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References: <4DA492C9.4329.AAE777@mike.dennison.ntlworld.com>
Date: Tue, 12 Apr 2011 18:13:31 +0100
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From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Insulating tape / Decca repair
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Its teh output transformer, so even at the 1kW level voltage won't be all
that high.  Assuming 50 ohms, only 320V or so peak.   ANd any heatshrink
sleeve will withstand that.   The fact that the original insulation was only
cloth with varnish reinforces that idea.

For tape, see if you can get soem of teh high temperature - it may be mylar,
or something else - transformer tape.  Thin, and yellow in my case, but you
see it a lot on SMPSU transformers so must be readily available

But it is all that critical holding teh turns firmly inplace.   I'd just be
tempted to slap some varnish over the turns to hold them.

Andy
.www.g4jnt.com



On 12 April 2011 17:58, Mike Dennison <mike.dennison@ntlworld.com> wrote:

> Further to my report on the failed combiner/output transformer in my
> Decca QRO Tx, I have dismantled the transformer. The really good news
> is that the huge (85mm dia) ferrite pot was not glued at all, and was
> easily unclamped. The internal wires were more difficult, being
> wrapped with cloth tape that seems to have been soaked in hard glue.
>
> I finally got to the secondary coil and found the shorted turn. That
> coil comprises three layers wound on top of each other, but the cloth
> wrapping was inadequate in one place, resulting in one turn from the
> top layer being adjacent to one on the bottom layer, and thus
> providing a higher voltage gradient than intended and shorting
> several turns.
>
> So all I have to do is to improve the construction (to avoid the
> voltage gradient) and re-assemble. I will wrap some sleeving over the
> slightly charred wires where the short occurred.
>
> I need some advice on what to use to replace the glue-soaked cloth
> tape. It needs to fix the turns and provide a high degree of
> insulation. I could use ordinary plastic tape but it may suffer from
> possible high temperatures (It is physically difficult to check
> whether the transformer gets hot whilst transmitting for long
> periods, but it's certainly likely).
>
> I have looked at the Maplin catalogue and they do "Silicone Self
> Fusing Repair Tape" which seems to resist high temperatures (up to
> 260 deg C) and high voltages (it actually says "7,000 volts/20mil
> (0.51mm) dielectric strength", which I don't really understand).
>
> Should I replace the wire, or perhaps just its insulation? It is
> solid, enamelled and insulated with some brown cloth or paper
> (similar to the glue-soaked cloth) and is in good condition away from
> the fault. Outside diameter is 2mm.
>
> Any suggestions or advice would be gratefully received.
>
> Mike, G3XDV
> ==========
>
>

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<div>Its teh output transformer, so even at the 1kW level voltage won&#39;=
t be all that high.=A0 Assuming 50 ohms, only 320V or so peak.=A0=A0 ANd=
 any heatshrink sleeve will withstand that.=A0=A0 The fact that the origin=
al insulation was only cloth with varnish reinforces that idea.</div>

<div>=A0</div>
<div>For tape, see if you can get soem of teh high temperature - it may be=
 mylar, or something else - transformer tape.=A0 Thin, and yellow in my ca=
se, but you see it a lot on SMPSU transformers so must be readily availabl=
e</div>

<div>=A0</div>
<div>But it is all that critical holding teh turns firmly inplace.=A0=A0=
 I&#39;d just be tempted to slap some varnish over the turns to hold them.=
</div>
<div>=A0</div>
<div>Andy</div>
<div>.<a href=3D"http://www.g4jnt.com">www.g4jnt.com</a></div>
<div><br><br>=A0</div>
<div class=3D"gmail_quote">On 12 April 2011 17:58, Mike Dennison <span dir=
=3D"ltr">&lt;<a href=3D"mailto:mike.dennison@ntlworld.com">mike.dennison@n=
tlworld.com</a>&gt;</span> wrote:<br>
<blockquote style=3D"BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8e=
x; PADDING-LEFT: 1ex" class=3D"gmail_quote">Further to my report on the fa=
iled combiner/output transformer in my<br>Decca QRO Tx, I have dismantled=
 the transformer. The really good news<br>
is that the huge (85mm dia) ferrite pot was not glued at all, and was<br>e=
asily unclamped. The internal wires were more difficult, being<br>wrapped=
 with cloth tape that seems to have been soaked in hard glue.<br><br>I fin=
ally got to the secondary coil and found the shorted turn. That<br>
coil comprises three layers wound on top of each other, but the cloth<br>w=
rapping was inadequate in one place, resulting in one turn from the<br>top=
 layer being adjacent to one on the bottom layer, and thus<br>providing a=
 higher voltage gradient than intended and shorting<br>
several turns.<br><br>So all I have to do is to improve the construction=
 (to avoid the<br>voltage gradient) and re-assemble. I will wrap some slee=
ving over the<br>slightly charred wires where the short occurred.<br><br>I=
 need some advice on what to use to replace the glue-soaked cloth<br>
tape. It needs to fix the turns and provide a high degree of<br>insulation=
. I could use ordinary plastic tape but it may suffer from<br>possible hig=
h temperatures (It is physically difficult to check<br>whether the transfo=
rmer gets hot whilst transmitting for long<br>
periods, but it&#39;s certainly likely).<br><br>I have looked at the Mapli=
n catalogue and they do &quot;Silicone Self<br>Fusing Repair Tape&quot; wh=
ich seems to resist high temperatures (up to<br>260 deg C) and high voltag=
es (it actually says &quot;7,000 volts/20mil<br>
(0.51mm) dielectric strength&quot;, which I don&#39;t really understand).<=
br><br>Should I replace the wire, or perhaps just its insulation? It is<br=
>solid, enamelled and insulated with some brown cloth or paper<br>(similar=
 to the glue-soaked cloth) and is in good condition away from<br>
the fault. Outside diameter is 2mm.<br><br>Any suggestions or advice would=
 be gratefully received.<br><br>Mike, G3XDV<br>=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D<br><br></blockquote></div><br>

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