Diode Replacement.

TM 10-3930-633-34

are replaced, be sure that positive diodes are

(1) The stator windings may be checked with

installed in heat sink and negative diodes in-

a 110-volt test lamp or an ohmmeter. If the lamp

stalled in end frame.

lights, or if the meter readings is low when

CAUTION

connected from any stator lead to the frame, the

Do not strike any of the diodes while

windings are grounded. If the lamp fails to light,

removing. Shock may cause damage to

or if meter reading is high when successively

the other good diodes.

connected between each pair of stator lead, the

h. Slipring Servicing.

windings are open.

(1) If the springs are dirty, they may be

(2) A short circuit in the stator windings is

cleaned and finished with 400 grain or finer

d i f f i c u l t to locate wit bout laboratory test

polishing cloth. Spin the rotor in a lathe, or

equipment due to the low resistance of the

otherwise spin the rotor, and hold the polishing

windings. However, if all other electrical checks

cloth against the sliprings until they are clean.

are normal and the generator fails to supply rated

CAUTION

output, shorted stator windings are indicated.

The rotor must be rotated in order that

f. Diode Checks. One method of checking

sliprings will be cleaned evenly. Cleaning

diodes is to use an ordinary ohmmeter. The lowest

the sliprings by hand without spinning

range scale on the ohmmeter should be used and

the rotor may result in flat spots on the

the ohmmeter should have a 1 volt cell. To

sliprings, causing brush noise.

determine the cell voltage, turn the selector to the

(2) Sliprings which are rough or out-of-round

lowest scale, and then connect the ohmmeter

should be trued in a lathe to .002 inch maximum

leads to a voltmeter. The voltmeter will indicate

indicator reading. Remove only enough material

the cell volt age,

to make the rings smooth and round. Finish with

(1) With the stator disconnected, check a

400 grain or finer polishing cloth and blow away

diode in the heat sink by connecting one of the

all dust.

ohmmeter leads to the heat sink, and the other

ohmmeter lead to the diode lead, and note

i. Bearing Replacement and Lubrication,

the reading. Then reverse the ohmmeter lead

(1) The bearing in the drive end frame can be

c o n n e c t i o n s and note the reading. If both

removed by detaching the retainer plate screws

readings are very low, or if both readings are very

and then pressing the bearing from the end frame.

high, the diode is defective. A good diode will give

If the bearing is in satisfactory condition, it may

one low reading and one high reading. Check the

be re-used, and it should be filled one-quarter full

other two diodes in the heat sink in the same

with special grade ball bearing lubricant before

manner.

reassembly.

(2) To check a diode mounted in the end

CAUTION

frame, connect one of the ohmmeter leads to the

Do not overfill, as this may cause the

end frame, and the other ohmmeter lead to the

bearing to overheat.

diode lead and note the reading. Then reverse the

(2) To install a new bearing, press in with a

ohmmeter lead connections, and note the reading.

tube or collar that just fits over the outer race. It

If both readings are very low, or if both readings

is recommended that a new retainer plate be

are very high, the diode is defective. A good diode

installed if the felt seal in the retainer plate is

will give one low reading and one high reading.

hardened or excessively worn.

Check the other two diodes in the end frame in the

(3) The bearing in the slipring end frame

same manner.

s h o u l d be replaced if its grease supply is

g. Diode Replacement.

e x h a u s t e d . No attempt should be made to

(1) Diodes are pressed into the end frame

relubricate and re-use the bearing. To remove the

and heat sink. To remove diodes, use a piece of

bearing from the slipring end frame, press out

tubing slightly larger than the diode to support

with a tube or collar that just fits inside the end

the heat sink or end frame, while pressing out the

frame housing. Press from the outside of the

old diode.

housing towards the inside.

(2) When pressing in a new diode, use a piece

(4) To install a new bearing, place a flat plate

of thin-wall tubing that will contact only the outer

over the bearing and press in from the outside

rim of the diode, to press the diodes in place.

towards the inside of the frame until the bearing

Support the heat sink and end frame with a larger

is flush with the outside of the end frame. Support

piece of tubing as before.

the inside of the frame with a hollow cylinder to

(3) Positive diodes are marked with red

prevent breakage of the end frame. Use extreme

printing, negative diodes with black, If all diodes

3-4