RE: Atomic Laboratories 71869 Tesla Coil (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: List moderator <mod1@xxxxxxxxxx>

Moderators note:

We don't send out images to the list due to size concerns.  However, I do
have space on the web server for them so I put Pete's images there.  You
can find them at 
http://www.pupman.com/current/pbirk

Chip

---------- Forwarded message ----------
Date: Tue, 24 Oct 2006 08:55:51 -0400
From: Peter Birk <birk@xxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: RE: Atomic Laboratories 71869 Tesla Coil

Dave,

Thanks for your help.  Here's a picture of the capacitor and a circuit
diagram.  I found a frequency counter in my MFJ Antenna Analyzer.  The
secondary coil dips at 245 kHz without the ball and 236 kHz with the ball.
The primary dips at 217 kHz.  The capacitor had some green crusting on one
terminal - usually a fatal sign on an electrolytic capacitor.  It does seem
to check out using what I call the RC method.  I use an ohmeter at the
highest setting aned a capacitoer decade box.  The dip in the ohmeter as the
capacitor charges matches very well with the .01 MFD in the decade box.  I
was pleased to find the coils in close agreement.  I'm not sure how much the
20 kHz difference matters.  Not sure how I could change it (?replace the
ball with a toroid).  As you can see in the earlier pictures the primary is
entirely encased in a rubbery doughnut.  Thanks again.

Pete

-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Monday, October 23, 2006 1:49 PM
To: tesla@xxxxxxxxxx
Subject: Re: Atomic Laboratories 71869 Tesla Coil


Original poster: Davetracer@xxxxxxx

In a message dated 10/22/2006 8:53:02 AM Mountain Standard Time,
tesla@xxxxxxxxxx writes:
Original poster: "Peter Birk" <birk@xxxxxxx>

Never sent pictures through a list.  Hope they're not to big.  As you can
see the metal case is labelled Atomic Laboratories division of CENCO.
Probably was purchased new when the school opened around 1975. Tried to
measure the resonant frequency of the secondary with an oscilloscope with a
built in signal generator.  First dip occurred around 250KHz.  Found the
metal ball on top was not making contact - with the ball connected it
dropped to 225KHz.  Guess I need to short the spark gap and check the
primary but I was thinking I should try seperating the coils first and
measuring their independent resonant frequencies.  My plan is to unsolder
the bottom of the secondary where it connects to the potted network and
connect it to a piece of well insulated stranded 16 or 14 gauge wire with a
clip for external grounding.

http://hot-streamer.com/temp/PeterBirk/

-Peter Birk


Dear Peter,

Thanks for sending the pictures!

     While I am not one of the experts on Tesla Coils, I noticed a
few things in your pictures, and I have a few questions.

     This looks like a straightforward spark-gap Tesla Coil. The
spark gap looks like about 1/8" to 1/16", so probably the transformer
is in the 5,000 to 7,500 volt range. Are there any numbers stamped on
it? (e.g., "7.5 Kv").

    I am wondering if this transformer could have shorted to frame
ground and giving you the sparks you are seeing at the power switch.
Admittedly, not likely, but worth checking. This unit is old enough
that insulation breakdowns are possible. Spin it up in a dark room
and look for corona discharges inside the box that shouldn't be.

     Are there any numbers on the capacitor? That would sure be
helpful. Alternatively, you could unhook the leads to it and measure
it directly. I'm wondering if the cap could have failed or whatever.

     I can't quite see enough to figure out the schematic. Is the
capacitor in parallel with the transformer? Does the capacitor then
fire across the spark gap through the primary?

     I don't see any purpose for the RC network. The lead from the
bottom of the secondary dives into this network. I'm wondering if its
total purpose is to provide some sort of protection for people
drawing arcs from the coil. I realize this is idiotic; spark gap TC's
really jolt you. A more likely purpose is tuning the coil, done in a
strange way. This coil dates to before modern tuning techniques.

     When the unit runs, what sort of discharge do you get at the top
of the secondary? Anything? You've mentioned a half inch discharge at
the switch (I bet that's real uncomfortable). Have you tried running
a good solid ground to the bottom of the secondary just to see what happens?

     Do you have a 'scope to see what freq the coil is working
at?  (Caution, I fried a scope this way, don't get it anywhere close).

     Thanks again for the pictures, Peter, they really help.

     -- thanks,

     David Small