Power Supply Description

11-5835-243-34/EE641-AA-MMl-010/E154

MTT/TO

31S3-4-110-1

tape motion is in the forward direction, DRIVE B

isolated transformer secondaries. The + 28-vdc

input power is chopped by the inverter/oscillator

goes positive, and the DRIVE A side of the bridge is

cut off. Operation of this side of the bridge is

and transformer coupled to four regulators. Figure

identical to the DRIVE A side described above. In

2-8 is a simplified block diagram of the power

this manner the dc motor is driven linearly in either

supply. Figure 2-9 illustrates the tape transport

direction without the use of relays or other else

power and ground system.

tromechanical devices.

c. Power Turn-on. The junction field effect

transistors (JFETs) Q8 and Q9 restrict the motion

of the motor during power turn-on. R18 and C4 form

an RC network which controls the transistor gate

voltage. At startup, the positive side of C4 is at

ground. This turns Q8 and Q9 which hold the drive

signal transistors Q1 and Q7 in the cut off state,

inhibiting motor motion. AS the charge on C4 in-

creases, the gate voltages increase until Q8 and Q9

are turned off. In this state Q8 and Q9 present a

high impedence and do not interfere with normal

operation of the power amplifier circuitry.

d. Elapsed Time Meter. The elapsed time meter

on the front panel is energized when the capstan

motor operates. Transistor Q4 (part of the OR

circuit made up of Q4, CR3 and CR4) returns the

meter to ground when there is a positive voltage at

either of the motor terminals..

2-11. Power Supply Description

a. General. The tape transport power supply

EL4RD007

(PSI, fig. 1-2) is a sealed replaceable unit. The

power supply is a dc-to-dc converter type with

Figure 2-8. Power Supply Functional Block Diagram.

2-15