Robbie wrote:

> If the resistor isn't working the motor current flows through the
> points alone.  When the resistor fails an oscilloscope will then
> indicate greatly increased voltage across the points, ...

When you see sparking never try to connect a scope on these points.
Most probably you will destroy the input circuit of your scope.
Sparking starts at a point when the voltage across two points, 1 mm
(0.04") apart, reaches a voltage of 1000 volts.

This is the way I tested the high voltage of color TV sets under
repair: when the high voltage connection to the picture tube started
sparking when it reached a distance from about 20 mm.(0.8") to the
chassis, the voltage was 20 kV(!) and was approved.

You have to put extreme safety precautions.  So don't do this unless
you are a qualified technician and you are not alone!

By the way, when you have installed new brushes and the sparking is
not gone completely, don't be upset.  It takes quite a time to let the
brushes settle to the surface of the collector.  When the collector is
some what damaged, sandpapering the collector under reduced speed will
often reduce the sparking.

Reducing the speed can be done with a light dimmer or a light bulb in
series with the motor.  Again this should only be done by experienced
(electro)-technicians!!

Cheers,
Willy van der Reijden,
The Netherlands

 [ I use a high-voltage oscilloscope probe designed for use with
 [ automobile ignition coils.  For testing fractional-horsepower
 [ motors, I use either a variable transformer (Variac or Powerstat)
 [ or a variable high-power DC supply, as appropriate.  -- Robbie