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

Tue May 18 23:05:50 PDT 2010

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
> 
> 
> 
> 
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