[Elecraft] Topband: Milt Sullivan, RL Drake Chief Engineer, SK

[Alan Bloom]

Hi Herb,

I'd have to open up my L75 to be sure, but I believe all I did was etch
away some of the copper around the outside edge of the board (to prevent
arcing around the edge of the board) and then sprayed it with Q-dope to
prevent arcing to the air (corona discharge) from the edges of the
copper.

It is also very important to use low-loss PC board material.  When I
tried it with standard FR4 PC board material, the capacitor started to
smoke after a few seconds with a couple hundred watts of RF.  We used
glass-teflon PC board, but other low-loss material like Duroid might
also work.  http://www.rogerscorp.com

Alan


On Fri, 2010-11-12 at 15:39 -0400, Herb Schoenbohm wrote:
> 
> > I was working on at the time.  For example, the MN2700 kilowatt antenna
> > tuner required some high-voltage, high-current fixed capacitors.  We
> > were quoted a ridiculously high price and a 6-month lead time by the
> > vendor.  So we figured out a way to make our own capacitors from Teflon
> > printed circuit board (for low loss / high Q) spray-painted with Q dope
> > to prevent corona discharge.
> >
> >
> > Alan N1AL
> >
> > Alan,  That component design of Milt was also  was used in the Drake 
> > L-7 amp.  I lost that cap several years ago in both the amp and the 
> > add on ones in the MN2700. For the amp a I found a door knob with 
> > enough current  and voltage to work.  But for the tuner there just is 
> > not enough space to mount the replacements fixed caps. Do you know of 
> > anyone who makes these flat board PC caps? Or could someone make a 
> > sandwitch out of some doble side copper PC board.  I think, however 
> > the edges would be a voltage breakdown problem and Milt's unique 
> > design had an island of insulation around the edges plus the Q dope.
> 
> Thanks,
> 
> Herb Schoenbohm, KV4FZ