Fig.
5-25
Fig.
5-26
Fig.
5-27
Fig.
5-28
Solder surface.
should be f
lat
c.
Checking field coil
Ground test:
5
To test the field coil for ground with an ohmmeter,
place one prod on the yoke
or
pole core and the other
prod
to
the field terminal.
An infinite meter reading should be obtained.
If
a
reading other than infinite is found, replace the field
coil and yoke assembly.
Short circuit test:
Connect the prods to each
of
the two leads.
If
there
is
no
flow
of
current, the coil
is
open circuit
and must be
replace~.
d. Checking brush holder
Check the brush holder for ground. Touch one prod
of
an ohmmeter to the insulated brush holder and
the other prod to the brush holder frame.
If
the meter reading
is
other than infinite, the brush
holder assembly
is
shorted and must be replaced.
Repeat this test for the other insulated brush hoJder.
Do
not
use
this test
on
the two grounded brush holders.
e.
Checking brushes and brush springs
Check the brushes and replace
if
they are worn down
more than one third
of
their original length.
Otherwise, the brush spring tension will
be
reduced,
leading
to
an increase in the brush-commutator contact
resistance. This will lower the torque and cause the
burnt commutator surface.
The spring tension
is
1.3 ~ 1.7
kg
(46 - 60 oz).
If
the tension
is
too low, replace the springs.
To replace the brush, proceed
as
follows:
l.
Remove the brush from the holder.
2. Smash the old brush
by
tapping
it
with small hummer
or pinching with pliers.
3. Clean the brush lead and insert the lead to small
chamfer side
of
new brush.
4. Solder the lead and brush together, using rosin
core solder.
Use
a
200
-wa
tt iron.
f.
Checking bushes
Check the clearance between the armature shaft and the
bush.
If
it
exceeds 0.2 mm (0.008 in), replace
the
bush.
5 8
,.
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