[TCML] Subject: Overheated Secondary

[bartb]

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