Sharp MZ-700 manual Power Supply

8.POWER SUPPLY

Power supply circuit description

(1)AC source power is rectified through the diode bridge (RB-156).

(2)Current flowing through the primary coil of T1 is switched by means of Q1 so as to induce electromotive force in the secondary coil. To protect voltage fluctua- tion on the primary side, Q2 is implemented, which will become active to deactivate Ql by dropping the base of Q1 to GND when voltage increase occurs on Q2.

(3)The electromotive force induced in the secondary coil is rectified through D21, and DC5V is derived from the network consisting of C22, C23, and L21.

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MZ-700

(4)For circuit stabilization against load fluctuation, we use IC21 (shunt regulator), PC1 (photocoupler), and Q3 (transistor). When the gate of IC21 is above 2.5V after sensing the 5V output by means of R22, VR21, and R23, it makes IC21 active and then PC1 active.

As the base voltage of Q3 increases with activation of PC1 photo-transistor, it makes Q3 active and drops the base ofQ1 to GND so as to turn offQ1.

Although it is possible to adjust the DC 5V voltage by means of VR21, it needs to put the CPU PWB into connection.

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TROUBLESHOOTING 1. Trouble kind

Because different location has to be repaired depending on a kind of trouble, you have to check, the output voltage in the first place. (Apply source voltage of 220 to 240V with a 3.75A load connected on the output.)

There are following four cases:

1)No output (OV)

2)Low output (1 to 4V)

3)High output (7 to 10V)

4)Abnormal increase of output ripple (several hundreds millivolts)

2. Finding where is in trouble

2-1. In the case of "No output (OV)"

N

Observe waveform between the collector and emitter of 01

(*1 )

01 or 01 in failure

021 in failure

,

01, R1 in failure	R5, 02, 03, R3,
	03 in failure

(*1): If the output voltage were to be still OV without blowing the fuse after replacement of Q1, proceed to steps described to the left of the troubleshooting flowchart.

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