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How to cure buzzing chokes/transformers?
How to cure buzzing chokes/transformers?
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Old 17th September 2018, 02:37 AM   #91
trobbins is offline trobbins  Australia
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Patrick used a messy two-pack varnish technique as the more practical diy alternative to a commercial bake-cool-epoxy vacuum impreg-overpressure impreg - bake process for output transformers. He found that provided stable reliable performance.

Not sure if that allowed him some benefits during winding build up, compared with doing a dry winding (and then vacuum impreg later).
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Old 17th September 2018, 05:12 PM   #92
maxhifi is offline maxhifi  Canada
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So yesterday I decided to apply a second coat to the choke. The first thing I did, was put the choke into the oven, to warm it up again. The toaster oven I bought got too hot, and the varnish on the bottom where it was in contact with a metal pan turned liquid - apparently it wasn't yet cured all the way though, it became a gooey mess.

I wiped off the gooey stuff, and gave it another dip in the vacuum tank. The second coat took well, and there were a lot less bubbles this time. I then brushed some extra varnish on the surface.

A day later, the surface has skinned over and dried well, but I can see liquid pockets of varnish where I brushed it on thick. I thought a thick coat would help tame vibration, but I think I created a problem.

I tested the choke electrically, and it still reads good.

I'm going to let it cure for a week now, and see how it dries. I think I made some mistakes in my haste to get a second coat on. Hopefully there are not any long term negative consequences.

1. Toaster oven thermostats cannot be trusted.

2. Varnish takes longer to dry than the instructions on the can would suggest, when it's used for impregnating. I probably should not have given it a second dip after noticing that it wasn't fully cured.

3. Brushing on thick is tempting, but a bad idea, since the coatings should be thin if they're going to cure properly.

Patrick Turner suggested Polyurethane for impregnation, perhaps I should have used that, instead of the spar varnish? I will see how this one cures after it's had a bit more time. Thankfully there's zero urgency to this project, it's my amplifier, my money, my time, and if I screw it up, I'm only answering to myself.. I wish all of my life was like this

Last edited by maxhifi; 17th September 2018 at 05:16 PM.
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Old 17th September 2018, 08:29 PM   #93
FauxFrench is online now FauxFrench  France
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Thanks once more for an informative report.

Back in the days when I did coil-impregnation, we used two component resins that cure chemically from inside and do not rely on resin components evaporating to the outside. The disadvantage was evidently that the surplus resin was wasted.

In your case you will have to be patient and only apply a thin layer at a time. When one layer has cured properly, you can progress to the next layer. It is a slower procedure but should leave a fine result in the end.

Some polymerization processes are accelerated by UV-radiation. Try leaving your transformers to dry in the sun.

Last edited by FauxFrench; 17th September 2018 at 08:33 PM.
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Old 17th September 2018, 10:29 PM   #94
TonyTecson is offline TonyTecson  Philippines
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How to cure buzzing chokes/transformers?
i use air drying polyurethane varnish and do sun baking, of course the hot manila sun made this possible...
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Old 17th September 2018, 10:46 PM   #95
maxhifi is offline maxhifi  Canada
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My workbench has an arm mounted fluorescent desk lamp, with a circular tube on it. Given the non stop rain we have been having lately, it's going to have to be the UV generator for now, I will just leave the transformer directly under it for a few days and see what happens.
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Old 19th September 2018, 04:29 PM   #96
maxhifi is offline maxhifi  Canada
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So the lamp on my workbench had the fluorescent ring, with an incandescent lamp in the middle. It's surplus from a doctor's office. Since baking under the UV/heat for a couple days, the transformer has become 99% dry. A bit of liquid varnish leaked out after the first ten hours, and it smelled very strongly like paint thinner. Now the smell is much reduced, and the varnish is curing.

I'm going to keep curing it for a few more days. Lesson learned here is I don't like this sort of varnish, the next one is going to use something with more predictable characteristics, like a bake to dry varnish.
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