Hitachi VT-FX795A, Руководство по обслуживанию, Страница: 16 / 69

3-4
Power problem
It is highly recommended that a variable isolation
transformer which can monitor current be used.
(Alternatively a variable AC source which moni-
tors current will do). Read directions below before
power is added!
Repair method #1
(Power must be off)
Short circuit in the secondary side. Check diode D013,
D015, D016, switching FET (Q001), control transistor
(Q002, Q029), diode (D006), and resistor (R014)
replace as necessary.
Disconnect 40V diode (D013), 12V diode (D015), and
5V diode (D016). Check the load continuity of 40V
line, 12V line, and 5V line through a tester (resistance
range).
If the tester indicates a lower resistance value around
0 ohm, the line is short-circuited.
Before repairing the switching power supply, find out
the short-circuited area of such line and repair it.
If the tester does not indicate any low resistance value
(around 0 ohm), no load is short-circuited and there is
no problem.
1] Check for any defective parts while the secondary
rectifying diodes are disconnected. Perform a diode
check in both forward and reverse directions through a
tester.
2] Remove the following components and check for
defects: snubber diode (D051), switching FET (Q001),
source resistor (R014), control transistor (Q002).
Repair method #2
Check the fuse 1.0A (F001), primary rectifying diodes
(D001-D004) as possible problems. Remove the
above mentioned parts and check them. The circuit
which turns on switching FET (Q001) may be
regarded as a possible cause, even if the load at the
secondary side is shorted, it can't be detected
because switching FET (Q001) isn't operating. Per-
form check according to the step 1] and 2] of repair
method #1 and check the following parts:
(Remove the part from PCB)
Switching FET (Q001), source resistor (R014), gate
resistor (R008) and start resistor (R004 and R005).
Repair method #3
A circuit to turn on switching FET (Q001) may not work
and this may be regarded as a cause of trouble. Even
if the load at the secondary side is short-circuited, it
cannot be detected because switching FET (Q001)
does not turn on. Therefore, perform check according
to the steps 1] and 2] of the repair method #1 and exe-
cute the under-mentioned parts breakage check.
(Remove the part from PCB.)
switching FET (Q001), source resistor (R014), control
transistor (Q002), gate resistor (R008) and start resis-
tor (R004 and R005).
Repair method #4
The feedback circuit which is monitored by the output
of voltage may not work and this may be regarded as
a possible cause, remove control transistor Q002 and
check for defects. More over, a photo coupler (IC001)
and transistor (Q031) may be defective, replace any
defective parts with factory originals.
Repair method #5
If the output voltage of the secondary side is slightly
high, the line load may be in the "OPEN" state and this
may be regarded as a cause of trouble. If there is no
output voltage on the secondary side, the rectifying
diodes (D013, D015).
Repair method
CHECK
Connect unit to the isolation trans-
former and slowly increase the AC sup-
ply while monitoring the current, if it
draws too much current (Be ware fuse
is rated for 1.0 amps), then turn off
supply and do repair method #1.
YES
#1
NO
Check whether the primary rectifying
DC of the Switching power supply has
an output. (Reading should be about
168V.)
NO
#2
YES
With the primary DC working check
the secondary 5V. NO
#3
YES
Are the 40V, 12V, and 5V than normal?
YES
#4
NO
Although the secondary 5V is working,
are any of the other voltages higher
than normal (40V, 12V)?
YES
#5
NO
There is no problem on the SW power
supply.