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 voltages.
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!

-- 
73 Warren K2ORS
                WD2XGJ
                WD2XSH/23
                WE2XEB/2
                WE2XGR/1




On Tue, Apr 12, 2011 at 1:13 PM, Andy Talbot <[email protected]> wrote:
> 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 <[email protected]> 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
>> ==========
>>
>
>