On 2018-09-07 20:53, Bob Engelhardt wrote:
To clarify: the armature has been removed and alive-or-dead tests
were done with an ohmmeter. This showed that one segment was
disconnected. There are no shorts of any sort - just this one open
winding.

Can you clarify if it's really an open commutator segment or an open
winding? These two are not the same thing.

Consider the commutator segments as follows:

.... A B - D E F G H ...

Let's say segment "C" does not work. What does that really mean? Is "C"
completely disconnected or are there just different resistance values
when measured between B-C and C-D because one side winding is open?

You can also compare the resistance B-D to some other pair that spans
the same number of segments (like D-F, E-G, F-H).

If approximately R(B-D) = R(E-G) then the windings are intact, and only
the commutator segment has lost connection.

These windings are usually made with a continuous construction where the
same piece of wire goes from one segment through a winding to the next
segment, and so on. So the whole rotor is would from one single
continuous piece of wire and only the 2 outermost ends are connected
together at a single commutator segment somewhere.

With this construction, if the winding is still good then so is the
piece of wire that goes to the non-working commutator segment, making
the (typically crimped) connection the most likely cause of failure.

If so, consider trying to reconnect the segment by e.g. removing some of
the lacquer and soldering through the badly crimped place rather that
trying to bridge the broken segment to a neighboring one.

If the winding is no longer still good, then again, first check if there
is a loose connection at the segment's crimp joint itself - if you find
the loose connection there, try to fix it and only if there is none to
be found, consider bridging the commutator segments.