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Re: [TCML] Subject: Overheated Secondary

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,

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.


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