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Web: http://www.pearl-hifi.com |
No. 200, 2137 33 Ave. SW; Calgary, AB; T2T 4X3 |
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Ph: +.1.403.244.4434 Fx: +.1.403.244.9026 |
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E-mail: custserv@pearl-hifi.com |
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Precision Electro-Acoustic Research Laboratory. |
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Hand-Builders of Fine Music-Reproduction Equipment |
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This document has been prepared as a public service and is intended for unrestricted distrubution.
Any and all trademarks and logotypes used herein are the property of their owners.
Please note that the links in the PEARL logotype above are “live” and can be used to direct your web browser to our site or to open an e-mail message window addressed to ourselves.
♦ PDF Cover Page ♦
Quad Electrostatic Loudspeaker
Service Data–Model ESL 57
General
The notes in this section are for the guidance of the engineer who has some experience of carrying out repairs to these speakers together with the necessary proper materials for the repair.
The QUAD electrostatic speaker consists of five components: two bass units, one treble unit, an audio transformer unit and an EHT supply unit. If any repairs are necessary, it should be ascertained which of the five components is the cause and that component should be either replaced complete or repaired as appropriate.
Fault Diagnosis
Loss of Sensitivity
Check the EHT voltage, which should be: Bass 6Kv + 7%; Treble 1.5Kv + 7%.
If low, check, by disconnecting, whether due to leakage in speaker unit or fault within EHT unit. Voltages must be checked only with electrostatic meters, as the current drawn by other types may itself damage the rectifiers.
Distortion
1.Make sure that the speaker is really at fault by comparison with a second electrostatic speaker, using a QUAD amplifier.
2.Check EHT voltage.
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3.Suspect intermittent breakdown in speaker units.
4.Suspect intermittent breakdown in audio transformer unit. (Note: a fault in this unit is very unlikely).
No Output at All
Suspect EHT unit or audio transformer unit after checking more obvious things like external connections, not forgetting the leads under the transformer unit connecting the input sockets.
Background Noise
One cause of background noise in the electrostatic loudspeaker is internal discharge of the EHT supply at times of high humidity, or high voltage, or both. This may be reduced by lowering the EHT voltage and a tap is provided on the EHT mains transformer for this purpose.
Before the EHT unit is touched the mains should be completely disconnected and the loudspeaker left to stand for two hours to ensure it is completely discharged.
The connections to the EHT rectifier block are normally taken from tags marked Common and 61 0V. The latter is the righthand end tag and next to it is a blank tag marked 590V to which should be transferred the lead normally connected to the 61 0V tag. Background noise may also be caused by discharge of the EHT from points external to the loudspeaker units, at the tags on the rectifier block for example, if a hair of felt or piece of fluff comes in reasonably close proximity to that point, or if a spike of solder or sharp point of wire permits corona discharge.
Where EHT leakage occurs via a bass unit, this is sometimes found to be discharging from one of the eyelets around the periphery of the bass unit plates, probably to one of the aluminium brackets. In such cases a satisfactory repair can be effected by slitting the polythene tape round the edges of the unit, opening the dustcover frames and insulating the leak by applying a single layer of similar polythene tape all round the periphery of the internal plates, on top of the existing sealing tape, and reassembling the dustcovers, again with polythene tape.
Other internal failures of insulation will probably necessitate replacing the loudspeaker unit affected.
Mechanical
If the dust seal covers should be torn, it may be necessary to replace the complete unit as there will have been ingress of dust, which causes loss of sensitivity of the unit concerned.
Dismantling the Speaker
The loudspeaker should be switched off for about two hours before the grilles are removed, so as to ensure the EHT unit has completely discharged.
The component loudspeaker units of the QUAD electrostatic loudspeaker must be handled with the utmost care, partly because when not supported by the rigid frame of the cabinet they are more liable to physical distortion which would reduce the small internal clearances, and partly because the dust covers are necessarily made of very thin and therefore fragile plastic film.
At the rear of the treble unit are four pins located in the wooden struts of the cabinet, and as these represent an additional hazard to the dust covers of the treble unit, the positioning of this unit requires particular care.
Soldered joints should be smoothed and rounded and all spikes of solder, wisps of wire, etc., removed as these would tend to cause arcing at the high internal voltages used.
Removing Front and Rear Expanded Metal Grilles
The rear grille is held only by the screws around its periphery. For the front grille it is necessary first to remove the side mouldings, the staples through the metal beneath them, and the screws under the baseboard.Then the bottom edge of the grille is lifted gently outwards and upwards until the top rear edge may be slipped out of its groove in the cabinet, when the whole grille will be free. Care must be taken not to strain the top curved section during removal or the metal may split.
Replacing the Front Grille
Replacement grilles are normally supplied cut and partly preformed so the procedure is as for refitting an existing grille. It may be found helpful when working single handed, having inserted the top back edge of the sheet into the slot in the cabinet, to hold the bottom edge of the grille under slight tension to the bottom of the wooden frame by means of elastic bands and simple hooks of wire, such as an opened paperclip, and then to use a bar of wood slightly longer than the width of the sheet, and with a good flat face, to bed the grille to the frame by moving the bar progressively down the face of the grille, tacking the sides as you go, finally securing the bottom edge with the screws removed from the old grille. Do not forget to fasten the earthing lead to the grille.
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