And I know what an HT shock feels like after an unfortunate incident with a James Captain when I was about 14. Quite a jolt.
Crispin,
You've only had ONE shock from HT? You must officially hold the forum record for the fewest such shocks.
I have a few ideas that I don't think have been shared yet, however, I'm not really optimistic any of them hold the answer if all you did between sparking and not sparking was clean the points.
In no particular order:
1. Is the correct notch in the cam ring sitting over the cam ring stop peg and is the stop peg visible and not broken? One of the pictures below shows two stop pegs and covers (though, there is only one in the mag). The other picture shows the stop peg location with the cover in place. If the stop peg is missing or broken off it is possible that the cam ring is rotated such that the points open when the brass on the slip ring is not in contact with the pickup brush.
2. It is possible that the connections of the plug wires within the brush holders have come loose or otherwise degraded. With so much trouble going on, it would not be a bad idea to renew these connections anyway, meaning, cut back a little wire for a fresh end for the pickup to screw into. (we assume you are using copper plug wire and not carbon-impregnated fabric filament plug wire).
3. Be sure the springs that hold the brushes in the holders are seated at the bottom of the holes in the holders. They may be pushed down with a very small screwdriver (I have also used a cut-off toothpick).
I think the idea of finding a bit of local help is a good one. It doesn't mean you couldn't do it yourself. Sometimes when we are frustrated we just go blind to the big picture and it can really help to have another set of eyes on the issue.
Richard L.