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Re: [Amps] Drake L-7 arcing on 80m

To: "Bill Dzurilla" <>, <>
Subject: Re: [Amps] Drake L-7 arcing on 80m
From: "Carl" <>
Date: Tue, 18 May 2010 22:47:50 -0400
List-post: <">>
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.


----- Original Message ----- 
From: "Bill Dzurilla" <>
To: <>
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:
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.


> Subject: [Amps] Drake L-7 arcing on 80m
> To:
> Message-ID: <>
> 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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