Message-ID: <C888AF6CE1784403BE7F646F9CE82C98@MaynePC>
From: <VE7SL@shaw.ca>
To: <rsgb_lf_group@blacksheep.org>
References: <4DA492C9.4329.AAE777@mike.dennison.ntlworld.com>
In-Reply-To: <4DA492C9.4329.AAE777@mike.dennison.ntlworld.com>
Date: Tue, 12 Apr 2011 10:14:22 -0700
MIME-Version: 1.0
Subject: LF: Re: Insulating tape / Decca repair
Content-Type: text/plain;
	format=flowed;
	charset="iso-8859-1";
	reply-type=original
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


Mike - 3M / Scotch makes a woven fiberglass-cloth tape (#27 ?) that Scott 
and I have used on previous output transformers...it's not cheap but is 
perfect for this purpose.

Steve / VE7SL


> 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
> ==========
>
>


WEB - "The VE7SL Radio Notebook":  http://members.shaw.ca/ve7sl