CHAPTER 5 FUNCTIONING OF EQUIPMENT - TM-11-6625-542-14-10021

TM 11-6625-542-14-1

CHAPTER 5

FUNCTIONING OF EQUIPMENT

fan. Thermal switch K101 is a safety switch connected

5-1. Block Diagram

in series with the primary winding of the power

(fig. 5-1)

transformer. In instances of fan failure or high ambient

The various circuits described in this chapter are made

room temperature (above 1280F) K101 will open, thus

with reference to figure 5-1, a functional block diagram of

protecting the instrument from damage due to excessive

the AN/USM-108A, and schematic diagrams (figs. 5-2

heat. The crystal oven will continue to operate when

through 5-8).

K101 opens. When the unit cools, K101 closes and

5-2. Power Transformer.

power is restored to the unit. Fuse F101 is used as a

a. The primary winding of power transformer

protective device against electrical overload in the circuit.

T101 consists of two separate windings. One of the

windings is tapped to furnish voltage to the ventilating

Figure 5-1 Functional block diagram.

(Located in back of manual)

Figure 5-2 Power supply, schematic diagram.

(Located in back of manual)

b. The secondary winding of T101 consists of

control. When this control is properly adjusted, the

seven separate windings. Three of these windings are

output voltage is exactly --150 volts. Capacitor C163

connected to selenuim rectifier bridges which feed the

filters the output. Variations in the load which tend to

regulated voltage supplies. Regulated voltages of +350,

change the output voltage cause the potential on the grid

+225, and -150 are furnished by the supplies. Voltage

of V144A to change proportionately. An error voltage

sources of --8 and --15 volts are derived from the -150

thus exists between the grids of the difference amplifier.

volts supply.

The error voltage is amplified by V144B and fed directly

to the grids of V157 and V167. The change in potential

5-3. Crystal Oven Transformer

at the grid of these tubes causes a corresponding

Transformer T102 is a separate transformer which

change in voltage at the plates. The voltage at the plates

supplies 6.3 volts ac to a thermostatically controlled

is coupled, in turn, through rectifier CR141 to

crystal oven. Lamp B201 indicates when the oven is on.

compensate for variations in the output voltage and to

Transformer T102 is directly fed from the ac powerline.

cause it to return to the --150 volts level.

Therefore, the ON-OFF power switch has no effect on

5-5. +225 Volts Supply.

the crystal oven control.

The - 150 volts supply is used as a reference voltage for

5-4. - 150 Volts Regulated Reference Voltage

the +255 volts supply. Resistors R135 and R137 drop

The --150 volts regulated power supply is fed from the

the - 150 volts supply to approximately zero on the grid of

185 volt secondary winding across terminals 11 and 12

V124. If the load should change, any tendency of the

of T101. Gas diode tube V149 acts as a reference

+225 volts supply to change correspondingly would

voltage source for the --150 volts supply and maintains a

cause V124 to amplify the change, or error voltage. The

fixed potential of -84 volts on the grid of V144 (one-half

output of V124 is directly coupled to cathode follower

of a difference-amplifier). The triode section of V144 is

V107A. Thus, the amplified error voltage appears on the

connected across voltage divider R141, R142 and R143.

grid of the V107A. Because the cathode of V107A

The potential on the grid of V144A depends upon the

follows the

setting of potentionmeter R143, the --150 volts adjust

5-1