TM 55-4920-244-14
WARNING
(5) Connect an ohmmeter across CW
and CCW terminals of potentiometer. Reading
Use trichloroethylene only
should be 50 ohms + 5 percent.
in adequately ventilated
(6) Turn shaft to extreme counter-
area. Avoid prolonged or
clockwise position. Connect ohmmeter across
repeated breathing of va-
CCW and SLIDER terminals.
por. Avoid prolonged or
(7) Turn shaft slowly through its
repeated c o n t a c t with
range to extreme clockwise position, simultane-
skin.
ously observing ohmmeter. Replace potentiome-
(2) Use a cotton swab or camel's
ter if ohmmeter shows open circuit at any time
hair brush dipped in trichloroethylene (item 2,
or if any indication is given other than a smooth
flow from minimum to maximum value. Sudden
and wipers.
jumps to higher values of resistance followed by
a return to lower value indicates dirt or corrosion
CAUTION
on element or slider. A fairly consistent reading
Do not apply any pres-
on ohmmeter over a large range of shaft rotation
sure to spring-type con-
indicates a short circuit over that portion of the
tacts. Any weakening of
element.
tension will cause prema-
e. Repair.
ture failure of rotary
(1) Remove small areas of corrosion
switch.
with a fine file and tin area after all evidence
of corrosion has been removed. Replace potenti-
(3) Place a drop or two of trichloro-
ometer if corrosion has weakened terminal or ex-
ethylene (item 2, table 1-1) between jaws of con-
tends into body of potentiometer.
tacts and rotate rotary switch through its entire
(2) Replace potentiometer if condi-
range several times to free any foreign matter in
tions outlined in d above exist.
contacts.
(4) Use a dry cotton swab or pipe
4-24. Rotary Switches (Figure 4-8).
cleaner to remove any residue from wiper.
d. Inspection and Testing.
The rotary switches are SW-1 (10), SW-
3 (16), SW-4 (16), SW-6 (34), SW-7 (12, fig
NOTE
4-9), and SW-8 (37, fig. 4-8). With the excep-
Steps (1) and (2) do not
tion of SW-1, all rotary switches are phenolic
apply to SW-1.
wafer, open construction type. Switch SW-1 is
(1) Make sure all terminals are at-
enclosed construction. All rotary switches have
tached firmly to phenolic wafer and will not
positive detent action in contact positions and are
turn.
break-before-make type.
(2) Make sure detent action is posi-
a. Removal.
tive at each position and that wiper is securely
attached to rotating portion of wafer.
(1) Remove deck assembly (para. 4-
(3) Using an ohmmeter capable of
32) .
accurately reading values of 1 ohm, check all
(2) Identify and tag wiring. Unsol-
contacts for a closed circuit value of less than 1
der and remove wiring from rotary switch.
ohm and an open circuit value of at least 100
(3) Loosen setscrew and remove
megohms.
knob from rotary switch.
e. Repair. Replace rotary switch if any of
(4) Remove seal nut which secures
the conditions described in d above exist.
rotary switch to deck. Remove switch.
b. Disassembly. No disassembly is re-
4-25. Toggle Switch (Figure 4-8).
quired. Replace entire rotary switch if defective.
Selector switch (SW-2) (98) is a double
c. Cleaning.
pole, double throw toggle switch used to trans-
(1) Use a fine camel's hair brush to
fer the thermocouple input of the potentiometer
remove foreign matter from rotary switches.
circuit between heater cable and check cable con-
nectors.
NOTE
a. Removal.
Steps (2) through (4)
(1) Remove deck assembly (para. 4-
do not apply to SW-1.
32).
4-22
