Corona advice - no not that one!


Morris Odell
 

Hi all,

A friend of mine is restoring a 547 and has noticed a hissing noise coming from the HV supply. This is most likely due to corona around the tripler connections. Can anyone suggest a source or recipe for "corona dope"? Some of my older Tek scopes have some black substance applied around he relevant areas - what is this stuff?

Thanks,

Morris


Keith
 

Hi Morris,
Corona dope is just varnish. Extra dry lacquer based varnish.

Or buy it special. It is easily available in many places, like this

https://www.techni-tool.com/product/10001262-4226-55ML?gclid=Cj0KCQjwjo2JBhCRARIsAFG667Uvcqurq0_GJgBY-AwlmFItnzNpBqz9PDLewxtBYkQyBAYthPHWxu8aAsEdEALw_wcB

Hope this helps

Keith
CoolBlueGlow


Keith
 

And, for completeness, here’s the MSDS

https://www.amplifiedparts.com/sites/default/files/associated_files/sds_-_s-c10-4702.pdf

As you can see from page three of the MSDS, it is basically just thickened nitrocellulose lacquer

K


Morris Odell
 

Thanks Keith,

To my delight I found that corona dope is available on the e-place down here in Australia. Alternatively I wonder if some of the wife's clear nail polish might work just as well?

M


Ed Breya
 

Before gooping it up, it may be good to check how the parts are installed. The original parts and construction would have been done according to HV rules, including leaving no sharp points on lead ends, and nice rounded solder globs for connections. If any parts were replaced at some point, this may not be the case. If so, a little cleanup and adherence to the rules may go a long way, and possibly eliminate the need for added materials. Also, cleanliness is important - thorough cleaning of the circuits to get rid of any dirt and grime and arc tracks may be enough. If you goop over a marginal situation, it may return.

Ed


Ed Breya
 

Just noticed in the OP about "black substance" applied. This could be the same polyurethane that Tek used to vacuum impregnate power transformers. It's very good stuff.

Ed


Chris Wilkson
 

Maybe the black substance is Tek black polyurethane.

Could it also be a black silicone RTV?

We have used black RTV to repair anode wires on monitors. 18kV typical on a Motorola chassis with a 20WP4 or 23JEP4 tube.
This works for corona discharge around cracked/loose rubber boots (for the HV diode), but in one case I had to remake the anode wire insulation after it turned to dust.
Ugly and thick, but effective!

On Tue, Aug 24, 2021 at 12:01 AM, Ed Breya wrote:

Just noticed in the OP about "black substance" applied. This could be the same
polyurethane that Tek used to vacuum impregnate power transformers. It's very
good stuff.

Ed


Keith
 

Old-fashioned analog CRT televisions back in the day used lacquer based corona dope...but they had maybe 15-20kV working in those bits.

RTV seems like a possible but messy option. Might have to be really thick? Example: Automobile high tension cables at 45+ kV require 4mm of silicone type sheathing to ensure isolation.

Whatever choice you make, Ed Breya was right on the money. Get rid of those sharp points! Turn out the lights and take a picture of the supply operating as-is. You can actually see the discharge is worst at these sharp points - which are really just mini-antennas.

IMO, nuke the points like Ed said, THEN goop it up with whatever.

Working without a net here, but A 547 is only like 10kV, right? Regular dope should do the trick, IMO.


Keith
CoolBlueGlow

full disclosure: I am the dumbest and least educated person on the Tek forum


Chris Wilkson
 

Good question: What is the HV level on the 547?

Yes, as Ed Breya pointed out, get rid of the lighting rods (sharp points)! And then cover it with your HV insulator goop of choice. Make sure it's a complete seal.
And paraphrasing....."Completely clean everything!!! If you goop over a marginal situation, it may return."


For the 23" CRT, I used a very scientific measurement of "looks about right" for RTV. Probably about 2-3mm thick.
I let it cure out for 2 weeks, just in case. And I was careful to use the stuff that is *NOT* acetic acid based!
Then tested in a pitch black room (with zero beam current). Only light source was the CRT cathode filament.
There was no visible corona and the "grounded screwdriver test" passed with no focused discharges (visible or audible).
HV stayed steady at 18-19kV.

YMMV.

On Tue, Aug 24, 2021 at 07:04 AM, Keith wrote:


Old-fashioned analog CRT televisions back in the day used lacquer based corona
dope...but they had maybe 15-20kV working in those bits.

RTV seems like a possible but messy option. Might have to be really thick?
Example: Automobile high tension cables at 45+ kV require 4mm of silicone type
sheathing to ensure isolation.

Whatever choice you make, Ed Breya was right on the money. Get rid of those
sharp points! Turn out the lights and take a picture of the supply operating
as-is. You can actually see the discharge is worst at these sharp points -
which are really just mini-antennas.

IMO, nuke the points like Ed said, THEN goop it up with whatever.

Working without a net here, but A 547 is only like 10kV, right? Regular dope
should do the trick, IMO.


Keith
CoolBlueGlow

full disclosure: I am the dumbest and least educated person on the Tek forum


 

"ONLY like 10kV" seems like hyperbolic understatement to me (but the scope's I've worked most on, the 475 and 2200-series, use unusually low cathode potentials)

-- Jeff Dutky


Dave Wise
 

535-545-547 et al CRT anode are 8000-some volts above chassis, totaling 10kV between anode and cathode.

If OP is referring to black on the tripler ceramic strip, that’s corona dope, but RTV would work too.

Dave Wise

From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Chris Wilkson via groups.io
Sent: Tuesday, August 24, 2021 5:49 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Corona advice - no not that one!

Maybe the black substance is Tek black polyurethane.

Could it also be a black silicone RTV?

We have used black RTV to repair anode wires on monitors. 18kV typical on a Motorola chassis with a 20WP4 or 23JEP4 tube.
This works for corona discharge around cracked/loose rubber boots (for the HV diode), but in one case I had to remake the anode wire insulation after it turned to dust.
Ugly and thick, but effective!


On Tue, Aug 24, 2021 at 12:01 AM, Ed Breya wrote:

Just noticed in the OP about "black substance" applied. This could be the same
polyurethane that Tek used to vacuum impregnate power transformers. It's very
good stuff.

Ed


 

On Tue, Aug 24, 2021 at 04:41 PM, Dave Wise wrote:


black on the tripler ceramic strip, that’s corona dope
Probably GC #47-2 by GC Electronics, Tek P/N 006-0541-00

/Håkan


Michael W. Lynch
 

On Tue, Aug 24, 2021 at 09:39 AM, Jeff Dutky wrote:


"ONLY like 10kV" seems like hyperbolic understatement to me (but the scope's
I've worked most on, the 475 and 2200-series, use unusually low cathode
potentials)

-- Jeff Dutky
Jeff,

Keep in mind that the HV Multiplier of the 465/475 supplies about 14kV PDA to the Anode terminal of the CRT..

--
Michael Lynch
Dardanelle, AR


 

Michael,

(taking a sip of coffee with visibly shaking hands) that advice would have been slightly more useful last fall, when I was actively poking around the HV section of a 475A.

-- Jeff Dutky


benx618(g)
 

How nice it is to read corona discussion devoid of the word virus.

Ben


Michael W. Lynch
 

On Tue, Aug 24, 2021 at 10:27 AM, Jeff Dutky wrote:


Michael,

(taking a sip of coffee with visibly shaking hands) that advice would have
been slightly more useful last fall, when I was actively poking around the HV
section of a 475A.

-- Jeff Dutky
Jeff,

Thankfully, those voltages are typically well insulated and hidden from view for the most part. But the CRT can get dirty and leak anode voltage where it should not go. The CRT can store considerable voltage, as you know. But the bottom line is, that HV multiplier can deliver a nasty shock under the right circumstances. It pays to be very careful with these HV sources. .

--
Michael Lynch
Dardanelle, AR


Michael W. Lynch
 

Regarding the use of "Black RTV", Certain RTV's are excellent insulators. Keep in mind that TEK used silicone potted HV transformers in the later Type 576.

Here is one recommended product: Dow Corning RTV 732 BLACK This product has excellent Electrical properties. Full Datasheet is available online here: https://www.farnell.com/datasheets/91600.pdf.

Here are some relevant electrical details.

"Electrical properties, cured 7 days in air at 25°C (77°F) and 50% relative humidity.

Dielectric strength: 21.6kV/mm per ASTM D149
Dielectric constant at 100Hz/100kHz: 2.80112 per ASTM D150
Dissipation factor at 100Hz/100kHz: 0.0015 per ASTM D150 "

There are likely more materials available that will do the job.

--
Michael Lynch
Dardanelle, AR


n4buq
 

It looks like Dow Corning RTV 732 uses acetoxy curing. Wouldn't that be ill-advised in this application?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Michael W. Lynch via groups.io" <mlynch003=yahoo.com@groups.io>
To: TekScopes@groups.io
Sent: Tuesday, August 24, 2021 11:10:19 AM
Subject: Re: [TekScopes] Corona advice - no not that one!

Regarding the use of "Black RTV", Certain RTV's are excellent insulators.
Keep in mind that TEK used silicone potted HV transformers in the later
Type 576.

Here is one recommended product: Dow Corning RTV 732 BLACK This product has
excellent Electrical properties. Full Datasheet is available online here:
https://www.farnell.com/datasheets/91600.pdf.

Here are some relevant electrical details.

"Electrical properties, cured 7 days in air at 25°C (77°F) and 50% relative
humidity.

Dielectric strength: 21.6kV/mm per ASTM D149
Dielectric constant at 100Hz/100kHz: 2.80112 per ASTM D150
Dissipation factor at 100Hz/100kHz: 0.0015 per ASTM D150 "

There are likely more materials available that will do the job.

--
Michael Lynch
Dardanelle, AR






Michael W. Lynch
 

On Tue, Aug 24, 2021 at 11:21 AM, n4buq wrote:


It looks like Dow Corning RTV 732 uses acetoxy curing. Wouldn't that be
ill-advised in this application?

Thanks,
Barry - N4BUQ
Barry,

Good point. This was recommended by a HV transformer manufacturer I would need to investigate the recommendation further and report back. Thanks for bringing this up.

--
Michael Lynch
Dardanelle, AR


Dick
 

Take a look at Corona Dope, by GC. Been around since the Earth
started to cool.

73, Dick, W1KSZ
________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Michael W. Lynch via groups.io <mlynch003=yahoo.com@groups.io>
Sent: Tuesday, August 24, 2021 9:29 AM
To: TekScopes@groups.io <TekScopes@groups.io>
Subject: Re: [TekScopes] Corona advice - no not that one!

On Tue, Aug 24, 2021 at 11:21 AM, n4buq wrote:


It looks like Dow Corning RTV 732 uses acetoxy curing. Wouldn't that be
ill-advised in this application?

Thanks,
Barry - N4BUQ
Barry,

Good point. This was recommended by a HV transformer manufacturer I would need to investigate the recommendation further and report back. Thanks for bringing this up.

--
Michael Lynch
Dardanelle, AR