Ah yes, the crackling Quad ESL. We are talking about the Original ESL (57) here, ok?
Background:
The coating, if original ,or properly restored, by someone with a bit of nous, will be Nylon (soluble nylon, or, methoxy-methyl-nylon). This is the "secret" of the original Quad diaphragm performance (fwiw).
This coating had two important properties:
1. It is very high resistance. Something around 109 to 1012 Ω. It takes time for such a diaphragm to charge up. Minutes to hours. Leave the speaker switched ON, like the manual says - ok?
2. The coating absorbs moisture, ever so slightly, from the air, so this surface resistance does vary a bit.
Problem:
Crackles! Yes, I remember.
Scenario 1: You have switched the speaker off while on vacation/holidays, perhaps? You switch it back ON. Crackles!! This is the charge slowly building up on the diaphragm and the distance from diaphragm to stators is changing gradually and it is doing this unevenly, as it takes time to spread. This causes the crackles. No problem, It will cease within a few minutes. Leave the speaker switched on after it ceases. This phenomenon can happen in one speaker, or both. Even a slight difference in humidity in one speaker could have caused this while it was off. Do not worry further.
Scenario 2: More serious. You may have a leak in the EHT supply of the speaker or from the panel frame to ground/earth. If so, the EHT is trying to "make up" the charge all the time and the charge distribution fluctuates across the diaphragm making it Crackle! This is a serious issue, since it means you have to track down the fault. Look for discharge corona in the back of the speaker near the EHT, or around the edges of the panels. You need to take the grilles off to do this, so be careful!! In a dark room, corona discharge will look like a faint purple glow. You may smell ozone as well. This is like the "smell" that's sniffed around photocopiers, when (IF) they're working hot.
Scenario 3: Very high humidity is causing the diaphragms to discharge unevenly into the surrounding air. If you've had pouring rain lately, see what happens in a week when things dry out. If not, move to a dryer climate? Alright, there's a secondary tap from the small mains transformer that you can use to drop the input voltage to the EHT ladder from ~610V AC to ~590 AC and this may well "cure" the problem. If you have no idea what I'm saying here, then do not touch the EHT unit. Find someone who knows what they are looking at to make this adjustment.
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