Section IV. VAP-AIR MODEL 26440001-04

TB 9-2920-225-34-1

If the regulator still does not function correctly

Table 2-4. Substitutes for D1-Model 03

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

each component. All the parts should be checked

with an ohmmeter and the capacitors checked with

a capacitance meter. The circuit board traces

should be checked with an ohmmeter to make sure

that there are no cracked traces. Traces with hair-

line cracks will show good at lower temperatures

and will tend to show open circuit at higher tem-

peratures. If the board seems good at room tem-

perature, but the regulator still fails, this could

be the cause.

Section IV. VAP-AIR MODEL 26440001-04

2-8. General. Figure 2-10 is a plan view of the

if cracked, broken or missing. Coat washers with

insulation compound per MILS8660, NSN 6850

regulator with the cover off. Figure 2-11 is an

end view of the regulator showing the receptacles

00-880-7616.

and the voltage adjustment points. Figure 212 is

c. Diode.

an exploded view of the diode mounting. Figure

(1) Remove four screws holding transistor

213 is a relay location. Figure 214 is a view of

and circiut board mounting plate, and move plate

the circuit board and transistor heat sink. Figure

to one side.

215 is a view of the circuit board showing the

(2) Place negative ohmmeter probe to the

electrical test points. There are 12 electrical test

topside of diode D4 (see fig. 2-12).

points. Figure 216 is an electrical schematic dia-

(3) Place positive probe to the bottom of D4.

gram of the regulator. Table 25 lists the resist-

Meter should indicate low resistance.

ance checks, and table 2-6 1ists the voltage checks.

(4) Reverse probes. Meter should indicate

Table 27 is a transistor substitution list.

high resistance.

2-9. Checkout and Repair.

(5) If other readings are obtained replace

a. Voltage Adjustment Rheostat (Rz) (Power

diode D4 (MR1122), NSN 5961001031519 (see

Disconnected).

fig. 2-12) .

(1) Place ohmmeter test probes to terminals

d. Relay.

of the rheostat (see fig. 2-10).

(1) Place ohmmeter probes across relay coil

(2) Rotate rheostat fully clockwise, reading

terminals (see fig. 2-13).

should be approximately 250 ohms.

(2) Place ohmmeter probes across terminals

(3) Rotate rheostat fully counterclockwise,

(see fig. 2-13). Meter should read infinity (open

reading should be less than 5 ohms.

circuit).

(4) If other readings are obtained, replace

(3) If other readings are obtained, replace

rheostat.

relay, NSN2920007359542.

b. Transistors.

(4) Reinstall transistor and circuit board

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

mounting plate.

2l0).

e. Detailed Circuit Checks.

(2) Position transistors as shown in figure

(1) Table 25 contains "power off'' resistance

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

checks of the circuit (see fig. 215 for test points).

(3) If resistance values cliffer from the values

(2) Table 2-6 contains "power on" voltage

shown in figure 22, replace the transistors

checks of the circuit (see fig. 215 for test points).

through cannibalization or requisition transistor

f. Functional Check. Check the performance of

Q4, Vap-Air part number 26316033-12 (2N3441),

the regulator on the test stand and make final volt-

NSN 5961000544141, or transistor Q5, Vap-

age adjustments. Voltage output should read 28

Air P N 2631603370 (2N3773), NSN 596100

volts. If the regulator does not function properly,

9295014 (see table 27 for substitutes).

perform the following:

(4) Check the mica insulating washers be-

( 1 ) Check wires and connections.

tween the transistors and mounting plate. Replace

2-13