NST Repair Was: [TCML] Don't forget the Terry Filter! neighbors

[bartb <bartb@xxxxxxxxxxxxxxxx>]

Hi All,

Regarding the NST that died. I melted wax (little over an hour of 
heating) and poured into a large metal coffee can (took 2 cans and 
filled the 2nd up about 1/2 full). I popped out the core and took it 
apart. No visible signs of arcing. Measured the hv windings and both 
were good. So likely, an air bubble near one of the hv windings caused a 
carbon track initiated by corona (my best guess). This morning, I put it 
back together and dry tested it on the work bench (measured 14kV across 
windings, so all is good again). I also made a slight mechanical 
upgrade. The hv windings are using 36 Awg and are very fine. I decided 
to solder 24 Awg to the 36Awg and wrap 1 turn of the 24 Awg (taping in 
place) just to make handling a bit stronger (that worked out great).

I mounted the core back in the NST case, remelted and poured the wax 
back in the case. I thought about using Vasoline as Gary Lau has used, 
but I just had too much to do today and wanted to simply get it back 
together. The NST ran fine since 2006 with the wax potting and the 
depot/repot process is so quick and easy now that's it's no big deal. 
This is my 3rd or 4th time doing this with this NST (but as it is a 
200mA NST, it's worth it to me). I think this is the first time it died 
since the first repairs in 2006.

With regards to depotting NST's, the initial black tar removal is a task 
not cut out for the faint of heart. But, once you get past that, the 
rest is gravy. As much as we recommend not depotting NST's due to the 
mess and difficulty, I'm very glad I did. This has turned out to be one 
heck of a transformer (equivalent to a PT). Initially, I removed 1/2 the 
shunts in each pack, but during the last depot in 2006, I inserted about 
3 or 4 laminations into each shunt pack to gain a little more current 
limiting (I think I measured 196mA on last measurements).

BTW, I'm using 15kV GTO out of each HV bushing. The bushing are epoxied 
to the case as well as the GTO wire inside the bushing itself. The 120V 
line into the case was also upgraded to a metal cord grip that is also 
epoxied to the case. I did this in 2006. My reason was to seal the case 
well in the event I had to repair it (I don't like the gasket design). 
During this depotting experience there were no leaks, so the choice to 
do that back in 2006 proved itself worthwhile.

Take care,
Bart


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