a NOS Quad ESL, never used, it would no longer meet spec. There is no way to stop the leaching of plasticizers over time whether the speaker is in use or not.
It’s quite hard to get this information: it’s not part of official specs. My measurements on a rebuilt treble cell:
86dB/2.83V/1m@1kHz (sensitivity). By the way, I read this figure in various locations: in line with expectations. But some other sources say few dBs less. Maybe due to aging cells ?
88dB/1W/1m@1kHz (yield). Thus 12.5Ohm at this frequency (vs about 10Ohm expected).
Handling deep bass
ESL 57 may not handle correctly heavy low bass frequencies at high volume.
What is ‘high volume‘ ? Maximum output is 100dB at 2m officially, between 70 and 7000Hz. Let’s consider this is overstimated of 3dB, and thus 103dB at 1m. This makes 20Vrms max, exactly voltage limiting option of a Quad 405.
Maximum output at 2m within 50 and 10000Hz is 93dB. This may not change max voltage at 50Hz since frequency response drops significantly at this extreme frequencies.
All that said, stay below 33Vpp (11.7Vrms) or 17W(8ohm) and/or install clipping option.