Re: MMC partial failiure

Hi Nick,

	Carefully look at the caps.  Almost always you will see that they have
swelled up or the case has split somewhere.  However, 5kV should be far
below any voltage breakdown. and three strings should not be stressed
either at this voltage.  I would definitely suspect something else is going
on...  Don't know what but maybe the gap is overheating or the variac is
burning a brush...  I have seen sync rotaries loose sync and do this...


At 12:56 PM 7/26/99 -0400, you wrote:
>Hi All,
>         my little coil's mmc seems to be undergoing some sort of weird 
>failiure mode.
>The coil functions normally, throwing nice 18" arcs and drawing 80ma -at- 5kV, 
>then it starts to pulse in and out of tune.  
>The secondary discharge stops almost totally and the spark gap makes the sort 
>of hiss that is does when the coil is very badly tuned.  After a few seconds 
>it pulses back into tune only to drop out again and repeat what I have 
>already described.
>My theory is that in one of the 3 strings of caps is a cap close to death and 
>that it is breaking down reducing the capacitance of the string and taking 
>the coil out of tune, then self healing a bit then breaking down again.  
>Unfortunately not having an accurate C meter I can't measure the strings to 
>check this.  
>The caps are 1600V 47nF philips 378 series in strings of 10 with 3 strings in 
>parallel  and the neon is a 6/60 (I said 80ma 5kV because it sags quite a lot 
>under load) 
>It is not a reso-cap so I don't see how a large enough voltage to damage the 
>caps is being generated off a 6/60 neon?
>Semi-baffled in GB
>Nick Field