Condensers - practical

Condensers - practical

This page describes how the condensers can be tested and then, if necessary, how they are replaced. The theory of what a condenser is and why it is needed in a magneto is described in our Technical Pages here
We carry out three tests on magneto condensers using modern digital electronic test meters.
1: Capacitance test. Easily carried out using a digital capacitance meter. Expect to see a value of 100 - 250nF (that's 0.100-0.250 micro Farads). The lower end for small magnetos, the upper end for larger magnetos with stronger magnets. This one is reading just over 161nF so that's a good result.
2: Insulation test. Again easily carried out using a digital insulation meter. Most have a facility to select the test voltage which should be chosen to represent the type of voltages the condenser is likely to see in normal operation. On this one, 250v would be a little low. It's probably not wise to  use the 1000v range. It is too high and will only serve to over stress the condenser and may damage an otherwise usable component. We use the 500v range. You should expect to see in excess of 10Mohms. This one is reading a shade under 35Mohms so that's a good result too.
3: Charge test. Press the ‘test’ button down on the insulation tester to apply 500v to the condenser. Keep the button pressed down while one of the leads is removed from the meter. Count six or seven seconds and then touch the free lead on the other one to short the condenser - made easier by fitting a brass extension on the test lead left in the meter as shown. There should be a noticeable ‘crack’ heard and a spark seen as the condenser discharges. If they are not present, then the condenser has lost it’s charge through internal insulation leakage and should be replaced. 
If the capacitor passes these tests, we put it in the oven for an hour at about 60°C and test again. This is because the insulation properties do deteriorate as the temperature goes up.

The mica condensers, although up to a hundred years old, more often than not test out OK. If they need to be replaced, it is usually because of physical damage rather than a loss of electrical performance.

As for the paper condensers, well to be honest we used to get so many failures when testing that we don’t even bother now – we always replace them with new ones. Even NOS ones fail.

CAUTION: Sometimes vintage test equipment becomes available but be careful. We have a combined coil and condenser tester which, when we used it to 'test' a condenser, it passed even though we had already rejected it. Further examination showed that the test meter only used 60v to test the insulation........ The tester makes a very nice looking ornament and a good conversation piece but it's not much use for anything else!

Replacing condensers

There are many different capacitors available. Some are suitable for use in magnetos, many are not. This picture shows some which have been installed by others and which have failed. For the reasons explained here, The Magneto Guys always fit EVOX RIFA 271M capacitors as replacements in the magnetos we overhaul. We would be the first to admit that these are not the only suitable replacements. Yes, other alternatives are available but we use the EVOX RIFA capacitors because experience over many years, with them fitted in thousands of magnetos convince us that these work well - we have had no electrical failures with them at all. So we stick with what we know works and see no need to experiment with any new options which come on the market.
We fit the EVOX RIFA condenser in a variety of ways. When used in the end of an armature, it is held in place with a resin compound. 
Sometimes, they are fitted into the original cases and potted.
Other times they are held in place with a metal bracket - often made from a length of welding rod. 
Note that some of these pictures show two capacitors used. This is because sometimes there is limited space and two dimensionally smaller capacitors fit better than one large one. The required total capacitance is achieved by simply wiring the two capacitors in parallel and adding the individual capacitances together.
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