I assumed not since it was built as it is, but that said a dead SB220 HV
transformer I pulled apart did indeed have insulated bolts. Should I try
doing so, maybe with a teflon tube as opposed to paper?
73 Dave
----- Original Message -----
From: <TexasRF@aol.com>
To: <n3db@radix.net>; <amps@contesting.com>; <km1h@jeremy.mv.com>
Sent: Saturday, May 03, 2008 8:24 PM
Subject: Re: [Amps] HV transformer issue
>
> Are these through bolts supposed to be insulated?
>
> 73/gw
>
>
> In a message dated 5/3/2008 7:20:02 P.M. Central Daylight Time,
> n3db@radix.net writes:
>
> Hello all,
>
> I am seeking input on a potential problem with my "almost new" B+
> transformer. It is a 240V input, output 4 taps from 2800-3000V rated 2
> amp full load.
> Transformer was tested in situ today, first stand-alone. Has a slight
> hum
> but not significant, and as mounted no detectable vibration on the
> laminations. AC output tested fine on all taps. Second test was with
> full wave bridge
> feeding a 12 x 800MF (series) homebrew cap bank attached- expected DC
> Voltage
> measured fine on the cap bank. 3rd test was with 100k ohm 250w R across
> the
> cap bank from B+ to B-, ~100 volt drop.
> Now the "problem": The transformer is a typical E I Core (roughly
> 9"x7"x7")
> with primary taps on one side fed to binding strip & secondary on the
> other
> side to an EU style strip opposite the mains. Core is held together with 4
> pieces of allthread, nuts on each side, around 1/4" in diameter.
> Transformer
> is not potted. With mains fed to the transformer & even with cap bank
> removed from the secondary side, the 4 allthread "bolts" get hot. Not a
> "little
> hot" but "blister your finger hot" with just a minute or 2 of mains
> voltage
> applied. I understand this was new when I bought it- that said, I can
> smell some
> heating varnish, apparently from the bolts. The laminations themselves
> stay
> completely cool, but granted it is so massive (84lbs) that it would take
> a
> good long while for it to heat up. I am not sure but the allthread
> appears to
> be standard steel. I have never run into this strange effect before-
> this
> can't be normal, can it? I am guessing this is hysteresis, magnet
> ostriction and heavy eddy currents in the bolts- does this sound
> plausible?
> What is going on here?
> Because they are getting so hot after just a short time under power I am
> concerned about using the transformer in the amp (GS35b 50 MHz in
> mid-build).
> It has been suggested that I pull one of the allthreads and check for
> hotspots
> on it, which I plan to try. If they are steel would it pay to replace
> them
> with non-magnetic stainless bolts & get rid of the allthread completely?
> Also, in its current configuration I cannot tell whether the bolts are
> actually
> a proper fit for the holes in the E I plates- it appears that it may be
> slightly undersized, and of course they are threaded the whole way, ans
> therefore
> may not be effectively transferring heat to the laminations.
> Any input would be most appreciated.
>
> 73 Dave N3DB
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