[TCML] Calibrating a sync spark gap

[Karl Thorkildsen <karl.thorkildsen@xxxxxxxxxxxxx>]

I'm in the process of building a synchronous spark gap.  I have 
modified a motor and built a rotor from G10.  My electrodes are 
mounted by drilling a hole the long way through a bolt, then drilling 
a hole sideways into the head and tapping for a set screw.  The bolts 
are mounted through the G10 with some washers as cooling fins, and 
titanium rods stuck through held in place with the set screws.  I'll 
upload some pictures later and link to them.

The problem is how to set the position correctly.  Here's what I did:

Hook up an NST to the gap and connect one channel of my scope to the 
AC line of the NST (actually going through a variac too).  I took a 
piece of hookup wire, alligator clips on the ends, and wrapped it 
around one of the output wires of the NST, about 5 wraps.  The ends 
of this wire go across the other channel of the scope, signal to 
gnd.  Basically I built a cheap current probe.

Now firing the thing up, I set the scope to trigger on the current 
probe.  Basically getting a pulse every time the gap lines up.  And 
looked at the position of the AC line voltage relative to the pulse.

Using this method I got a fairly consistent display on the scope and 
was able to adjust the position by rotating the motor in the mount.

Question, I think I remember reading that I should sync to about 85% 
of peak.  In other words it should fire a little early, is that right?
- Karl

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