Re: 576 curve trace scope problem

Bob Koller <testtech@...>

The offending "brown" potted Tek transformers were I believe potted using an urethane. The problem with whatever they used was it seems to become lossy over time, suppressing the oscillations. I tried making comparative inductance and Q  measurements, at 30KHz, on both an older, bad and a newer good, transformer to no avail. The measurements were made a much lower power levels than operation. Perhaps the potting material restricts the ferrite core in some way. Who knows..

From: Dr Mark Vaughan
To: TekScopes@...
Sent: Tuesday, August 2, 2011 10:25 AM
Subject: RE: [TekScopes] Re: 576 curve trace scope problem

I’ve used the silicon two part potting compounds by weighing, or volumetrically measuring the mix depending on what’s written on the two containers it comes in. We’ve also kept the stuff on the shelf in the workshop and it’s still been OK about two years down the road.
The epoxy potting compounds also survive very long times but those with fillers you’ll usually find the fillers settle and are very difficult to pull back into the mix, though settling to this state seems to take about five plus years. I had large buckets of this stuff from a bankruptcy auction, what I have left is now twenty years old, but very difficult to mix, but still goes hard and has the same dielectric strength.
I now typically use a two part polyurethane potting compound which is much cheaper, and again keeps, comes from RS in two equal cans, I use a plastic cup, pour one in with a marker stick for depth, lift the stick and pour the other half to the same height. Again it keeps pretty well and it’s dielectric strength is good.
Many manufacturers will put a shelf life on stuff, but that usually has plenty of spare time, so their estimate is based on an idea how long it might last, against what is acceptable to the customer, and a need to sell more.
For transformers, we usually put them on a vibrating table, with  a vacuum pump, pull vacuum then release it, and pull it again, this helps pull air bubbles out and refill with compound as the vacuum is released. We used to do it this way for transformers with outputs operating typically at 40 to 50KV, with some tests at 300kV. I prefer epoxy or polyurethane for these.
Hope that helps.
Dr. Mark Vaughan Ph'D., B.Eng. M0VAU
Managing Director
Vaughan Industries Ltd., reg in UK no 2561068
Water Care Technology Ltd, reg in UK no 4129351
Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH UK.
Phone/Fax 44 (0) 1872 561288
RSGB DRM111 (Cornwall)
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Ed Breya
Sent: 02 August 2011 17:41
To: TekScopes@...
Subject: [TekScopes] Re: 576 curve trace scope problem
Yes, as I understand, the silicone stuff gradually polymerizes on its own over time - even without exposure to air or catalysts. The two-part or catalyzed types should last a long time if kept tightly sealed, but time marches on. Refrigeration should help to extend the life, if other family members don't object. I used to keep a small frig in the garage, dedicated to only storing adhesives and beer, but over many years, most of the adhesives still spoiled - the beer, however, was always properly used up before expiration.


--- In TekScopes@..., Chuck Harris wrote:
> I can do precise weights without problem, but I am somewhat concerned about
> how the silicone resin handles exposure to the air. I read that it has a
> definite shelf life, and that it gets much shorter after opening the can.

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