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Re: [OM] Re: [OT]Transformers (was Noisy T32 (wax potting))

Subject: Re: [OM] Re: [OT]Transformers (was Noisy T32 (wax potting))
From: Jim Brokaw <jbrokaw@xxxxxxxxxxx>
Date: Fri, 17 Oct 2003 12:33:04 -0700
on 10/17/03 10:50 AM, Joe Gwinn at joegwinn@xxxxxxxxxxx wrote:

> At 3:17 PM +0000 10/17/03, olympus-digest wrote:
>> Date: Fri, 17 Oct 2003 10:06:25 +0100
>> From: "Julian Davies" <julian_davies@xxxxxxxxxxxxxx>
>> Subject: [OM] Re: [OT]Transformers (was Noisy T32 (wax potting))
>> 
>> The problem is the drilling, not the bolting. By putting a drill through a
>> transformer, the laminations will be shorted and  eddy current will be
>> increased.
>> Transformers designed to be bolted have the laminations punched and deburred
>> prior to being coated all over for insulation, then  assembled with the
>> holes aligned to allow the bolt or rivet. The laminations stay insulated and
>> the bolt does not contribute an eddy - current path of its own.
> 
> Yes, that's certainly how it's done in production.
> 
> In Moose's case, if I recall there was already a hole in the laminations, and
> all he had to do was to provide the bolt.
> 
> But for retrofitting a small power transformer, I wouldn't worry about a new
> hole or two, even if it does raise the loss slightly.  I suppose leaving space
> so the bolt can have a insulating sleeve would help, as then any connection
> would have to be via the burrs.
> 
> I would be far more worried about accidentally damaging the transformer while
> drilling, unless I could remove the core from the winding.  If the core is
> removable, then it ought to be possible to deburr the holes.  But this is a
> lot of work.
> 
> Anyway, this is why I preferred wax potting.  No disassemby needed, and wax
> won't hurt anything.
> 
> CH suggested using real electrical potting varnish, which would certainly work
> very well, but as he said it's hard to get these materials, at least in small
> quantities.  But CH does give me an idea -- ordinary shellac, available at
> paint stores everywhere, would work and again isn't going to hurt anything.
> Shellac can also be thin enough that it will simply soak into the transformer.
> Remove the shellac by soaking in alcohol.
> 
> 
>> Julian
>> 
>> Who got quite familiar with this designing and building valve amplifiers in
>> a previous life.
> 
> Maybe it matters for an audio output amp, which has to handle a wideband (20
> to 20000 Hz) signal without distortion, but for a one-frequency power
> transformer it likely matters far less, so long as the power loss isn't
> excessive.
> 
> Valve amps are making a comeback in hi-fi circles (for the mellow tube sound)
> and never died for guitar amps (for a better grade of distortion), so a new
> career awaits you.
> 
> Joe Gwinn

CH also mentioned that the T-32 transformer is a ferrite-core part. Hence no
laminations, so the noise must be coming from something else. I'd be
inclined to closely check the solder joints for vibration induced
'cold-solder' effect, maybe touch them up anyway, then flood the coils with
something that might damp out any vibration in the wire loops. I suppose wax
or shellac or whatever could be used. If it isn't sounding like a 747 taking
off I don't think I would make the effort... if you can hear it at least you
know its working. <g>
-- 

Jim Brokaw
OM-'s of all sorts, and no OM-oney...




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