The death of a classic - First look

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Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Finn and All,

Today I got the failed strings from Finn's cap.  It sure does smell!  I
love the smell of burning electronics :o)))

I tested a number of the resistors and they were all perfectly in spec.
They showed no signs of stress.  The resistor I was concerned with before
was in the middle of a pretty bad fire.  The fire damage to the boards is
very profound.  Seeing the boards first hand and testing the resistors, I
can find no evidence the resistors caused the failure.  They seemed to be
perfectly good and adequate.

I disassembled a number of caps that were not involved in the fire or were
in the "good" string.  The is no evidence of end plate or current heating
damage.  They appeared to run cool without any thermal stress.

However.........  All of the cap's internal plates and dielectrics are
"machine gunned".

http://hot-streamer-dot-com/temp/finn-001.jpg

There are literally thousands of holes blown in the dielectric layers that
have self healed.  Some areas are very bad.  Considering the large number
of caps in Finn's MMC, catastrophic failure was almost certain.  One cap
had a section blown out of the case but did not burn up.  It was a disaster
that never happened..., but it tells a story of serious problems...

I will have to get to a high power scope tomorrow, but there are three
possibilities:

1.  Over voltage on the cap caused millions of dielectric breakdowns.  A
"few" did not self heal and ignited the caps

2.  Ion damage to the dielectrics caused weak spots that eventually failed.

3.  High currents in the internal metalized plate degraded and failed the
thin metal center internal layer.  

Finn stated a very possible clue:

"Gap voltage is 32 kV (most of the time, but I am sure I`ve hit it with up
to 45kV for quite some time as well)"

45kV is 40% over the caps rating.  "I" would only go to 25% on EMMCs.  If a
few caps in a string breakdown and go to zero volts, it can put even more
voltage stress on the others in the string causing further breakdowns.  I
note that some areas have slight corrosion suggesting that the damage is
older than a few days.

When I can see the areas under high magnification tomorrow, I can determine
which it was.  But it sure "looks" like the first possibility.

BTW - No doubt about MMC's ability to self heal now!!!  Even though it
"eventually" failed.  Finn's MMC easily has millions of self healing events!

Unfortunately, the rest of Finn's cap is certainly heavily damaged too.
Time to take it out of service and put it in a trophy case so It can be
admired for generations to come :-))  Maybe if you explained to the museum
that it was one of the first of it's kind, they would accept it as a
display item.  100 years from now, it will be a big piece of Tesla coiling
history!

Cheers,

	Terry