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Brightspark Magnetos
NEW ...
Take a look
at some of the equipment we use in our workshop for magneto servicing
and overhauls.
Our pick-up and slip ring (PUSR)
tester is used in conjunction with the
Magalyser coil tester to check
for leakage through the insulation of slip rings and pick-ups. The
low-tension pulses from the Magalyser are fed to an ignition coil mounted in
the bottom of the PUSR tester, and the HT output of the ignition coil is fed
back to the Magalyser's spark gap.
The top plate of the PUSR
tester has an earthed fixture for the slip ring so that the hole through the
slip ring and the outer surfaces of its flanges are exposed to earthed
metal, just as they are when mounted on a magneto armature. That fixture is
doubled-ended and snugly fits Lucas slip rings with one end, and BTHs with
the other. The top plate is also formed with two rows of holes and fittings
so that one, or a pair, of various common pick-ups can be attached to the
earthed plate so that they are mounted similarly to when they are on their
magneto. The top plate, complete with any pick-ups and slip ring mounted on
it, is quickly detachable so that the whole lot can be placed in an oven.
The HT output from the ignition coil also goes to a
five-position switch, which can connect it to:-
(1) the brush of the pick-up mounted in the first row;
(2) nothing;
(3) the brass track of the slip ring;
(4) nothing; or
(5) the brush of the pick-up mounted in the second row.
So, with the PUSR tester, we can see what effect
connecting a slip ring or pick-up into a spark-generating circuit (operating
at about 8 to 10 kilovolts) has on the current required to produce the
spark, and so we can simulate the effect that the slip ring or pick-up has
on the LT current that its magneto needs to produce.
(Please click the play button)
Conveniently, when the switch is in a "nothing" position
(2 or 4), the minimum LT current required from the Magalyser for the PUSR
tester's coil to produce consistent sparks at the Magalyser's 5.5 mm 3-point
test gap is 1.0 amp. When the switch is moved to the other positions
(1, 3, or 5), then slightly more current, perhaps up to 1.1 A, may be
required when a good pick-up or slip ring is in place. However, if the insulation of
the pick-up or slip ring is leaky, considerably more LT current is required
in order to achieve consistent sparks at the spark gap. In the example shown
in the video above, both pick-ups needed 10% extra current, when cold, which
is just passable. However, the slip ring needed 70% extra current, when
cold, and was therefore leaking very badly.
(Please click the play button)
Pick-ups and slip rings often only exhibit leakage when
warmed to operating temperature, and it is therefore very important to test
them hot. The second video is a repeat of the earlier test, but after the
pick-ups, slip ring and top plate of the PUSR tester have been warmed in an
oven to about 50°C. In the example, the pick-ups
then needed 25% and 30% extra current, showing that they were going leaky at
elevated temperature. With the slip ring, it wasn't possible to get any
sparks at all with the maximum current that the Magalyser could produce. All
the slip ring did was make a noise and kill off the sparks.
Although the majority of "it
won't start hot" magnetos have a fault with the condenser, especially if it
is an original Lucas paper condenser, or a later substitute paper condenser,
a significant proportion of these problems are additionally or alternatively
caused by winding insulation, pick-up(s) or a slip ring that goes leaky when
hot.
Footnote 1: We're not sure where the slip
ring used in the videos came from, but it has clearly never been used in a
magneto. Regrettably, many new high-tension parts for magnetos that are put
on the market are moulded out of materials without the required dielectric
strength and are not up to the job. When buying slip rings and pick-ups,
beware! High quality slip rings and pick-ups on which we conduct quality
control tests are available from our
online shop.
Footnote 2: Slip rings and pick-ups need to be tested at the
voltages to which they are exposed in operation, i.e. about 8 to 10
kilovolts. Merely using an insulation tester, such as a Megger, is
insufficient unless it can test at such high voltages.
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