[Amps] snapping capacitor case

[John T. M. Lyles]

Hi Jim

Thats a wierd one.
I assume that the GE capacitor has two studs on it, and that one is 
going to ground, or close to ground with a current sensing resistor 
off ground. It might be possible that the internal wire to the foil 
is broken and that the thing is breaking down inside the package, to 
get that return path to your charging supply  (for both ripple 
current and the DC fixed potential across the break). So then the 
snapping is inside the cap, and sounds like it is external. Have you 
witnessed said sparking visually? Only other possibility might be 
that there is something else nearby which actually has the high 
potential and is breaking down TO the capacitor case as it is too 
close?

If the internal wire is open, the capacitor is history, as the peak 
current into it requires a good conductor to work well. I would think 
that a high potter on the cap would prove out this, as it would be 
erratic on charging current swing, it might be snapping.

Be careful! We have much bigger GE caps with 4 uF and 40 kV on them 
here, and when they spark internally, they are quite amazing, the 
fire, the explosion, the smoke. We replaced about 300 of them last 
year, which were > 10 years old, as they were beginning to bother the 
fire department crew with the drills they had to attend to. GE oil is 
flammable, given enough heat to reach flash point. Only the finest 
PCB style are the best for fire retarding.

[JIM, By the way, I have the bigger modulator deck and power supply 
for the Johnson 500 almost done, except point to point wiring. 
Getting > 200 lbs of iron into a small rack under the Johnson was a 
chore. The 500 itself hasn't been completely restored yet. I have 
2500 VDC for the RF and 2000 for the modulators in my design with a 
~2:1 RCA modulation transformer. The bottom covers of the chassis 
were missing so that cooling isn't right for the 4-400A, so I am 
looking for some. Any ideas? I may have to fabricate them myself. ]

73
John
K5PRO
New Mexico


>
>From: JimMoci at aol.com
>To: amps at contesting.com
>Subject: [Amps] HV Capacitor arcing problem????
>Hello all~
>
>I am having a problem with my homebrew amplifier and am looking for some
>advice. It is a 4-1000A amp in a rack with the HV power supply mounted to the
>bottom of the rack.
>
>As of late I started to hear an intemittent snap or arc. After some
>investigation I discovered that this arc was coming from the side of the
>metal case of my 32uF 4500V oil filled GE cap (it is not coming from the
>screw terminals). It just seems to arc to the bottom of the rack cabinet and
>happens anywhere from every 5 to 10 minutes. It is mounted with a bracket to
>the bottom of the rack which is painted steel. I sanded the paint down to
>bare metal to be sure there was a good electrical connection and this has
>reduced the frequency of the arcing. Unfortunately when it does arc now, it
>is an even louder SNAP! I even soldered a ground strap to the cap and
>grounded it to the chassis with no effect.
>
>What do I do next ???