will shellacking transformer eliminate mechanical hum?

Hi,
I posted this thread in tube amps.
http://www.audiokarma.org/forums/showthread.php?t=279319, but I'm not getting a lot of responses leading to a solution.

Basically, I've fully restored a Fisher X-101-C integrated amp. Everything that should be replaced has been replaced. That being said, the only flaw is a mechanical hum coming out of the power transformer. I've scuttled a unit to eliminate a humming transformer on another project, but these amps are rare and doing that won't be so easy.

Vibration damping doesn't fix it. Isolators don't fix it. The sound emits from the wire holes. Has anyone shellacked the core of a power transformer and eliminated mechanical hum?

Have you eliminated it another way?

Can you point to resources?
Thanks in advance.
audiodon

It can. I had a Sony TA-F60 with a switching power supply that 'sang' on occasion. I used GC service cement. It took a few applications. You want to make sure that, whatever potting compound you use, it's thin enough to penetrate the windings. If it's just applied to the outside layers and the clacking windings are inside, you might block them permanently from getting any potting.

Well, here's the chronicle of my unsuccessful attempt of sealing the transformer in a shellac bath to eliminate the mechanical buzz.
http://www.audiokarma.org/forums/showthread.php?p=3481000#post3481000

I'm still open to suggestions here.

If you found a nice-looking outer case and there is enough space for it, you could try potting it in tar. Messy to undo if you don't like the result, though.

Find an old motor shop or transformer shop that will "shellac" or "varnish" the transformer for real, not the Home Depot version.

Does the transformer make sound from the moment it gets powered up, or only after it's good and warm? Reason I ask is I have an older ARC hybrid preamp and it was making a little noise on occasion when it got warmed up after a few hours run time in a mostly open cabinet. I put an ultra quiet 33cfm fan behind it to cool everything down and now it makes no mechanical sound.

Arkay has the right idea. Look here --

http://en.wikipedia.org/wiki/Magnetostriction

-- for a discussion of the phenomenon. It is called "magnetostriction." Frequently, the word "singing" is also used, though this more applies to a transformer that is vibrating in such as way as to reproduce the music flowing through its windings.

Fred
repairing audio since 1972

* * * * *

Fred Longworth said:

....this more applies to a transformer that is vibrating in such as way as to reproduce the music flowing through its windings.

Click to expand...

I have an old Raymer amp that will do that.

This power supply transformer buzzes at the same frequency, which is a mechanical buzz, and does not go into the audio chain, from the moment I power the unit on, until the moment I turn it off. I can short an input, switch to that input, turn it all the way up with the tubes good and warm, and I will not hear the buzz through the audio circuitry at all.
I already warmed the transformer core in a toaster oven, dipped it in warm shellac kept warm and soaked it for 36 hours, removed and drained off the shellac, baked the transformer to dry the shellac, reassembled the transformer carefully, shellacking parts as I reassembled them again so I would not introduce vibration elements, and rebaked the finished unit.

It does not sing. Unless one constant note like a starter on a flourescent light is a song.

Potting it may be one solution. Replacing it with a duplicate may be another. I'd actually like to find a replacement and then repot this one so I can discover if that fixes it. I've gone this far already.

Oh, and if I put my ear to the hole where the primary, secondary and filament wires come out of the core, I can hear that it's louder there.

After shellacking, over a month of constant use later, the buzz continues to diminish. Not sure why, but I'm certainly not complaining. Not gone, but the laptop fan is now far more prominent in my office system.

Have you tried tightening the bolts? There are the bolts that mount the transformer but I mean the ones that hold it together.

Sometimes potting a core will increase the losses. It's a magnetostriction effect and suppressing it will demand some power.

It's possible that the flux density is too high and you can lower it by reducing the power line voltage. Realize these old units were built to run on 115 Volts or so, not the 120 Volts we give them. Plus, to save money they ran the cores pretty high in flux to begin with.

If you want to try to lower the power line, you can conveniently do it with a 6.3 Volt filament transformer with its secondary connected to buck the power line. That should drop it to about 114 Volts and make the transformer quieter.

The reduced line voltage will probably make the transformer run cooler and slightly extend your tube life as well since the voltages will be proportionately lower, and probably closer to original design spec.

Did you physically check the tightness of the windings to the lamination core? Manufacturers often used wooden wedges similar to an axe handle types. Over the years they dry out and shrink and the windings will move about on the core and hum. This should be the first thing checked on a humming PT. Replacing the wedges with thicker ones to secure the windings tightly to the core then varnishing has worked for me.

The slowly diminishing hum could be that the old wedges are slowly swelling due to the varnish bath and/or increased humidity. You may ultimately end up with a solid fix too. As noted earlier, tightening the shell bolts is important. Further, if there are extra unfilled holes, add more bolts to them. Before you bolt everything up, you can also put a thick piece of cardboard between the shell and the windings on each side. When bolted up & tightened, the cardboard will squish and hold the windings down.

What happens is that the windings are either not impregnated with varnish, or the varnish has gotten old and dried out. What then happens (yes, you are correct) is that magnetostriction effect then causes the windings to vibrate in there. This is bad; it will eventually wear the enamel off the wire, and the tx will fail. I was having a bad time with some oriental transformers that I had that had been impregnated with polyester resin, which gets brittle (rather than soft) when it gets hot.

What I did was get a little venturi-type vacuum pump for a few clams on the 'bay, and hook it up to a vacuum flask that I made from a pot from the thrift store and a piece of 1/4" polycarbonate for a lid. I cleaned the lid well and put a bead of acrylic caulk around the edge, waxed the lip of the pot up real well with waxed paper, and skooged thing on there till it dried. That's a technical term. I heat the transformer in the oven, and the varnish in a dedicated little pan on the stove, both until they're hot but not too hot to touch. If the transformer is in a can, I just put it directly in the flask upside down. If it's endbells that could leak, I put it in a tin can that it can fit and put that in the flask. Varnish gets poured in until it covers the core by a quarter inch or so. Lid goes on, air gets turned on to the vacuum pump, increasing the pressure and watching the varnish bubble. The pump will pull about 28-29 inches of vacuum, but at about 26 or 27 inches the varnish will start to boil, which makes quite a mess.

The pump pulls about twice as much air at about 80lb pressure to get to 26" of suck as my little pancake compressor can deliver, but if I charge it all the way up to 140lb and leave it on, I can get the flask down to just above varnish boil and hold it there for about half a minute. The varnish stops fizzing after about 15-20 seconds, so we're good. When I'm down below 70lb in the compressor, I reduce the air gradually, turn off the compressor, pop the lid, and pour out all the excess varnish. I put the tx over newspaper on two sticks for any more that drains out.

If you don't have a compressor, this trick may not be feasable for you; but if you do, it's a few bucks and a couple hours of work to be fixed for restoring any old transformer for the rest of your life.

The gauge I use is attached to one of those squeeze-type vacuum pumps, which are also cheap and was my first try. Theoretically, It will also pull enough vacuum, but you'll be squeezing like a maniac on the thing for half an hour to do it. I ain't that strong, but fortunately that pump has a gauge and comes with a bunch of hose and fittings and stuff that I use to make the rig with. The nose of the squeeze pump is just connected to the main vacuum line by a tee fitting.

I don't use transformer varnish, cuz they want me to buy five gallons min, and ship it across the blue Pacific, which is a monumental PITW. I use cheap spar varnish, which is actually almost the same stuff. In fact, I use spar from the shop that's getting a little old and doesn't brush out nice like fresh. Who cares?

I have now treated five buzzing transformers, and every one quieted right down and runs a little cooler than before (varnish is a better conductor of heat than air, I guess).

I should have tried this dodge years earlier.

Aloha,

Poinz
AudioTropic

Poindexter said:

......What I did was get a little venturi-type vacuum pump........

Click to expand...

While, I knew a vacuum would help a lot, I thought it was beyond me to create such a setup. I didn't think about it until you mentioned it above, but I actually have a cheap venturi vacuum pump. Now I have a reason to keep it....... I originally wanted it to bleed brakes with, but it didn't work so well for my dad, when I gave it to him to try.

How loud is it? Is there an amount of hum that is normal?

Bleeding brakes and auto AC units is what these things are marketed for. Good thing; if they were DIY hi-fi gear, they'd cost a hundred bucks. Each. If you go on ebay and search up 'vacuum pump', you'll find both kinds. The hand one is useful for the gauge, which again would cost you $30-50 if you bought it alone. I thought of threading it directly into the polycarbonate lid of the vessel, but it works as is, and the snarl of gear and tubing has a hippie charm for me. I think the problem most folks have with the venturi pumps is that they underestimate the amount of air that they actually take, which is a 'king lot!

You're right; the vacuum is the key. I tried impregnating by soaking with very mixed results, and the 'shellacking the outside of the lams' method alluded to in the beginning of the thread is pure golden luck if it works at all. I can now use various cheapo Hammond (Hummond) transformers that were a nightmare before.

Aloha,

Poinz
AudioTropic

Is there an amount of hum that is normal?

Yes. None. Never deviate from this view. If a transformer hums mechanically, it will fail early.

P

This is great info. Thanks!

I'm glad to see it get picked up on. There are a ton of these late-model production tube components out there free for the asking, and really the best way of using them is to grab the iron, which is of pretty good quality, and fly the rest. These transformers do tend to have dried out impregnation though, and being able to 'rejuvenate' them is a godsend for the penurious DIY guy and the world at large. Great music for everybody!

Aloha,

Poinz
AudioTropic