Overall, a very interesting and informative series of posts, Magnetoman. A few comments/queries:-
The black cylinder at 4:00 is a carbon brush that completes the primary circuit back to earth when the points close. To do this, the carbon brush has to make good contact with the surface it rubs against.
Could you explain this peculiarity of the Bosch ZEV? In all other models of rotating-coil magneto I've examined, that black cylinder (the earth brush) is part of the secondary
circuit, and it's doing its business primarily when the points are open
to provide continuity for the plug current from the magneto body (and therefore the sparking plug body) back to the armature body bypassing the armature bearings and their insulators.
To make sure the material used for the aftermarket slip ring isn't conductive I wrapped a number of turns of stainless steel wire around it and measured the resistance between the wire and the armature housing. ... Because the slip ring in turn is connected to the coil, this test also measured whether there was any leakage from the coil to the armature. The resistance was over 11 GOhms with a 2500 volt potential applied, which is comparable (i.e. about half) to the voltage it will be subjected to in actual operation. This amount of leakage is completely negligible, so the material in the slip ring is good, as is the insulation between the coil and the armature.
I'm not so sure you should draw the conclusion from this test that the slip ring is good. 11 gigaohms at 2500 volts can certainly break down to a far lower resistance at the higher operating voltage of the slip ring. Breakdown of aftermarket slip rings is indeed a problem, but in my experience it often only rears its head at elevated temperature. A slip ring may test good at room temperature, but start breaking down at operating temperature.
I am also concerned that, with this test, you are applying the 2500 volts not only to the slip ring conductor and the HT tail of the HT winding, but also to the whole of the HT winding and more worryingly the LT winding. In operation, the insulation around the LT winding sees voltages of perhaps 250 V, and it might be fair enough to test it to three times that voltage, but testing it to ten times that voltage is, IMHO, asking for trouble. Even if it passes the test, the test may itself trigger the onset of a later breakdown. If you were a professional restorer and if you were to do that, I think you ought, as you have said in the past, to give your customers their money back.
I use an Eisemann tester for extended tests of coils ... I ran the coil for an hour on this tester with the gap set to 5 mm (6 kV). The coil works fine so I will use it instead of winding one of my own.
The Eisemann tester looks like a nice piece of kit. But does it get the armature up to operating temperature, or do you heat the armature elsewhere and then test it on the Eisemann while it's still warm?
Currently another supplier is advertising replacement condensers that are of a type not rated for pulsed high current applications, and whose dielectric layers are made of a porous oxide. Even if they survive the current pulses of a stressed magneto, no extended environmental testing has been done of them, so there is no way to even guess how long they might last in the ozone-rich atmosphere near the points before delamination or breakdown of the oxide might set in.
If you are going to have yet another tedious
go about the Brightspark EasyCap, it might help people if you named names instead of leaving them wondering. The fact is EasyCaps have undergone extended environmental testing; they carry on working; they are guaranteed; if one were to fail, it is a ten minute job to replace it
without the need to pull the armature apart and mill out the old one
as you have described in detail above; and last but not least the customers and their bikes are happy.
The Fall and Winter 2011 issues of The Antique Motorcycle magazine contain a two-part article by physicist Dr. Charles Falco explaining the science of why the wax paper condensers that Lucas used in their post-WWII magnetos have a limited lifetime, ...
Yes, that was a very interesting article. One thing I did find puzzling about it was Dr Falco's reasoning that a breakdown of the waxed paper dielectric caused an increase
in the equivalent series
resistance of the capacitor, when schoolboy physics would suggest it causes a decrease
in the equivalent parallel
resistance of the capacitor. Did you understand that?
... if you use any capacitor other than these Panasonics it is at your peril.
Crikey, that's not much help then, since they're out of production. Do you know whether Dr. Falco has any plans to test any capacitors which are in production?
The pack I used in the Bosch was the same as this one, minus the mounting bracket, and I installed it in the end cap using the minimum amount of 3000 psi epoxy necessary to insure it would stay in place.
Yes, if securing a capacitor in the end cap with resin, it is important to use the minimum necessary. We have seen some where the resin totally fills the end cap, bridges the gap between it and the windings, and encapsulates the tails of the winding. Removing a dud capacitor without also damaging the winding insulation or the tails is then, ummm, difficult to say the least. Some people use a blob of bathroom sealant, which seems to work quite well.
After reassembling the armature with the new capacitors I checked the output using one of my Merc-o-Tronic 9800 testers. The blue arc in the window of the tester shows the operation of the coil is reliable at a test current of just over 1 Amp supplied to the primary. This is as it should be, so the completed armature passes this test.
Again, do you recommend a room temperature test or a hot test?
But as I said at the start, a very interesting and informative series of posts. Keep up the good work!