[TCML] Subject: Overheated Secondary

[mark olson]

At 11:35 PM 1/3/08 -0800, you wrote:
Thanks again Bart,

I will begin repairs tomorrow (Saturday). Surely I will post the results of 
this venture.  Most of the tar is gone,
but even with gasoline I am not sure that it is possible to get all of the 
residue out.  I have been a mechanic
of one kind or another for most of 50 years, so gasoline is no strange 
solvent for me, but it is rather expensive.

later later,

Marko


>Hi Marko,
>
>Inner windings are connected to the core (which is center tap 
>configuration). Each secondary puts out about 6kVrms in your tranny. The 
>outer portion of the secondary winding is the high voltage end. This is 
>the area which usually shorts and your tranny appears to be no exception. 
>Typically a 2 or 3 layer removal will solve the problem. Some damage 
>occurs just in the depotting process itself from mechanical stress of 
>removing the tar. But, after a few layers of unwinding the damaged area of 
>the effected secondary, it's good as new again.
>
>I like to use new GTO cable on the hv outer secondary windings (which the 
>tranny originally used). It's good for 15kV insulation and perfect for NST 
>rebuilds. I bought like 100 feet of this stuff a year ago and it's come in 
>real handy (not all that expensive either). You may have found some gasket 
>or cardboard material in the potting compound. Insert something similar 
>when you repot the tranny. Allow the cardboard to keep the GTO cable out 
>of harms way (mechanically and electrically meaning more space away from 
>core and case is good).
>
>Removing 1/2 the shunts will put your transformer into major power mode. 
>The tranny will push a lot of current to the cap bank. It's not linear (if 
>you remove about 1/2 the shunts your current may go up x3). However, 
>that's what I did on my 12/60 and achieved about 200mA (big jump in 
>current). My shunts where divided into two sets. Yours looks a bit 
>different, but regardless, remove 1/3 to 1/2 of the shunts to boost the 
>current. The difference is night and day.
>
>Something I did was to remove several secondary layers to reduce the 
>output voltage. When I removed 1/2 the shunts, I got a big current boost 
>and reduced the voltage to a manageable level. Don't remove more than say 
>15 layers however. That will reduce your voltage down to about 10.5kVrms 
>and it's nice to keep the output voltage near the original value as much 
>as possible. However, if you find yourself unwinding more than you 
>actually wanted to, don't worry about it too much. Simply adjust the gap 
>for the new lower arc voltage. It will do fine and due to the current 
>increase, much better than previously realized.
>
>Try the tranny dry during shunt reduction. If you like it, pot it with 
>your flavor of potting compound (wax, oil, whatever). Use trial and error 
>with the shunts. If it seems like a bit much, add a few more shunt slabs 
>in there until you find what your coils happy with. But definitely, while 
>you have it apart, now is the time to beef it up by removing 1/3 to 1/2 of 
>the shunts. That's exactly what I would do.
>
>Take care,
>Bart
>
>>Thanks Bart,
>>
>>My photos are here:  http://members.cox.net/kc5gym/
>>
>>I have a question or two, maybe a million.
>>There is a picture of the shunt in my hand. I should remove 1/2 of these?
>>
>>I was not able to preserve the original "angel hair" wire terminations on 
>>the secondaries.  Are the inner windings
>>connected to center tap or are they the hv ends?  I suspect the outer 
>>ends are the outputs.
>>
>>Thanks
>>Marko
>
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