Power Supply Circuit Description; Trouble Kind; Finding Where Is Trouble; Troubleshooting - Sharp MZ-700 Service Manual

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 Q 1 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.
TROUBLESHOOTING
1. Trouble kind
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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)
021 in failure
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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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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
24
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.
,