DomainKey-Signature: a=rsa-sha1; c=nofws;
        d=gmail.com; s=gamma;
        h=mime-version:in-reply-to:references:date:message-id:subject:from:to
         :content-type:content-transfer-encoding;
        b=k2iU4YI+uo+YNQlQFnhuWY2jJ8EkVXi4PgPuc18gff9UUZy9m3vg5UKomTI3RGqIHB
         6oI5mmLpM/5w1mqs5g9WCXKJEonmmta8IEm7b0f1Cjl0bVN5G8dV8ESr75Ix7D56Iylm
         FcOqhGy346TkPQ7eSvF63DygA8q0Itk5sLWFw=
MIME-Version: 1.0
In-Reply-To: <BANLkTikpMpq000P94AQL8CJJWdgDjkyW6A@mail.gmail.com>
References: <4DA492C9.4329.AAE777@mike.dennison.ntlworld.com>
	<BANLkTikpMpq000P94AQL8CJJWdgDjkyW6A@mail.gmail.com>
Date: Tue, 12 Apr 2011 14:39:26 -0400
Message-ID: <BANLkTimUAN7gpntczY+vb7n6R6ZVgPK+Kw@mail.gmail.com>
From: Warren Ziegler <wd2xgj@gmail.com>
To: rsgb_lf_group@blacksheep.org
DomainKey-Status: good (testing) 
Subject: Re: LF: Insulating tape / Decca repair
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


Mike,

    Andy is spot on as for normal operating conditions, but if Murphy
has his way (as he usually does) there are a couple of scenarios where
the 320V peak can be greatly exceeded.
One would be a nearby lightning strike which could induce substantial volt=
ages.
Another is mis-tuning.
Both have happened to me!
The 1st case (lightning) is obvious.
The 2nd case (admittedly operator error) was when I had my synthesizer
tuned to 185 kHz for use with another transmitter and antenna, I
transferred it back to the Decca and its antenna tuned to 137 with
reprogramming the synthesizer . The result burned out the output
transformer and burned the teflon SO-239 connector ( and the PL-259
attached to it)!

Go for fresh wire and the best insulation you can get, it may be more
tolerant of mis-adventures!

--=20
73 Warren K2ORS
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 WD2XGJ
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 WD2XSH/23
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 WE2XEB/2
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 WE2XGR/1




On Tue, Apr 12, 2011 at 1:13 PM, Andy Talbot <andy.g4jnt@gmail.com> wrote:
> Its teh output transformer, so even at the 1kW level voltage won't be al=
l
> that high.=A0 Assuming 50 ohms, only 320V or so peak.=A0=A0 ANd any heat=
shrink
> sleeve will withstand that.=A0=A0 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 my=
lar,
> or something else - transformer tape.=A0 Thin, and yellow in my case, bu=
t you
> see it a lot on SMPSU transformers so must be readily available
>
> But it is all that critical holding teh turns firmly inplace.=A0=A0 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
>> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>>
>
>