Topband: Need for capacitor

daraymond at iowatelecom.net daraymond at iowatelecom.net
Fri Dec 28 16:16:08 EST 2018


Greetings topbanders. . .

While CQing early this morning I began noticing a faint but troubling smell in the shack.  It smelled like over heated wiring or electrical components.  It continued to get stronger as I realized the odor was coming from the Alpha 99 amp.  Indeed, a few moments later it tripped out.  I dug into it this afternoon and discovered a leaky 75 pF/6 kV cap which is one of five such caps in a parallel bank that is pulled in on the band switch to achieve tank resonance only on 160m.  I have removed the bad cap making the amp operational once again with the air variable tuning cap making up the lost C (and there’s still a little C to spare on the air-variable).  After some searching I cannot find a 75 pF/6 kV cap.  Mouser does not have anything even close in terms of the high voltage rating (hopefully I didn’t something).  The closet thing Newark has is 47 pF which I have ordered.  I’m guessing the 47 pF is probably OK but how important is equal current distribution across the five caps?  Maybe I don’t need to be concerned about it?  If I really do need the correct 75 pF/6 kV cap does anyone have ideas of a source?    Replying privately is fine.   See you all in the Stew.        73. . .Dave, W0FLS


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