TM 5-4520-241-14
(4) Open the control box cover. Remove nuts (1, fig. 6-3), lock washers (2), and flat washers (3) securing label
(4) to the PC board (16). Pull off the label for access to connection points on the board.
(5) Connect the power plug.
CAUTION
Set up tests and make all test connections before switching the HEATER-OFF-FAN
switch to HEATER. All test readings must be made with only momentary actuation
of the switch to the HEATER position.
(6) Connect a DC voltmeter to V5 (negative) and to the center pin of the transistor (positive). It may be
necessary to scrape away the varnish coating to ensure good contact. (Refer to Appendix D, Expendable Supplies and
Materials List.) Momentarily move the switch to HEATER. The voltmeter must read approximately 30 volts. If it reads
less than 28 volts, replace the PC board and transistor as an assembly (see below).
(7) Connect the voltmeter to V5 (negative) and V4 (positive). Momentarily move the switch to HEATER. The
voltage should be 13 volts. If it is less than 12 volts, replace the transistor. If it is more than 14 volts, replace the PC
board and transistor as an assembly.
(8) Connect the voltmeter to V5 (negative) and to V2 (positive). Momentarily move the switch to HEATER. The
meter should read approximately 18 volts. If it does not, replace the PC board and transistor as an assembly.
(9) If the PC board and transistor passed the tests in steps (3) through (8), neither part should be removed from
the control box control unless required for replacement of other parts.
(10) Check continuity through the bimetallic heater/sensor (33, fig. 6-3) to decide if it must be replaced. Refer to
figure 6-5 and check continuity between V4 and V5, and between V2 and V3. There should be resistance across each
pair of terminals, but if either is open, replace the bimetallic heater/sensor as an assembly.
(11) Check continuity through the reset switch (25, fig. 6-3). A white plastic knob extends through the switch.
When the knob is up (toward the bimetallic heater/sensor) there should be continuity across the switch terminals. Use a
pencil or small screwdriver blade to push down the white knob, simulating time delay shutdown. There must be no
continuity across the switch terminals. Replace the switch if it does not operate in this manner.
b. Removal and Disassembly.
(1) Remove the control box from the heater by releasing the two stud turnlock fasteners. Disconnect the male
plug.
6-5
