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Re: [TCML] Subject: Overheated Secondary

Bart, I was wondering if you would know off hand if it is feasible to completely remove the shunts of  a 15/60 NST  without burning up the secondary winding? Any estimate as to what the current output might equal when used in a TC? Any comparison to the durability of  MOT's? Looking for a winter project. Wyatt

-------------- Original message -------------- 
From: bartb <bartb@xxxxxxxxxxxxxxxx> 

> 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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