QUAD ESL 63 neon bulb flashes only once when plugged in


Last week I traded a pair of my ESL 57s with someone for a pair of ESL 63s. It made him happy. Unfortunately the 63s appear to be malfunctioning.
One ESL 63 plays even softer than the other especially compared to my 988's. I gather they should play equally loud?

Following Sheldon's advice in an older thread, I unsoldered the T2800B from the clampboard, but that didn't work out.
That's why I now suspect that the EHT unit is not building enough voltage. Thanks to Sheldon, I learned that bad diodes often turn out to be the cause.
I replaced all 8 of them with new GP0230E3 diodes, but without any result.
Before I proceed to the next step; replacing the capacitors on the EHTs, I would like to consult you first.

- What I find remarkable is that the neon lights that should light up with a certain regularity, remain off. They only light up once; when the ESLs are turned on.
-The ESLs don't make any disturbing noises.
Any ideas?
Thanks for reading this.
It could mean you simply have very little leakage, but the ESL 63s should play about equally loud.

If you have a multimeter and know its input resistance when set to measuring voltages, you could make a very high-ohmic voltage divider with a series chain of 10 Mohm resistors and the meter and use it to measure whether the 5.25 kV is indeed 5.25 kV.

The ESL 63 service documentation includes a decision tree for fault diagnosis, maybe that could help.

Try not to connect yourself to the high voltage supply while it is on, keep in mind that the output voltage drops very slowly after turn-off and most of all, keep in mind that the secondary side of the step-up transformers can kill you when there is a signal on the primary.
Thanks both for your suggestions. I checked the neon bulbs and they are Ok.

Because the speakers are technically about the same ESL63 and the ESL988 should play equally loud I suppose?

Sorry, I don’t have the right equipment to measure high voltages.

And it’s outside my comfort zone. Quad ESLs are lovely but are not worth dying for. I rather spend €70 at capacitors to make sure the EHT’s are fine.

If a non flashing neon bulb means there’s hardly leakage and suggests the EHT’s and panels are ok, it doesn’t explain why there is such low audio output.

So the question is, if the capacitors on the EHT boards are worn out, could the high voltage drop to such a low level that there isn’t even enough power to lit the neon lights? Or meanwhile could bad panels consume the remaining EHT power?
Thanks Naresh for the advise.

I replaced one bulb without effect and without being electrocuted. The diodes are all new.
To narrow down the problem I 'll better invest in new capacitors first and then see what happens.

If the neon bulbs still won't flash after the caps are replaced, there has to be another problem.
Because although it hasn't rained for weeks humidity is still 58% and flashing should be normal.

Against all odds, the possibility still exists that even under the stress of 5,25 kV all panels will prove to be ok...
Is it possible that the panels were rebuilt by someone before? do the diaphragms look darkly coating with a graphite type coating? what you are describing sounds like the diaphragms aren't charging up. IT could be the power supplies, but both playing softly really sounds like a diaphragm coating issue

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Thanks for your reply Sheldon.

I hope you can keep your shop in Grantham isolated from the smog of these terrible wildfires🔥.

Having the plates removed the cloth looks really tidy. As if the 63s are never been undressed since their assembly in Huntingdon.
I have to strip them anyway to check the dust covers. Who knows what's hidden underneath.

Meanwhile I ordered the capacitors for the EHT and added two IC 1’s ( NE555P) to the basket, just in case. The parts will probably arrive this weekend.
I’ll keep you posted on the progress. Have a nice day.

esl 63.jpg
For what I can see these are never opened before. The glue under the velcro was crackling and the tape has no irregularities what so ever. There are no markings of any rebuild. I neither discover darkish coatings. So I think we can rule out the diagram coatings?

I just wonder: Can a bad C25 result in low audio output, causing the same kind of problems?

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Hello people.

It took a while but yesterday I finally had the time to replace the capacitors of the EHT unit. Only Ceramic Vishay 3Kv were available, but I don't think that's a problem. It took some care to bend the wires to neatly fit in the holes. The nasty job was getting the wax back on board.

First I put the revised EHT unit in the speaker with the lowest volume, to see if anything would change. Well it did. The speaker sounded almost twice as loud as the first loudest one. Same recipe for the other one. Above expectation now both ESLs sound equally loud and good. Distortions are gone. The neon bulbs are both blinking every 5 seconds. (Humidity 56 %)

It appears that:

-The EHT voltage can drop below the level needed to make the neon bulb blink.
-Too Low EHT output results in the distortion of sounds.
-The EHTs of one and the same set may wear very unevenly, resulting in pronounced differences in loudness.
-Because I replaced the diodes in advance, I can't tell if some were also part of the problem.
-I must be very lucky that none of the panels makes disturbing noises, even under the stress of 5,25KV:))

Thanks all for your input and advices.
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