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— QUAD ESL57 Loudspeakers Rebuild —

This time no tweaks or mods but a simple refurbishment of a 1960s pair of QUAD ESL57 electrostatic speakers following the QUAD instruction manual and applying a bit of acquired knowledge and common sense


QUAD ESL Loudspeakers damaged treble panel


Both speakers had very low output which was mainly due to the HT units having failed so the Treble panels which had obviously arced over in past were no longer arcing — Not much output but also no arcing

These speakers came with a known history so although in very bad condition how they got that way was at least known — The Treble panels had been replaced by QUAD several times upto 1970 when a Bass unit was also replaced — The owner had purchased two spare Treble panels which were presented with the speakers

Note the old Treble unit above which was replaced by QUAD has no silver grey paint overspray which the originals would have had — It has been painted grey by hand around the edges to cover the view of the light wood frame through the grille

On the left the ESL57 frames with the panels - HT units and transformers removed — By completely stripping them down they could be thoroughly washed and the frames be repainted

It may seem like a lot of work but the treble panels needed replacing as did the dust covers and PVC tape on the bass panels and the HT units needed to be repaired so why not ?

QUAD ESL 57 loud speaker Frames
QUAD ESL57 Loudspeaker treble panel
Because the owner had "blown" many treble panels since new he had purchased 2 spares in 1970 when QUAD fitted new treble panels and the mod intended to prevent them from getting damaged again

Having New old stock Treble panels would make this repair easier but the Bass Panel Dust covers were split and one panel was possibly damaged . . .

The failed HT units had also been replaced by QUAD in 1970 – I Repaired them using better high voltage resistors and uprated modern high voltage diodes — The cases had originally been filled with bees wax which was easy to remove although it has to be heated very slowly to avoid melting the plastic cases

After testing for some time the HT units were potted with Robnor Resins polyurethane compound which is specially designed for high voltage and also looks good in the black plastic cases — New IEC 6A input connectors were fitted and we have effectively two new units

QUAD ESL 57 Loud speaker HT supply
QUAD ESL57 Loudspeaker Audio transformers HT supply
The transformers tested okay but the resistors were out of tolerance and they all were changed for higher voltage types

Although QUAD had put some resistors in series the voltage rating of each could have been higher — The cases were cleaned and resprayed and the original wiring cleaned up well

The transformers had the retro fit treble panel protection to which I fitted slightly different component values based on previous work with my own ESL57s — thus the 270kΩ resistor is 2 x 150kΩ in series and the C value changed

With repaired HT power supplies and transformers the bass panels could now be checked individualy — Only one panel showed signs of having a fault and this was cured with conductive paint around the HT rivets — All the bass panels had their dust covers removed and were cleaned and fitted with new dust covers and wide polythene tape

The fronts of the panels (that's the back of the treble panel) were lightly sprayed silver grey as the originals would have been

Each panel was again tested before being assembled back into the newly painted frames

QUAD ESL57 loudspeaker Bass and Treble panels
QUAD ESL57 Loudspeakers  repaired
New HT units and panels in place and transformers ready to connect — The treble panels are wire ended and connect directly to the tags of the transformer — The bass panels have solder tags and are wired in parallel and in phase and then connect to the transformer with HT cable
The dust covers are very thin and easily damaged by solder or flux splashes or the end of the wires

When fitting the wires between the bass panels card helps prevent solder splash damage and the ends of the stiff HT wires are taped to prevent them from cutting the dust covers if they move

Fitting QUAD ESL57 loudsppaeker Bass panel
QUAD ESL57 Loudspeakers Audio Transformer
The Transformer being connected to the panels — Note the dust covers at this stage are not stretched
Testing the ESL57s before refitting the cleaned felt pads behind the Treble panels and applying a final layer silver grey paint to the front

The dust covers have been heat shrunk into place now that units have their curvature due to the frame and a few pulses and tones were passed through them to establish the frequency response and output level were okay

QUAD ESL57 Loudspeakers dust covers heat shrunk
QUAD ESL 57 Loudspeakers almost finished
The light spray of silver grey paint makes the appearance looking through the grilles more uniform

Other than that I don't know why the were sprayed but it is an original feature of the early ESL57s

Finished apart from the front and rear Grilles — The front grilles were a bit dented but this was rolled out using a "Rolling Pin"

The "horse hair" matting on the rear covers was still held in place by the tar blobs and they were vacuum cleaned with a "Vacuum Cleaner "

Looking and sounding clean and bright and for a few more years I hope

Refurbished QUAD ESL57 Loudspeakers rear

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