Section V. VAP-AIR MODEL 26440001-05

TB 9-2920-225-34-1

Section V. VAP-AIR MODEL 26440001-05

(3) Place positive probe to the bottom side

2-10. General. Figure 217 is a plan view of the

of CR3. Meter should indicate low resistance.

regulator with the cover off. Figure 218 is the

(4) Reverse probes. Meter should indicate

end view of the regulator showing the receptacles

high resistance.

and the voltage adjustment point. Figure 2-19 is

(5) If other readings are obtained replace

an exploded view of the diode mounting. Figure

diode CR3 (MR1122), NSN 5961-00-103-1519

2-20 is a relay location drawing. Figure 2-21

(see fig. 2-19).

is a view of the circuit board and transistor heat

sink. Figure 222 is a view of the circuit board

d. Relay.

showing the electrical test points. There are 12

(1) Place ohmmeter probes across relay coil

electrical test points. Figure 223 is an electrical

terminals (see fig. 220 ). Meter should read ap-

schematic diagram of the regulator. Table 28

proximately 60 ohms.

lists the resistance checks. Table 2-9 lists the

(2) Place ohmmeter probes across contactor

voltage checks. Table 2-10 is a transistor substi-

terminals (see fig. 220). Meter should read

tution list.

infinity (open circuit ).

(3) If other readings are obtained, replace

2-11. Checkout and Repair.

relay, Vap-Air part number 27124377, NSN 2920-

a. Voltage Adjustment Rheostat (R2) (power

00735-9542.

disconnected).

(4) Reinstall transistor and circuit board

mounting plate.

(1) Place ohmmeter test probes to the violet

and brown wire terminals of the rheostat (see

e. Detailed Circuit Checks.

fig. 2-17) .

(1) Table 2-8 contains "power off" resist-

(2) Rotate rheostat fully clockwise, reading

ance checks of the circuit (see fig. 222 for test

should be approximately 250 ohms.

points).

(3) Rotate rheostat fully counterclockwise,

(2) Table 29 contains "power on" voltage

reading should be less than 5 ohms.

checks of the circuit (see fig. 222 for test points).

(4) If other readings are obtained, replace

f. Functional Checks. Check the performance

rheostat.

of the regulator on the test stand and make final

b. Transistors.

voltage adjustments. Voltage output should read

(1) Remove transistors Q4 and Q5 (see fig.

28 volts. If the regulator does not function prop-

2-17).

erly, perform the following:

(1) Check wires and connections.

(2) Position transistors as shown in figure

( 2 ) Replace circuit board, Vap-Air part

2-1, and check each separately (see fig. 22).

number 27534930.

( 3 ) If resistance values differ from the

(3) Perform bench test procedures.

above, replace the transistors through cannibaliza-

tion or requisition transistor Q4, Vap-Air part

NOTE

number 26316033-12 (2N3441), NSN 5961-00

054-4141, or transistor Q5, Vap-Air part num-

After final repair, check regulator on

ber 2631603370 (2N3773), NSN 5961-00929

the test stand.

5014 (see table 210 for substitutes).

If the regulator still does not function correctly

(4) Check the mica insulating washers be-

it will be necessary, if local skills permit, to test

tween the transistors and mounting plate. Re-

each component. All the parts should be checked

place if cracked, broken or missing. Coat washers

with an ohmmeter and the capacitors checked with

with insulation compound per MIL-S-8660, NSN

a capacitance meter. The circuit board traces

685000880-7616.

should be checked with an ohmmeter to make

c. Diode.

sure that there are no cracked traces. Traces with

(1) Remove four screws holding transistor

hairline cracks will show good at lower tempera-

and circuit board mounting plate, and move plate

tures and will tend to show open circuit at higher

to one side.

temperatures. If the board seems good at room

temperature, but the regulator still fails, this

(2) Place negative ohmmeter probe to the

could be the cause.

top side of diode CR3 (see fig. 2-19).

2-22