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Old 9th December 2005, 11:09 AM   #1
stevers is offline stevers  France
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Join Date: Nov 2005
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Default Tizzy FREDs

This might help someone, I've spent days trawling the web (and Quad) for answers. I got a lot of help from Wpkae87@aol.com, thanks if you're about.
My ESL63s went more and more low output and tizzy over a couple of years.
I nearly rebuilt the elements but couldn't see anything wrong with them, arcing etc.
The big problem was measuring the EHT (6Kv). With a simple probe (500 Meg in series with 10 meg, DVM 10 meg input impedance across the 10 meg, giving 100:1) I was getting a reading of 1.8Kv and no-one, including Quad, could tell me what it should be with that loading.
Eventually I built a slightly modified version of the SDS EHT supply, using 3P02 diodes (3Kv PIV) to avoid lots of 4007s.
This design lends itself to simple Veroboard construction and can be made easily to fit in the existing space. I transplanted the little neon board although I've no idea what jujube medicine it does. The EHT off this board measures 4Kv with my probe and the speakers sound as new. Hope this is useful.
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Old 9th December 2005, 11:27 AM   #2
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I have a Testec HV probe with an input impedance of about 1000M, good Quad supplies read about 4.6kV.

The neon lights when the voltage across it reaches about 90v. The rate at which it flashes indicates something about the leakage of EHT from the diaphragms. The Quad diagnostic flow chart implies that a flash rate 'much greater than once per second' would be an issue. I presume if you can see flashes rather than a flicker then that's OK.

Paul
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Old 9th December 2005, 01:03 PM   #3
stevers is offline stevers  France
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That's helpful re the volts. The neon now flashes once every ten seconds. I don't quite understand why there isn't a faint 'pop' (90/6000 volts worth) each time it squirts its 90V worth but, presumably, the electrons trickle slowly all over the membrane like Golden Syrup.
Was there one of these in the original ESL ?
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Old 9th December 2005, 05:24 PM   #4
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Aside from being a leakage indicator, the neon lamp also isolates the diaphragms from the EHT supply except for the brief time they are being charged.

The original ESLs did not have this feature, but there are modified boards available from Anders Enquist which include the circuit.

Enquist modified EHT supply for ESL57
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