How to cure buzzing chokes/transformers?

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I would think that surface adherence would be the most critical thing to achieve, and that a standard polyurethane would be best vs lacquer or other less flexible coatings
The vacuum should do wonders for getting in everywhere when the coating has been thinned similar to that of say milk.
What do the companies use that make high end industrial transformers?

Nice to see that some buzzing old parts have been revitalized.
 
I had posted a bit back that I made Q-Dope by dissolving polystyrene packing peanuts in a solvent, but I named the wrong solvent. It should be toluene. Toluene is still available at local hardware and big box stores, but bear in mind that it is not terribly good for your health so use it with proper ventilation.

But I have a question. Ham radio operators seem to swear by Q-Dope for their projects (mainly RF coils) since polystyrene is low dielectric.
Toluene is a quite aggressive solvent and attacks most plastics (that´s why it easily dissolves polystyrene) and will damage many modern wire enamels, specially the atoms thin "self stripping" type, so better avoid it for transformers.

Ham radio guys have no big problem because they just brush their RF coils, which to boot have few turns, are v"open" and most important, one turn does not touch the next in general or at most side by side, very lightly, while transformer wires areb crushed one against the other.

Only wire enamel which stands it (and all others, inluding acetone) is Epoxy, but since you never know what did manufacturer use, better play it safe.

I had a whole batch of transformers ruined, lost a lot of money, because supplier furnished by mistake 1 spool of self stripping wire among the high temperature, electric motor approved type I use.

Visually it looks the same, so I only noticed it when *all* started buzzing and overheating on installation ... too late.

Mineral spirit on the contrary, plain wood varnish solvent, is very mild and attacks nothing ... not even polystyrene, go figure.

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What do the companies use that make high end industrial transformers?

There are commercially available transformer varnishes, such as:
41eKjovG7lL.jpg
 
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Hi Terry, what would be another name for Toluene? brand maybe...?

i was using shellac flakes dissolved in denatured alcohol earlier on, but shifted to polyurethane when it became available to me...

Hey Tony, I have a question for you. Did you ever wind transformers for single ended 300B amplifiers? I know the western electric copies seem to all originate from Asia, maybe you had some exposure to them. I'm curious what's inside those replica 171A transformers, and if it's possible to wind one at home.
 
I finished the first coating of the first choke yesterday, and gave it a try today.

I would call it an unqualified success. The buzzing is 95% gone, and now all I can hear is the power transformer hum. I checked it against the other amplifier, and on that one the choke buzz still dominates.

While in the past you could hear it anywhere in the room, now you have to be within 12 inches, with the furnace shut down to kill ambient noise.

I think that due to the high magnetic forces in this choke, to get better would be very difficult if not impossible.

Still, I'm going to give it two more treatments, just for insurance.

Thanks to all who have contributed to this thread. I will update once I've made more progress.

At the moment I cannot do the power transformers, since the vendor sent the wrong vacuum chamber, and the one I have is physically too small to fit a power transformer.

Note: that blob on the bobbin is wax from the original setup, I didn't try to clean it off for fear of damaging the fragile paper. Same with the staining on the laminations, that's original varnish which was deteriorating and is now locked in. That paper would disintegrate by touching it before, now it's like I laminated it with plastic.
 

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Hey Tony, I have a question for you. Did you ever wind transformers for single ended 300B amplifiers? I know the western electric copies seem to all originate from Asia, maybe you had some exposure to them. I'm curious what's inside those replica 171A transformers, and if it's possible to wind one at home.

i have not seen a 171A OPT, but i did wind my own for single ended...
Bud Purvine coached me a lot...and the web page of Patrick Turner contains a lot of informations on audio output transformers..for-sale-3-output-transformer-notes
 
Well done, maxhifi. And, thanks for the reports. They will serve as inspiration for many in the future as it is on the edge of audio knowledge.

Even brand new and good quality transformers I have bought are not fully silent. I guess it is due to magnetostriction. We can't change physics but the way you describe the result, your choke is as good as new (noise-wise).
 
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I remember Patrick from rec.audio.tubes, years ago. He was a very talented guy with strong opinions. I actually asked his advice about this amplifier's transformers, he suggested potting. I will go back to his website, I forgot he wound his own transformers.

So for the project concerned, the transformers are operating as per RCA design, with all original voltages and currents. If I built the amp myself I would have used the transformers more conservatively, that's for sure - but they aren't into saturation and overload.

I did realize that there's no temperature rating associated with wood varnish. For the choke I am not too worried, but the power transformer does get hot. I think lacking data the only way to go is emperical I will bake the old and not valuable chokes I used as test subjects at 200C for a few hours, let it cool down, and inspect the varnish for damage. If it is fine, I can do the transformers.. They won't get past 85 on the worst case.
 
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).
 
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 :)
 
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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.
 
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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.
 
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.
 
Here is the first amplifier, completed. The transformers have both been vacuum impregnated. Result is that buzz is 100% gone, and hum is about 90% gone. It used to sound like a defective magnetic fluorescent ballast, now it sounds like nothing unless you're up close. The one in the middle is the audio output transformer, and I didn't touch it, and don't plan to.

It does make a bit more noise at full power, but under real world conditions this will be entirely masked by the sound of the speakers.

I didn't paint the transformers. The varnish could LOOK a bit nicer, but I was mostly focused on performance, and I believe I got rewarded nicely.

I have the first transformer for the second amplifier drying now. Each time I do this, I refine the process a bit - I will probably be good at it by the time everything is done.
 

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Thanks a lot for one more useful report. We pick up the "cherries" without getting varnish on our fingers. Nice to know that a good result can be achieved this way.

Vacuum and vibration or only vacuum?

Vacuum and heat. I pre-heated the transformer to 75C for an hour before submerging it, and then kept the vacuum chamber heated to reduce the viscosity of the varnish.

In the later iterations, I figured out some details, for example, suspend the transformer to dry with the least visible corner pointing down, because the varnish will build up a bit thicker where it's all draining off.

I did manually agitate the chamber during the final phase of out-gassing, to provoke any final bubbles to come out, but I never did develop any sort of vibration table.

Another lesson here, is the full curing process takes weeks. I wouldn't put a transformer back into service until two weeks, at which point the varnish becomes absolutely hard. During curing, I heat it under an incandescent light bulb. (goose neck lamp)

There was a lot of trial and error in getting it to work consistently, and frankly this process only makes sense if you really like the device you're working on - it's very messy and time consuming, if I had to pay myself an hourly rate similar to what I get at work, I would have paid for new transformers many times over. Thing is though, I'm happy that it IS possible to do at home with only a moderate amount of equipment and skill.

This RCA amp and its partner are my old friends though, I think all of us have some equipment we will never sell, and this is it for me. I've tried so many tactics to reduce the hum noise, including locating them in far corner, and even building a sound proof box. All of these tactics though weren't good for keeping the amps cool, and I more or less just lived with it until trying this solution.
 
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There was a lot of trial and error in getting it to work consistently, and frankly this process only makes sense if you really like the device you're working on - it's very messy and time consuming, if I had to pay myself an hourly rate similar to what I get at work, I would have paid for new transformers many times over. Thing is though, I'm happy that it IS possible to do at home with only a moderate amount of equipment and skill.

Many thanks for your elaborate reply.
Unfortunately, living from designing and building electronics has become very difficult with the price level today. Why don't we just buy what we need and spend our time on something else? Because we, as dedicated engineers with various background, have an urge to understand how technical elements work. We like to control the technical constructions and not just do arbitrary experiments. This is what will keep you going (in particular your brain) when you get elder without having to work and intellectual maintenance is important.
What you made yourself will stay in your memory much longer than what you just bought.
 
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