[Amps] Drake L-7 arcing on 80m - with video

Wed May 19 10:49:15 PDT 2010

Carl and all,

I may have lucked out this time.  I was able to obtain full 1200 watts output on both 80 and 160 meters simply by refusing to turn the Load capacitor to less than 4.5.  I left the Load cap on 5 and tuned the Plate cap for maximum output while slowly increasing drive.  The video camera was rolling, but no arcing, and no sign of excessive redness in the tubes.

Still not sure everything is 100%.  It seems as though the amp wants to see less loading, because output went up when I turned the Load cap from 5 to 4.5.  You can see this in yesterday's video.  At the moment of the arc, I was turning the Plate cap to the left and output was increasing.  The arc occurred with Plate at 4 or perhaps even 3.5, so today I just would not go below 4.5.

So am I good to go or what?  Several folks said right from the start that my problem was under-coupling, and it seems they were correct.

73 and many thanks for all the responses and emails,
Bill NZ5N 

--- On Wed, 5/19/10, Carl <km1h at jeremy.mv.com> wrote:

> From: Carl <km1h at jeremy.mv.com>
> Subject: Re: [Amps] Drake L-7 arcing on 80m - with video
> To: "Bill Dzurilla" <billdz.geo at yahoo.com>, amps at contesting.com
> Date: Wednesday, May 19, 2010, 10:07 AM
> These old eyes didnt capture all the
> detail in the video I guess!
> Unfortunately staked plates cant be removed to smooth and
> polish. 
> Alternatives would be to retrofit a different cap unless a
> Drake can be 
> found. Or smooth the bad plates as well as possible and
> give them a spray 
> coat of acrylic clear from Plasticote or similar.
> 
> Carl
> KM1H
> 
> -----------------
> 
> 
> Many thanks to all who watched the video and emailed me
> with ideas.  I have 
> watched it 50 times and I'm now convinced that the arc
> occurred in the plate 
> cap stator that has the white graffiti-like traces, as
> described in a prior 
> post.  The flashes on the coil are just
> reflections.  If you look carefully 
> through the plastic bubble on back of the plate current
> meter, you can see 
> the arc originating in the plate tuning capacitor.
> 
> Suggested fixes have included:
> * replacing the mica caps mounted underneath the rear wafer
> of the 
> bandswitch,
> * replacing the plate blocking capacitors,
> * polishing or totally removing the stator with the white
> traces,
> * Carl's suggestion below to try a piece of insulation
> between the coil and 
> the cap.
> 
> Have not yet tried any of these suggestions, but please
> keep the ideas 
> coming.
> 
> 73, Bill NZ5N
> 
> --- On Tue, 5/18/10, Carl <km1h at jeremy.mv.com>
> wrote:
> 
> > From: Carl <km1h at jeremy.mv.com>
> > Subject: Re: [Amps] Drake L-7 arcing on 80m
> > To: "Bill Dzurilla" <billdz.geo at yahoo.com>,
> amps at contesting.com
> > Date: Tuesday, May 18, 2010, 10:47 PM
> > That is strange. Try placing a thin
> > insulator on top of the cap: teflon, polystyrene,
> > fiberglass, etc and see if its just the coil or the
> cap
> > arcing independently and not from one to the other.
> >
> > Also check all RF connections for crystallized solder
> due
> > to RF heating. If so suck or wick it off and reflow
> new
> > solder.
> >
> > Carl
> > KM1H
> >
> >
> >
> >
> >
> > ----- Original Message ----- From: "Bill Dzurilla"
> <billdz.geo at yahoo.com>
> > To: <amps at contesting.com>
> > Sent: Tuesday, May 18, 2010 10:26 PM
> > Subject: Re: [Amps] Drake L-7 arcing on 80m
> >
> >
> > Thanks to all who responded on and off line. As
> > suggested by Carl, I removed the cover and tried it
> > again. Rig was connected to a Heathkit Cantenna dummy
> > load. As suggested by several, I set the Load control
> > to 5 at the start, to prevent under-coupling. I made
> a
> > video of the procedure and, as you can see at:
> > http://www.youtube.com/watch?v=iJRyNQnkEWA
> > there was arcing again. The arc occurs near the end
> > of the video clip, at the 1:18 mark. You can see it
> on
> > the first 2 turns of the coil that is on top of the
> Plate
> > capacitor. Not sure if this is a reflection or if the
> > coil itself was arcing. I did not see any arcing from
> > the capacitor.
> >
> > Not sure how visible it is on YouTube, but I have
> watched
> > this several times in slow motion. You can see a bit
> > of the rotor of the Load capacitor on the left side of
> the
> > screen, and also a bit of the output meter. At least
> in
> > slo-mo, you can see output going up as I start to turn
> the
> > Load cap to the left, then the tubes start to redden
> and the
> > arc occurs.
> >
> > If the turning of the Load cap to the left resulted
> in
> > under-coupling and caused the arc, I'm not sure how
> to
> > proceed, because output was very low with the Load cap
> set
> > at 5.
> >
> > Comments, ideas, advice?
> >
> > 73, Bill NZ5N
> >
> > > Take the cover off, bypass the interlock, and do
> a
> > visual when in >operation.
> > > There could be a switch problem or fixed cap
> causing
> > the variable to arc.
> >
> > > Carl
> > > KM1H
> >
> > > Subject: [Amps] Drake L-7 arcing on 80m
> > > To: amps at contesting.com
> > > Message-ID: <28539.87346.qm at web51103.mail.re2.yahoo.com>
> > > Content-Type: text/plain; charset=us-ascii
> > >
> > > Hello,
> > >
> > > I recently acquired an L-7 amp and I tried it
> out
> > > yesterday. I used my Pro II as the exciter, an
> amp
> > > keyer, and a dummy load.
> > >
> > > There seems to be some problem on 80,
> unfortunately.
> > > On all bands, I started with 20 watts drive and
> > slowly
> > > increased the drive while tuning for max output.
> > > When
> > > I got up to about 600 watts out on 80, I heard
> and
> > saw
> > > through the meter a blue arc. This happened twice
> on
> > > 80, and I did not want to try it again. No arcing
> at
> > > all on 40, 20, 15, or 10.
> > >
> > > I opened up the amp and everything looks OK,
> except
> > for one
> > > of the 25 or so stators on the plate tuning
> capacitor
> > has
> > > graffiti-like white traces on it. I suspect this
> is
> > > the source of the 80m arcing, as the rotor does
> not go
> > over
> > > the spot with the traces when set to 40m and up.
> I
> > > have not tried the amp on 160m, but if my theory
> is
> > correct
> > > it would arc on 160 as well as 80.
> > >
> > > The capacitor plates are not dented or deformed
> in
> > any
> > > way. Have not tried the amp on the WARC bands.
> > >
> > > So, do you think the stator is the source of the
> > > problem? If yes, is there any fix?
> > >
> > > 73, Bill NZ5N
> >
> >
> >
> >
> > _______________________________________________
> > Amps mailing list
> > Amps at contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
> 
> 
> 
> 