Quad ESL 57: hum on one treble panel

[Fabien Lefebvre]

Hello,
I have completed refurbishing a pair of ESL57, which means all panels except one: a treble panel, which appeared to be good and besides already refurbished (don't know when).
This treble panel makes some background noise, some kind of hum. If un-connected from power supply, this noise disappears. But output power reduces significantly almost immediately and more and more as time passes (timescale in seconds). I changed power supply (the full EHT block) but no improvement.
On the treble panel I refurbished, this is not the case: no hum and even after removing power supply connection, output power still stays high for a long time.
Any idea ? I have one but I would like to get confirmation.

I tried this: add a layer of insulating tape on the outside of the stators (I had to remove the dust cover for this). But I did not test yet if it improves behaviour.

Thank you.

 

[Calvin]

Hi,

sounds like a seriously leaking panel, that draws too much current from the HV-supply. Try to measure the current or test on leakage with a blinker circuit.
If it prooves to be leaky, inspect the panel closely and try to find out if there´s some dust, humid grease or a particle at or within the panel that may cause the leakage.

jauu
Calvin

 

[Franz Gysi]

I think, it is very important to avoid leakage current not to destroy the HT-psu tranny.

My local dealer warned me, that he has seen a lot of burned trannies. His conclusion was, that the trannies are made for 220VAC and not 230VAC.

I think, this conclusion is not correct.

The tranny is marked with 0.5mA (at about 610VAC). And the fuse is a 100mA type.

So, every leakage current could imho be dangerous for the tranny, specially when feeded with 230VAC.

I strongly recommend the blinking neon bulbs to control leakage.

Franz

 

[Fabien Lefebvre]

I strongly recommend the blinking neon bulbs to control leakage.

Franz

Any reference for this blinking neon bulb to suggest ?

 

[Franz Gysi]

Any reference for this blinking neon bulb to suggest ?

Yes, here:

http://www.diyaudio.com/forums/plan...57-panel-refurbish-diy-not-2.html#post2343083

I used very "normal" NE-2 bulbs.

100V types without series resistor.

And I connected them simply in the HT-line. One for each bass panel and one for the treble panel.

Here you can see the bulbs:

[IMGDEAD]http://image.n0t.de/f-fe1505495248bd6cd5fef650c5028749.jpg[/IMGDEAD]

Franz

 

[Fabien Lefebvre]

Hi,

sounds like a seriously leaking panel, that draws too much current from the HV-supply. Try to measure the current or test on leakage with a blinker circuit.
If it prooves to be leaky, inspect the panel closely and try to find out if there´s some dust, humid grease or a particle at or within the panel that may cause the leakage.

jauu
Calvin

I removed dust covers to re-insulate the external side of the stators.
No improvement: the panel is certainly leaking. Knowing that there is no rivet at the peripheral (fixed by some strong tape) there remains one option from what I read on the net: coating is too conductive and might be in fact graphite based.
It looks like I have to re-build this panel completely. Anyway I will open it and see if I find some dust inside.

 

[Calvin]

Hi,

the blinker consists of a small neon flash bulb, in parallel with a small film cap and a high ohmic resistor in connected in series to the bulb||cap.
The bulb may have a flashover treshold of less than 100V, the cap may be 100nF and >100V, the resistor may be 10MOhm. If the HV-supply already features a high ohmic resistor You may use that one instead.
Since the bulb is rather a shortcut when flashing You shouldn´t omit with the resistor in any case, because it reduces the current to a small and safe value and reduces the stress on the HV-supplies components. The small cap then supplies for the charging current through the bulb and it guarantees a clearly visible flash.

jauu
Calvin

 

[Fabien Lefebvre]

I opened the panel. No obvious sign of dust or whatever could create some leakage.

The coating itself is rather dark; it looks like graphite. I measured its conductivity: 1e7 Ohm/square, which is much lower than what can be achieved with original coating, soluble nylon: from 2 x 10^9 to 1 x 10^12 ohms per unit square according to quadesl.org.

See here.

This panel is really noisy, in the sense this hum is audible at a distance of 1m. In my opinion this is due to the chosen coating.

 

[Calvin]

Hi,

this could be the case indeed. AFAIR the caps in the HV-supply are of rather low value (<<100nF) and a low ohmic coating might be a too heavy load for this.

jauu
Calvin

 

[Fabien Lefebvre]

Hi,

this could be the case indeed. AFAIR the caps in the HV-supply are of rather low value (<<100nF) and a low ohmic coating might be a too heavy load for this.

jauu
Calvin

The caps I use are 10nF (ceramic, 3kV).

 

[Fabien Lefebvre]

The treble panel is now re-built, using Elvamide. And a new sheet of mylar stretched with more than 50kg.
No hum any more...

I still have some leakage but not that much and localized on a bass panel, near one metal support. If I push the bass panel this disappears. Some simple insulation and it will be gone.
I read somewhere "you don't imagine what can conduct at 6kV". This is indeed truth !
My advice for this: take your multimeter and check voltage at these locations. You might be surprised. One precision:I am using high voltage probe thus with input load > 1000MOhm. This might be important. I did not check with standard probe (10MOhm).

 

[alexberg]

Fabien,
what is your recipe for Elvamide?
My experience shows that alcohol soluble nylon does not stay on membrane
(at least without some nasty additives)
BTW naphtalene, widely used before against the mole, is much more
dangerous in respect to phenol. The second one is not cancerogenic

 

[Fabien Lefebvre]

Fabien,
what is your recipe for Elvamide?
My experience shows that alcohol soluble nylon does not stay on membrane
(at least without some nasty additives)
BTW naphtalene, widely used before against the mole, is much more
dangerous in respect to phenol. The second one is not cancerogenic

Indeed I read that some people had issues with Elvamide.

My experience is completely different: I am using Elvamide with Ethanol (widely available in drug store) and this works great !

Difference is certainly in preparation: I am using a hot stirring plate each time.
I wash the mylar with acetone as well before applying Elvamide.
Finally adherence is really good. For example when I want to remove it from
outside of the panel or because I fell I did not put enough, it's not that easy even with acetone.
I am using as well a precise scale for preparation but I do not think this is key.
Note that even with a hot stirring plate preparation can be as long as one hour for perfect dillution. But not a problem as the device is making the job for you. Thus I imagine that some people lack patience if doing it by hand.