The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: roadrocket on 17.06. 2016 17:12

Title: No spark after new slipring
Post by: roadrocket on 17.06. 2016 17:12

I have just replaced the slipring on my trusty (yes) K2F, since a hefty groove was worn in the old one. After reassembly I have no spark. I am rather sure that I have not broken any components, and have taken off safety screws and earth brush during assembly. When I put on the new slipring, I just let the copper wire from the armature poke into the new slipring, and did not observe any particular moves here - could this be the issue? Are there any obvious parts that need to be isolated from each other that I might have upset? The paper gasket between points housing and main housing is really worse for wear, but the shimming of the bearings is good. I had good use of a very trick inner race extractor.

Otto in Denmark
Title: Re: No spark after new slipring
Post by: groily on 17.06. 2016 21:34
I think, Otto, it would be a good first move to take off an HT pick-up and measure the resistance from the brass track of the slipring to the mag body (you might have to turn the mag/engine slowly to get it into the line of sight). You want to see about 5K ohms in round numbers - this will prove whether the slipring is actually touching the wire spike from the coil. It needs to!
Sometimes when there is no reading the spike is nearly touching, and you would still get a spark (as it will jump the gap); but if the spike has got crushed and not entered into the very small hole at the bottom of  the larger hole in the boss of the slipring, then you could have no sparks.
('Nearly' touching isn't really good enough though, as arcing will cause deterioration of the slipring and/or the extreme tip of the HT spike and things can only get worse.)

Quite often, the insulation round the base of the spike on a coil is quite chunky - and either comes up against the bottom of the wider section of the hole inside the slipring before the spike has penetrated far enough to make proper contact with the brass segment, or it won't enter the larger diameter hole at all and things get rather squashed and out of shape. In which case, the fatter part of the hole needs enlarging, preferably with a drill held by hand, to make it the right size to accept the insulated bit of the wire.

Occasionally, the length of bare spike is simply too short to make good contact inside a replacement slipring. In that case, life is harder - but it is possible to solder an extension onto the too-short spike, preferably by pushing a piece of single-strand wire up into the insulated section (to secure it) and then carefully soldering where the bare section starts.

If there is any part of the bare HT take-off wire visible, there is the risk of the spark jumping to the nearest metal object (the brass end cheek or something in the magneto body when the instrument is assembled). A layer or two of heatshrink tubing -which enters into the larger hole - will usually be enough to resolve that.

If there is no connectivity between the slipring and the coil, unfortunately you're going to have to use that trick puller one more time to investigate!
Title: Re: No spark after new slipring
Post by: roadrocket on 17.06. 2016 21:43
Thanks Groily!

I have just read a very long post on another board about condensors, and also learned a little about sliprings, so I think I'm left without connectivity between slipring and armature, but will investigate toorrow. I hope this is just it!

Otto in DK
Title: Re: No spark after new slipring
Post by: roadrocket on 18.06. 2016 10:37
Now I'm at a loss! I have disassembled the mag, and taken the slipring off again. I could not measure any conductivity from the spike to anywhere on the armature, including the two wires that go the condenser. I have not damaged anything during dismantling. The spike is easily long enough to reach into the slipring, and it does not bend, so must poke into the hole in the brass segment. Should there be conduvtivity from the spike to the armature? Everything looks sound, and the mag was rebuilt by a pro ten years ago. It worked great before I embarked on changing the slipring.

Otto in DK
Title: Re: No spark after new slipring
Post by: a10gf on 18.06. 2016 11:14
Here's the circuit, to measure ohm\conductivity.

My measurements "Primary winding approx 0,5 ohm, secondary approx 5,1 kohm"
Title: Re: No spark after new slipring
Post by: roadrocket on 18.06. 2016 12:51
Thanks, but I need it even more idiot proof. I have assemled the whole thing again, I meausure 6800 ohms from where the earth brush bear to the spike, and thats not bad I guess. With the slipring i measure the same to the brass segment, som I'm good with the spike/slipring connection. I have not damaged anything I'm certain, so I think I'm looking for something else... What have i done wrong? When I turn the assembly with a drill and poke a screwdriver into the pickup hole I usually have a million sparks, now I only have a very faint little spark when the screwdriver is very close to the slipring. I really can't imagine that I have damaged anything, and the armature looks as sound as it did ten years ago. I don't think it is likely that the condenser gets tired all of a sudden, but can you fry it somehow?
Title: Re: No spark after new slipring
Post by: roadrocket on 18.06. 2016 21:07
Oh well, now it works!  *smile* But exactly what I did to do that is beyond me. I put i back together one more time, and then I had two fat sparks just turning by hand. Back on the bike it fired right up. The ghost of old Joseph was havning fun.

Otto in DK
Title: Re: No spark after new slipring
Post by: groily on 20.06. 2016 11:17
It's good it works!
Could it be you had the probe of the meter on a bit of HT spike still covered in insulating varnish? If so, you'd probably get no reading unless you scraped the spike clean. Oftentimes, the spike is only shiny clean at the sharp end as it were, where it needs to touch the slipring to make the connection.
Your 6800 ohms sounds fine by the way - depends who did the wind, and what recipe they used. There are a lot out there in that range, but as A10GF says, 5K is more typical. Doesn't matter really, as long as the turns ratio and the wire gauges are complementary and about right, which I'm sure they will be. There's a lot to be said for having a goodly load of copper on there. It is often said that Lucas laid more wire on their Competition coils anyway - I've seen 6K+ ohms on originals before now. And at the lower end, nearer 4 or 4.5K on some rewinds.