TTB-9-2920-225-34-1 Regulator Check Out and Repair 100 Amp 2920-00-900-7993 Download

TB 9-2920-225-34-1 Section V. VAP-AIR MODEL 26440001-05 2-10. General. Figure 2–17 is a plan view of the regulator with the cover off. Figure 2–18 is the end view of the regulator showing the receptacles and the voltage adjustment point. Figure 2-19 is an exploded view of the diode mounting. Figure 2-20 is a relay location drawing. Figure 2-21 is a view of the circuit board and transistor heat sink. Figure 2–22 is a view of the circuit board showing the electrical test points. There are 12 electrical test points. Figure 2–23 is an electrical schematic diagram of the regulator. Table 2–8 lists the resistance checks. Table 2-9 lists the voltage checks. Table 2-10 is a transistor substi- tution list. 2-11. Checkout and Repair. a. Voltage Adjustment Rheostat (R2) (power disconnected). (1) Place ohmmeter test probes to the violet and brown wire terminals of the rheostat (see fig. 2-17) . (2) Rotate rheostat fully clockwise, reading should be approximately 250 ohms. (3) Rotate rheostat fully counterclockwise, reading should be less than 5 ohms. (4) If other readings are obtained, replace rheostat. b. Transistors. (1) Remove transistors Q4 and Q5 (see fig. 2-17). (2) Position transistors as shown in figure 2-1, and check each separately (see fig. 2–2). (3) If resistance values differ from the above, replace the transistors through cannibaliza- tion or requisition transistor Q4, Vap-Air part number 26316033-12 (2N3441), NSN 5961-00– 054-4141, or transistor Q5, Vap-Air part num- ber 26316033–70 (2N3773), NSN 5961-00–929– 5014 (see table 2–10 for substitutes). (4) Check the mica insulating washers be- tween the transistors and mounting plate. Re- place if cracked, broken or missing. Coat washers with insulation compound per MIL-S-8660, NSN 6850–00–880-7616. c. Diode. (1) Remove four screws holding transistor and circuit board mounting plate, and move plate to one side. (2) Place negative ohmmeter probe to the top side of diode CR3 (see fig. 2-19). 2-22 (3) Place positive probe to the bottom side of CR3. Meter should indicate low resistance. (4) Reverse probes. Meter should indicate high resistance. (5) If other readings are obtained replace diode CR3 (MR1122), NSN 5961-00-103-1519 (see fig. 2-19). d. Relay. (1) Place ohmmeter probes across relay coil terminals (see fig. 2–20 ). Meter should read ap- proximately 60 ohms. (2) Place ohmmeter probes across contactor terminals (see fig. 2–20). Meter should read infinity (open circuit ). (3) If other readings are obtained, replace relay, Vap-Air part number 27124377, NSN 2920- 00–735-9542. (4) Reinstall transistor and circuit board mounting plate. e. Detailed Circuit Checks. (1) Table 2-8 contains “power off” resist- ance checks of the circuit (see fig. 2–22 for test points). (2) Table 2–9 contains “power on” voltage checks of the circuit (see fig. 2–22 for test points). f. Functional Checks. Check the performance of the regulator on the test stand and make final voltage adjustments. Voltage output should read 28 volts. If the regulator does not function prop- erly, perform the following: (1) Check wires and connections. (2) Replace circuit board, Vap-Air part number 27534930. (3) Perform bench test procedures. NOTE After final repair, check regulator on the test stand. If the regulator still does not function correctly 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 hairline cracks will show good at lower tempera- tures and will tend to show open circuit at higher temperatures. If the board seems good at room temperature, but the regulator still fails, this could be the cause.