Toshiba CF35E50, Technical Training Manual

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5. PROTECTION CIRCUIT DIAGNOSIS PROCEDURE
• 125V line becomes over voltage, thyristor D862 comes in to failure, and when Q863 does not turn on, D888 is short-circuited
and intermittent oscillation occurs. To protect the circuit a double protection system is employed.
• When the overvoltage protection circuit is working. never turn on the power with the protection circuit disabled.
High voltage will be stepped up and secondary breakdown may occur.
Operation of protection circuit
for Thyrister D862. (SR81
Relay turns on but immediately
turns off.)
Power on with D870
opened. (Do not turn on
for a long period.)
With SW turned on,
check 125V line voltage
with oscilloscope.
SR81 turns on for a short time
but immediately turns off.
Higher than 130V.
Higher than 35V.
Higher than 24.1V.
Less than 130V
Less than 35V
Less than 24.1V
With SW turned on,
check 24.5V line voltage
with oscilloscope.
With SW turned on,
check voltage across C471
with oscilloscope.
Check C867, D862, D878,
Q863, R874, R875, R877,
and repair.
Check over-current protection circuit,
H output and repair D870, Q870,
R870 - R873, R876, R881.
Check over-voltage protection circuit
(125V) and D846, D878, Q801, Q841,
Q845, Q862 and repair.
Check broken pattern in feedback loop.
Check over voltage protection circuit
(24,5V) (125V rectification line opened,
or 24.5V line leaded light.)
Check rectification line (T862 -C889),
Q610, pattern connectors connected to
Q610, and repair.
Check X-ray protection circuit, H output
circuit, X-ray protection detector circuit,
and repair.