The table below describes various symptoms that can occur if there is a short on one of the power supply lines. These types of problems occur when major components being supplied fail by shorting. An example would be if the horizontal output were to fail, the set might shut down as if the B+ line were shorted.
he Stereo LED can only be used as an indicator if the set is
in Tuner Mode. If a failure occurs when the unit is not in the Tuner mode, the
Stereo LED cannot be used as a failure indicator [Click on picture to Enlarge}
POWER SUPPLY PROTECTION
LATCH
Placing a HIGH on Q653/B activates the latch. This turns ON
Q653, which turns ON Q652. Once Q652 turns ON its conduction keeps Q653 ON
until power is disconnected. This places a LOW at Q648/B, turning it OFF. This
causes RY600 Power Relay to open and the set turns OFF.
+135 OVER CURRENT PROTECTION
Over current is detected by monitoring the voltage across
R652. When this voltage, which rises as more current is drawn, gets to a level
that causes Q651 to turn ON, the latch will be activated.
Vcc RECTIFIED LOW VOLTAGE PROTECTION
The signal from T605/3 is sent through R601 and rectified by
D603. This voltage is filtered by C618. It is important that this voltage be at
a certain level since it is used to control regulation. Therefore a circuit is
constructed where Q623 is normally ON. This transistor is ON because D607 should
always have sufficient bias to turn ON. This will place Q623/C at close to
ground level. This keeps D604 and D608 from conducting and activating the latch
circuit. If this voltage were to fall below a certain level, D607 would turn
OFF, causing Q623 to turn OFF. When Q623 is OFF, the +12 volt line would cause
D604 and D608 to turn ON, which would activate the latch.
FOLDBACK
The foldback circuit is a type of protection circuit that
uses a winding on T605 PIT to monitor the current drawn by its secondary. This
voltage is rectified and called Vcc Rect. Q624 is setup with the Vcc Rectifier
voltage input to the emitter. Its base is connected to the Rectified +12 volt
line from the secondary side of the supply, through D605 and R609. D605 is a
zener diode whose breakover voltage is 13 volts. This circuit will protect against
two failures:
The first is a rise in the Vcc Rect. This voltage is
developed by the foldback winding of T605. This winding is used as a current
mirror. This means that the more current drawn on the secondary of T605 PIT,
the higher the voltage at T605/3. When too much current is drawn from the
secondary, Q624 would turn ON because of the increase in voltage across R610.
If this occurs the latch and soft start circuits will be activated. Soft start
is activated to lower the output supply voltage by increasing the switching frequency.
The latch’s purpose is to shut the supply down. Secondly, if there were a loss of the +12 volt line then D605
would breakover. This would cause sufficient current flow through
R610 and Q624 would turn ON. If this occurs, the latch and soft start circuits
will be activated.
